Geology of the Jemez Area, Chapter 12: The Recent Past
The previous chapter may be found here.
Soda Dam. Looking east from 35.792N
The resurgence of the Valles caldera and the eruption of the
ring domes was not the end of geological history in the Jemez.
Geology is an ongoing process. In this chapter, we will look at
geological processes in the last half million years — the very
recent past, in geological terms.
Potassium-argon dating becomes inaccurate for igneous rocks
younger than about 100,000 years in age, because there isn't time
for enough argon to accumulate to permit an accurate measurement.
Carbon-14 dating is accurate only for ages of less than about
50,000 years. Other methods of dating are either inherently
uncertain. As a result, events occurring between 100,000 and
50,000 years ago can be difficult to date accurately.
An important source of information on life during this time
period is caves, where animal and plant remains are preserved by
cool temperatures and dry conditions away from sunlight. Such
remains are not actually considered fossils, since there has not
been time for significant mineralization to take place. Little of
the original organic material has been replaced by minerals
carried by groundwater.
- A final burst of volcanism
- The El Cajete Pumice
- The Battleship Rock Ignimbrite
- The Banco Bonito Flow
- El Cajete Lake
- Hydrothermal Activity
- Sulfur and volcanoes
- Hydrothermal alteration
- Erosional landforms
- Weathering and soil
- Desert soils
- Alluvial fans
- Toreva blocks in White Rock
- Borrego Mesa
- Vallecitos de los Indios
- A river runs through it
- Zeolite ledges
- The Sierra Ladrones Formation
- Old alluvium
- Terrace gravels
Relief map of the Jemez with suspected rock glaciers
highlighted in red and Shinarump lag field in yellow.
The geological record indicates that the Earth's climate is
usually mild and stable. However, there have been several
intervals of geological time when the climate became unstable,
with large ice sheets developing near the poles and periodically
advancing over much of the temperate zone. These intervals of
geological time are known as ice ages.
There have been at least five ice ages during the history of the
Earth. Each occurred when the balance between heat arriving from
the Sun and heat radiated away from the Earth was upset. Since
astronomers have concluded that the Sun has steadily grown
brighter over the last 4.5 billion years, decreases in the solar
input are not a likely cause of ice ages (though there is evidence
that shorter cold spells may be due to changes in solar activity.)
However, the motion of the continents can affect solar heating,
because continents reflect more of the Sun's radiation into space.
When continents are covered by epicontinental seas, this effect is
reduced. It is also reduced when continents drift away from the
equator, where most of the solar radiation falls. But the most
likely cause of ice ages is reduced levels of carbon dioxide in
the atmosphere. Carbon dioxide is a greenhouse gas; it lets the
relatively short-wavelength radiation from the Sun pass through to
heat the Earth, but traps the longer-wavelength radiation
re-emitted by the Earth. Carbon dioxide levels increase when
volcanic activity is high or when vegetation dies off and
decomposes. It is reduced when vegetation flourishes or when large
areas of fresh silicate rock are exposed at the surface, allowing
carbon dioxide to combine with sodium, calcium, and potassium in
The first ice age occurred around 2.1 billion years ago, in the
Paleoproterozoic, and likely was caused by reduced carbon dioxide
levels from the proliferation of cyanobacteria. The second
occurred in the Neoproterozoic, 770 to 590 million years ago, and
was the worst ice age the Earth has experienced. It was likely
caused by continuing decline in carbon dioxide levels and the
movement of the continents towards the poles. This Varangian
glaciation, as it is called, may have covered almost the entire
earth in an ice sheet -- the "snowball Earth." When the ice
finally retreated, an explosion of new forms of life took place,
marking the beginning of the Phanerozoic Era. Two less spectacular
ice ages occurred in the Ordovician-Silurian time interval, 480 to
420 million years ago, and in the Carboniferous-Permian time
interval, 330 to 250 million years ago.
All these ice ages were characterized by periodic advances and
retreats of the ice sheets. This is probably because the advance
of the ice reduced vegetation cover and allowed carbon dioxide to
build up and trap more heat. When the ice retreated, vegetation
began proliferating again and removed the carbon dioxide from the
atmosphere, allowing the climate to cool again.
We are living in an ice age now, likely triggered by the
collision of the subcontinent of India with Asia and the exposure
of vast amounts of fresh silicate rock in the Himalayan Mountains.
Beginning about 2.6 million years ago, which marks the beginning
of the current Quaternary period, ice began building up in
Antarctica, which has remained mostly covered with an ice sheet
ever since. Fossil algae called dicoasters became extinct
and, almost simultaneously, the first modern horses appeared, and
this moment in time is formally defined as the start of the
Pleistocene, the final epoch preceding our modern Holocene epoch.
Greenland soon iced over as well, and there have been at least
fourteen distinct glacial periods since then, each lasting about
100,000 years. These were separated by relatively brief
interglacial periods of warmer weather in which the ice sheets
retreated. We are living in such an interglacial period today, the
Holocene, which began 11,700 years ago. Estimates of when the next
glacial period will occur range from within centuries to over
50,000 years, depending on various assumptions about the causes of
glacial periods (which are still uncertain) and the amount of
carbon dioxide that will be released into the atmosphere by
ourselves and our descendants.
During the Quaternary glacial periods, ice sheets have spread
over much of North America and have had a profound effect in
shaping the topography of the northern and more mountainous parts
of the continent. The Great Lakes were produced by glaciation, as
was Long Island. However, ice sheets did not extend as far south
as the Jemez Mountains, and there is little evidence that large
glaciers ever formed there. Valleys in the Jemez generally have
the V-shape characteristic of valleys cut by rivers rather than
the distinctive U-shape of valleys cut by glaciers.
However, there are several boulder fields in the higher terrain
of the Jemez that have been interpreted as remnants of rock
glaciers. Rock glaciers form when a boulder field is
saturated with water which freezes to ice, and the ice does not
completely melt during the summer months. The outer layer of
boulders acts to insulate the ice. Such permanent ice is called permafrost.
The ice accumulates within the boulder field to the point where
the field becomes ductile, and the mixture of rock and ice can
slowly flow downhill. Current conditions in the Jemez are much too
warm to permit permafrost, even at the maximum elevations of
around 3400 meters (11,000'), but temperatures were significantly
lower during past glacial periods and may have been conducive to
One of the likeliest candidates for an inactive rock glacier is
located on the
west flank of Cerros del Abrigo.
Rock glacier. Looking east from 35
55.928N 106 30.163W
This boulder field shows definite flow features in the satellite
photograph. It is also quite striking at close range.
For one thing, the toe of the field is quite steep.
Toe of possible rock glacier. 35
55.867N 106 29.344W
The flanks likewise are remarkably steep.
Flank of possible rock glacier. 35
55.850N 106 29.316W
Particularly striking is the smooth slope of the hillside on
which this glacier sits. There are no scattered boulders beyond a
few feet of the edge of the flow.
Views from atop the boulder field:
55.864N 106 29.302W
The pictures don’t quite convey the bowl shape of the glacier
surface. One gets the impression that the center was significantly
deflated when the ice melted away, which is probably about right.
The rocks are covered with lichen. This suggests the flow has not
been active in a very long time. There is actually such a thing as
lichenometric dating, based on the diameters of individual
lichen colonies, though it’s very uncertain. But one can make a
guess that the flow has not moved in at least two thousand years.
I did notice one patch at the south edge that looked very fresh.
This small area had either moved recently, or a forest fire burned
off the lichen. Some of the boulders were spalled, as if they had
been subjected to fire.
You can see a logging road crossing the flow in the upper
photograph. Here’s a panorama looking west from the edge of that
Rock glacier. 35
55.878N 106 29.236W
Behind the trees at left is Rendondo Peak and Redondito. At
center is the rugged terrain of debris flows, megabreccia, and
low-volume Deer Canyon Rhyolite flows northeast of Redondo Peak.
For comparison, here is an active rock glacier on the flank of
Mt. Lincoln, near Alma, Colorado.
Active rock glacier on Mt. Lincoln,
Colorado. Looking west from 39
The rock glacier sits in a hanging valley, which is a result of
conventional glacier activity. During the height of the last
glaciation, a glacier ran down this valley and joined a
larger glacier in the foreground valley. Now the valley is
occupied by the rock glacier, which has the same characteristic
steep front as the inactive rock glacier on Cerros del Abrigos.
Other interpretations of some of the boulder fields of the Jemez
are that they are debris flows or lag deposits. However,
the Cerros del Abrigo field, with its steep front and flanks, its
lack of any matrix between the boulders, and the indications of
pressure ridges in the satellite photograph, is best interpreted
as an inactive rock glacier.
The period from 500,000 to 60,000 years ago marked a pause in
volcanic activity in the Jemez. Sometime during this pause, but at
least 200,000 years ago, the first bison arrived in North America.
The appearance in the fossil record of Bison antiquus, the
direct ancestor of the modern bison, defines the beginning of the
Rancholabrean land mammal age of North America, which is named
after the famous tar pits at Rancho
La Brea in what is now Los Angeles.
A final burst of
The Jemez is not an extinct volcanic field, and will likely erupt
again someday. There are, in fact, indications that a new cycle of
volcanic activity has already commenced — though I hasten to add
that there is little likelihood we will see an eruption in our
The most recent eruptions in the Jemez are those that produced
the El Cajete Pumice, the Battleship Rock Ignimbrite, and the
Banco Bonito Flow, all of which have similar chemistry, seem to
have originated from closely spaced vents, and are likely the
result of a fresh batch of magma rising into the Valles magma
chamber. Collectively they are known as the East Fork Member of
the Valles Rhyolite.
The El Cajete
Relief map of the Jemez with El Cajete beds highlighted
Sometime in the last 100,000 years, a vent opened south of
Redondo Peak at El
Cajete. The exact date is quite uncertain. This is because
very young rocks are very difficult to date precisely by the usual
radioisotope methods. There simply isn't time in a hundred
thousand years for enough argon to accumulate for a reliable
radiiometric date. Charcoal from burnt logs preserved in the
volcanic beds from this eruption shows no measurable 14C,
which means the eruption took place at least 42,000 years
ago. Other dating methods, such as fission track, electron
spin resonance, thermoluminescence, and optically stimulated
thermoluminescence, attempt to measure accumulated radiation
damage to a rock to determine the time since the rock was hot
enough to erase such damage. 21Ne dating is slightly
different in that it measures accumulation of 21Ne in
rocks from cosmic ray bombardment rather than the accumulated
damage from the rock's own radioactivity. These methods yield
highly discordant results, varying from 140,000 years from fission
track dating to as little as 26,000 years from 21Ne
dating. The current accepted estimate, based mostly on
electron spin resonance measurements, is that the eruption took
place around 55,000 years ago.
The eruption produced an eruption column that blanketed much of
the southern Jemez with pumice beds. The map above shows only
those areas where the beds are most substantial. El Cajete pumice
occurs as isolated drifts and thin beds over much of the southern
Jemez and covers many of the Bandelier Tuff mesas in western
Bandelier National Monument. Some beds occur as far east as the
Cerros del Rio.
Today, El Cajete is a large meadow south of Redondo Peak.
El Cajete. 35
50.200N 106 33.278W
The flat area is mostly alluvium, carried in by water after the
vent formed, but there is a patch towards the center that is an
original deposit of El Cajete pumice. There are also some deep
pits in the far side of the flats, used as barbeque pits for
guests when this was part of a private ranch.
At left in the photograph is a rim of pumice thrown up around the
vent. At right is Redondo Peak. The low ridge in the second and
third frames is the east end of the Banco Bonito obsidian flow,
which we'll have more to say about later on. The vent for the El
Cajete pumice is though to be directly under El Cajete flats, but
the vent for the Banco Bonito flow is further west, not far from
the edge of the flow seen here.
Although the El Cajete eruption produced widespread air fall
pumice beds, the area immediately around the vent has coarser
eruption products. These include large clasts of snowflake
Snowflake obsidian is so named because of the white patches.
These can vary in size from fairly small, as in these samples, to
an inch across.
The white patches are typically cristobalite, a form of
silica that is stable at high temperature. This poses a bit of a
puzzle, as the temperatures measured in modern rhyolite magmas are
not high enough to be in the stability range for cristobalite. The
explanation is given by Ostwald's step rule, which says
that supercooled liquids most easily crystallize to their most
disordered solid form. Cristobalite is less ordered than quartz,
and so a relatively cool rhyolite magma has an easier time
crystallizing to cristobalite than quartz.
These clasts are likely remnants of an obsidian dome that formed
on the east side of the vent. Geologists have found evidence for
at least three such glassy domes, each of which marked a pause in
the explosive eruptions of pumice and ignimbrites.
Large clasts of vesicular rhyolite are also found close to the
Vesicular rhyolite. 35
50.118N 106 32.746W
Vesicular rhyolite is not bubbly enough to be classified as
pumice, nor massive enough to be just plain rhyolite. The chunks I
hefted felt like tuff. This chunk is larger than a basketball and
is an impressive distance from the vent; given that it’s in a
sheetwash deposit, and given the lay of the land, it probably
originally landed even further from the vent.
Thick beds of El Cajete air fall pumice are exposed in road cuts
along State Road 4 near the source vent.
South Mountain Rhyolite, El Cajete Pumice, and Banco
Bonito obsidian in road cut. Near 35
49.675N 106 35.413W
Here the pumice lies on top of South Mountain Rhyolite and in
turn is buried under Banco Bonito obsidian. Here's a sample of the
This sample shows clumps of biotite and hornblende. This is in
contrast to the Bandelier Tuff pumices (Guaje and Tsankawi) which
contain no biotite but have numerous phenocrysts of quartz and
sanidine. Some geologists interpret the mafic minerals as remnants
of the deep crustal rocks from which this magma formed that did not
El Cajete Pumice. 35
49.675N 106 35.413W
There is an impressive bed of El Cajete pumice in a driveway cut
as one begins the ascent out of the Vallecitos de los Indios onto
the south caldera rim along Forest Road 10.
El Cajete pumice. Near 35
48.194N 106 35.400W
El Cajete Pumice beds in the southern Jemez are relatively easily
worked with simple agricultural implements and tend to hold
moisture, and archaeologists have found that they played an
important role in Native American settlements in the area. More
recently, they have been extensively mined for use as abrasives,
as an additive in concrete, for water filtration, and as the stone
used to manufacture stone-washed jeans.
Relief map of the Jemez with Battleship Rock Flow
exposures highlighted in red.
The eruption of the El Cajete Pumice was followed by a series of
pyroclastic flows that generally followed existing river valleys.
These are most prominently exposed at Battleship Rock and along Cañon
de San Diego to the north, where the Jemez River has cut
back through the ignimbrite to reestablish its drainage.
Battleship Rock. 35
49.694N 106 38.551W
Battleship Rock itself marks the confluence of the Jemez and East
Fork Jemez rivers, which have cut through the ignimbrite deposited
in their valleys to leave this dramatic erosional remnant. The
rock is a welded ignimbrite showing classical fiamme:
Battleship Rock Ignimbrite. 35
49.694N 106 38.551W
The rock is full of lithic fragments (bits of older rock caught
up in the pyroclastic flow). The elongated brown patches are bits
of soft rock, perhaps pumice or tuff (though the color is rather
dark for that), that were softened and flattened as the very hot
pyroclastic flow settled onto the surface and welded together.
The ignimbrite is quite glassy north of Battleship Rock, so that
the canyon walls glisten under the right lighting conditions.
Battleship Rock Ignimbrite. Looking southeast from near
50 759N 106 38.075W
Battleship Rock Ignimbrite at a different time of day.
Looking northeast from 35
50.645N 106 38.185W
Further north, the flow becomes less densely welded and resembles
Battleship Rock Ignimbrite at La Cueva. 35 52.289N
The Banco Bonito
Relief map of the Jemez with Banco Bonito exposures
highlighted in red.
The most recent eruption in the Jemez produced the Banco Bonito
Flow, which fills much of the southwest moat of the Valles
caldera. This flow shows up clearly on satellite
images and relief
maps, because it is young enough to have experienced little
Like the El Cajete Pumice, the Banco Bonito Flow is well exposed
on State Road 4 near its source vent.
Banco Bonito obsidian in road cut. Near 35
49.675N 106 35.413W
The flow is largely obsidian, though this is extensively
devitrified, especially in the upper parts of the flow.
The relief map shows that the flow has pressure ridges and a
number of large explosion pits. One of these is located on Forest
Service land just east of State Road 4. Here's a view from the rim
of the pit.
Bonito explosion pit seen from rim. Near 35
51.268N 106 37.358W
Here's the view
from the center of the pit itself.
Banco Bonito explosion pit seen from center. Near 35
51.268N 106 37.358W
This kind of explosion pit probably does not represent a source
vent, since these pits are distributed more or less randomly
across the surface of the flow. Similar features are seen at Newberry
Volcano, Oregon, and at other young obsidian flows. The
likely explanation is that they are phreatomagmatic explosion
craters, formed when groundwater accumulates within the upper part
of the flow while its lower portion is still very hot. If drought
lowers the water table, the decrease in pressure can allow the
remaining groundwater to flash into steam and blow out very large
craters like this one.
Support for this theory comes from the presence of broken blocks
of devitrified obsidian in and around the crater.
Blocks of Banco Bonito devitrified obsidian. Near 35
51.268N 106 37.358W
Some very young explosion craters of this kind are found in
Yellowstone, and the formation of one such crater was actually
witnessed by geologists nearby. (Fortunately, they were not so
close that they were in any serious danger.)
El Cajete Lake
Relief map of the Jemez with El Cajete Lake extent shown
The third known lake in the Valles caldera, following the early
lake and the San Antonio/South Mountain lakes, was the El Cajete
lake. This was formed when the El Cajete and other East Fork
Member eruptions filled the southwest moat and dammed the East
Fork Jemez River. Lake deposits from this most recent lake are
extensive in the Valle Grande, and sometimes contain El Cajete
highlighted in red.
Relief map of the Jemez with El Cajete Lake exposures
highlighted in red.
An exposure of these beds is found southeast of Cerro la Jara.
El Cajete lake beds. Near 35
51.155N 106 29.655W
El Cajete Lake also produced lake bars and terraces of loose
gravel. One of these is well exposed in the road cut on the south
flank of Cerro Pinon.
Old lake terrance of El Cajete Lake. 35
53.353N 106 29.714W
The terrace is composed of loose gravel with considerable sand
and clay, containing occasional bits of El Cajete pumice.
The Jemez volcanic field is far from dead. It has experienced
volcanism for at least the last fourteen million years, and, as
we've seen, the most recent volcanic activity may have occurred
just 55,000 years ago.
There is evidence that there is still a magma chamber beneath the
Valles caldera. One clue is that seismic waves are seen to slow
down as they pass beneath the caldera. This low-velocity zone
is interpreted as a magma chamber that is only partially
crystallized. Seismic waves passing through the low-velocity zone
also experience higher attenuation than in normal rock.
Another clue is the continuing high rate of heat flow from deep
underground, manifest both in a high temperature gradient in holes
drilled in and around the caldera, and in hydrothermal activity in
the same general area. Hydrothermal activity in the modern Jemez
mostly takes the form of hot springs, though there are a
few fumaroles in the Sulfur Springs area.
The most accessible hydrothermal feature of the Jemez is Soda
Dam. This is a large natural dam of travertine,
calcite deposited by hot water from nearby hot springs.
Soda Dam. Looking east from 35.792N
There are numerous small hot springs in the area, but the largest
are just across State Road 4 to the west. These originally drained
into a plumbing system within the dam. The calcite-rich water then
emerged from a long fracture running the length of the dam, and
the calcite was deposited on the dam surface. Highway engineers
demolished the west end of the dam to improve State Road 4 in 1960
inadvertently destroying its plumbing system; the dam is now
slowly eroding away.
Here's one of the feeder springs exposed by the demolition
Soda Dam feeder spring. 35.792N
The green around the spring flow is partly algae, partly sulfur
bacteria feeding on the sulfur compounds in the spring water. The
smell of sulfur is noticeable throughout this area.
Here's the exposed calcite interior of the dam where it was
Soda Dam exposed interior. 35.792N
The interior is quite coarsely crystalline.
Soda Dam is geologically young, probably less than 5000 years
old. Calcite from the spring waters has cemented together beds of
eroded gneiss from nearby outcrops, which have now been cut by the
Soda Dam debris beds. Near 35.792N
There are a number of older travertine deposits in this area.
Those near river level on the opposite side of the Jemez River are
probably 60 to 110 thousand years old, while those well
above the level of Soda Dam in the nearby canyon walls are around
0.48 to 1.0 million years old. These show that the hydrothermal
system here has been long-lived.
The location of the springs is significant. This is the area
where the Jemez Fault crosses Canyon de San Diego, and the fault
zone provides a natural pathway for ground water.
Old travertine is also found along the San Jose Fault, which runs
along the west side of Borrego Mesa.
Breccia cemented with travertine. Somewhere near 35
40.618N 106 38.688W
This is one of several stacks of broken rock cemented together
with travertine that are found in this area.
Further down slope there are occasional chunks of travertine in the
Travertine. Near 35
40.837N 106 38.771W
The identification of this rock as travertine is confirmed both
by its flow features and by the acid test for calcite, which rules
out a flow-banded rhyolite (the only other real possibility.)
The age of the travertine deposits along the face of Borrego Mesa
is unknown. However, stacks like those shown earlier seem to
postdate the landslides, making them less than 50,000 years old.
Springs in the Jemez area fall into three categories. The most
numerous are cold springs, which are recharged by precipitation
that does not penetrate deeply enough to be affected by the deep
magma chamber. These are similar to cold springs found in all but
the most arid climates throughout the world. Their chief
significance in the Jemez is that they tend to follow boundaries
between volcanic flows, where the rock is more permeable, and thus
can serve as clues when mapping flows.
The hottest springs in the Jemez area are found within the Valles
caldera itself, such as at Sulphur
Sulfur Springs. 35.908N
Sulfur Springs is one of the most geothermally active areas of
the Jemez, which was once mined for sulfur and later developed as
a hot springs resort. Accounts vary on its fate: Some report
that the resort failed during the Great Depression, while others
report that it burned down in the 1960s. Now all that is left are
the surviving buildings of the resort, abandoned trailers, and the
Detroitus visible in the second frame.
Here the groundwater has been superheated by the very hot rock
beneath the caldera floor, and it is full of sulfur compounds that
give it a very low pH. These acid-sulfate hot springs tend
to rapidly alter rock in their vicinity to clays, iron oxides, and
sinter (amorphous silica.) The area immediately around the
springs is underlain by Deer Canyon Rhyolite tuffs, but these have
been altered to light clay minerals by the hydrothermal activity
in the area. The smell of sulfur is unmistakable.
West of the resort is a bank of heavily altered Deer Canyon
Altered Deer Creek Rhyolite tuff. 35
54.521N 106 36.977W
Another area of hot springs is found at the mouth of Alamo
Canyon. This is a rocky area with bubbles emerging from
several places. In some cases, there was what appeared to be
sulfur deposits around the springs.
Springs at mouth of Alamo Canyon. 35.919N
It is hard to imagine that this is anything but sulfur deposited
by the spring. However, at the time I visited the area, the water
was icy cold. I tried several vents; only one was lukewarm rather
than cold. I also found that the water had no noticeable taste. My
visit was in May, and the hot springs may have been diluted by
heavy spring runoff. However, one road log for the area
reports that it is fairly normal for the spring water here to be
Outside the caldera, one finds warm springs that are dominated by
chloride rather than sulfur compounds and have a neutral to
slightly alkaline pH. The springs in the Jemez Springs area fall
into this category. The water from these springs is meteoric
water (water from rain and snow that has not been long
The distinction between chloride and acid-sulfate springs is not
a sharp one, and many springs show both chloride and acid
chemistry. The springs around Soda Dam are moderately
acid-sulfate, but neither as hot nor as low in pH as the springs
at Sulphur Springs.
Sulfur is a moderately common element in the earth's crust, and
it is particularly associated with volcanic activity.
Sulfur has a complex chemistry compared with elements like
silicon, oxygen, or even iron with its two oxidation states.
Sulfur has many oxidation states, ranging from donating six
electrons in sulfates to accepting two electrons in some sulfides.
Most magmas contain some sulfur, but because sulfur has a strong
affinity for iron, the amount of sulfur in a magma is closely tied
to its iron content. However, sulfur is highly incompatible with
silicates in all but the sulfate state, where it forms rare
sulphosilicate minerals. Because of this incompatibility, the
sulfur in a magma will often separate into a distinct sulfide
phase rich in iron and other chalcophile
(sulfur-compatible) elements, and this phase can produce rich ore
One of the surest signs that fresh magma has risen beneath a
volcano is increased emission of sulfur dioxide, SO2.
This has a characteristic sharp smell and dissolves in water to
produce an acid solution of sulfurous acid. When further oxidized
to SO3, as is likely to occur at a slow rate when
exposed to air, it dissolves in water to produce powerful sulfuric
acid. Other gaseous sulfur compounds emitted by volcanoes include
hydrogen sulfide and sulfur vapor. Hydrogen sulfide has a strong
odor of rotten eggs, while sulfur vapor is typically deposited as
solid sulfur crystals near fumaroles.
The most sulfur-rich magmas are alkaline basalt magmas, which can
contain up to 5000 parts per million of sulfur. A value of 1000
parts per million is typical of more common basalts.
We saw earlier that hydrothermal alteration is common in the
Paliza Canyon Formation, particularly in its lower beds exposed in
canyon bottoms and the base of the Valles topographic rim.
Hydrothermal alteration is not easy to date, but this episode of
alteration probably immediately followed the eruption of the
Bearhead Rhyolite around 7 million years ago. A second period of
hydrothermal alteration began with the Toledo and Valles eruptions
and continues to the present day.
Here is an a rather spectacularly colored rock in an area of the
Valle Jaramillo/Redondo graben saddle mapped as Deer Canyon
Altered Deer Canyon Rhyolite. 35
54.159N 106 33.490W
The vivid green color is due to epidote. The rock is
still very solid, which rules out certain other forms of
alteration. There are visible quartz crystals, as well as
transparent lath-like crystals whose identity I’m not mineralogist
enough to identify. Hydrothermal alteration is pervasive in the
Deer Canyon and Redondo Creek Formations.
Epidote is a sorosilicate mineral with composition Ca2Al3(AlSiO4)(Si2O7)O(OH).
It typically has a green color due to substitution of ferric iron
for some of the aluminum.
Epidote sample from Colorado.
Sorosilicates are a class of silicate minerals contains pairs of
silica tetrahedra joined at one corner. Each pair thus has the
formula Si2O7. Epidote also contains
isolated silica tetrahedra, giving it a rather complex structure.
It is a quite common mineral and can be a major component of some
metamorphic rocks formed at low pressure and relatively low
Cerrp Pedernal. Looking west from 36
04.664N 106 25.280W
The landforms we see in the Jemez today are usually eroded,
sometimes heavily. Northern New Mexico has undergone regional
uplift over the last few million years, with the result that
erosion has accelerated and is now destroying rock beds that, in
some cases, are nearly 1.8 billion years old.
It seems kind of sad.
Erosion begins with the weathering of solid rock, which in some
cases produces a layer of soil over the bedrock. Soils show great
variety, even in as small a region as the Jemez Mountains, and I
can only touch on a few aspects of soil formation.
The lower elevations around the Jemez are semiarid and produce
characteristic desert soils. These are poor in organic matter and
are alkaline, since the scanty rainfall is not efficient at
leaching weathering products such as sodium, potassium, or
calcium. Calcium, in particular, tends to accumulate in the
subsurface as calcium carbonate to form a hard layer called caliche.
We saw examples of this in Chapter 6, where the older basalts of
the Lobato Formation are often encrusted with caliche.
Another characteristic of desert soil is desert pavement.
This is well developed on the rim of White Rock Canyon.
Desert pavement. Note quarter for scale.
The numerous exposed basalt clasts are an example of a lag
deposit. The remaining soil around the larger clasts is often
cemented, sometimes by hardy cyanobacteria that colonize the soil.
This is fairly delicate terrain, and I'm sorry to say that the
popularity of the area for hiking pretty much guarantees this
desert pavement will eventually be pounded into oblivion. But I
also can't bring myself to tell people to stay away; this is
beauty I can't resist sharing. I'm proud of my home.
Even if there are creepy-crawlies along the trail.
This is Orthoporus
ornatus, the desert millipede. Though uncommon,
tarantulas are occasionally encountered in the White Rock area,
along with many smaller species of spider.
This reminds me of an experience I had while out walking at
twilight to the local grocery store. It was a pleasant warm
evening, with the light fading fast. I had a head lamp lit so I
could see my way.
I am accustomed to the occasional glint of light from the
ground, from some crystal face oriented just right. But as I
walked along, I saw a pair of greenish glints that rapidly
flashed at me as I walked past. Curious, I moved closer, and saw
that it was two spiders. Their compound eyes had given multiple
green reflections as I moved by.
As I continued my walk, I spotted many more. The spiders seemed
to be mostly of a single species, not large (the biggest had a
leg span of perhaps an inch), brown striped, and clearly some
kind of hunting spider. Of course, I was also a crepuscular
hunter in search of prey, though my prey was an unwary gallon of
milk with which to make my breakfast. Still.
The spiders were beautiful. It was the kind of magic moment I wish
my wife or children had been along for.
Beyond the shadow of the ship,
I watched the water-snakes:
They moved in tracks of shining white
And when they reared, the elfish light
Fell off in hoary flakes.
Within the shadow of the ship
I watched their rich attire:
Blue, glossy green, and velvet black,
Then coiled and swam; and every track
Was a flash of golden fire.
O happy living things! no tongue
Their beauty might declare:
A spring of love gushed from my heart,
And I blessed them unaware:
Sure my kind saint took pity on me,
And I blessed them unaware.
The self-same moment I could pray;
And from my neck so free
The Albatross fell off, and sank
Like lead into the sea.
— Samuel Taylor Coleridge, Rime of the Ancient Mariner
A significant agent of erosion in the Southwest is wildfire.
Las Conchas fire of 2011. Looking northwest from
49.342N 106 13.467W
Under natural conditions, the Jemez Mountains are subject to
frequent, but usually low-intensity, wildfires. Geologists have
taken borings from older trees and stumps in the Jemez and
matched burn scars with tree rings to find the dates of major
fires. It appears that, prior to about 1890, widespread fires
took place every five to fifteen years. These were mostly
surface fires, which burned off grass, shrubs, and tree
seedlings but rarely destroyed more mature trees. The frequency
of fires dropped precipitously in the final years of the 19th
century, probably due to livestock grazing that removed grassy
Human efforts since 1900 to reduce the occurrence of fires have
likely had the perverse effect of making fires less frequent,
but more destructive. Without the frequent burning out of ground
debris and younger trees, fuel builds up until uncontrollable
crown fires destroy the forest ecosystem. The trend towards a
warmer and drier climate accentuates the hazard of wildfire.
Destructive fires on the Pajarito Plateau include the La Mesa
fire of 1977 (15,444 acres), the 1996 Dome fire (16,516 acres),
the 1998 Oso Complex fire (5,185 acres), the 2000 Cerro Grande
fire (48,000 acres), and the 2011 Las Conchas fire (150,000
But even before the arrival of humans in New Mexico, wildfires
contributed significantly to erosion. The more intense fires are
capable of speeding the weathering of some kinds of rock.
Fire-scorched boulder on Las Conchas.
47.838N 106 31.687W
This boulder is in an area that was burned over by the 2011 Las
Conchas fire, a particularly hot and destructive fire, and the
rock show a thin layer of soot. There is also some indication of
exfoliation, the shedding of an outer layer of rock, from
the heat of the fire. It seems likely that this is a significant
cause of weathering over geological time scales in an area where
natural wildfires are a regular occurrence.
Fire also increases erosion by stripping the ground of
vegetative cover. This allows raindrops to hit bare soil and
rainfall to run downhill without the moderating influence of
vegetation. This in turn leads to flash floods, in which
a summer monsoon thunderstorm dropping heavy rain in a brief
period can lead to channels further down the watershed being
swept by sudden and violent floods. These can cause spectacular
erosion in geologically insignificant intervals of time.
Relief map of the Jemez with colluvium deposits
highlighted in red
The earliest stage of erosion in
mountainous areas is the formation of colluvium at the
base of cliffs. In the Jemez area, this is particularly obvious
around mesas of the Tsherige Member, Bandelier Tuff, which
typically are surrounded by talus slopes cut into the
underlying Otowi Member that are covered with clasts of Tsherige
Colluvium at the base of a mesa of Tsherige Member,
Bandelier Tuff. 35.801N
Scree at base of Tsankawi Mesa. 35
48.055N 106 13.433W
Colluvium is unconsolidated fragments of rock and soil that
accumulate at the base of a slope from relatively gentle
processes, such as rainwater flowing over the surface. Colluvium
does not include landslide deposits, which are not a gentle
process, nor alluvial fans produced by the concentrated action
of water. If the colluvium is mostly rock shards produced by
weathering of the overhanging cliffs, it is sometimes called scree.
The Otowi Member is often difficult to distinguish from a
colluvium-mantled talus slope at a distance, since it is usually
present only under cliffs of more durable Tshirege Member and it
tends to weather to a gentle slope. As one hikes down the trail
on the south side of Pueblo Canyon onto the talus slope, one
sees that it is indeed deeply mantled with soil.
Colluvium on south wall of Pueblo
52.890N 106 15.945W
Further complicating the picture is the fact that, because
colluvium is composed of weathered fragments of nearby rocks,
the two can sometimes be hard to tell apart.
Colluvium formed from Otowi Member in
San Antonio Canyon. 35
56.572N 106 38.791W
The colluvium here looks like a tuff, but it’s very poorly
consolidated. The geologic map maps this area as colluvium, but
shows a bank of Otowi Member just up the hill. So this is reworked
tuff, formed from sediments eroded off the original tuff
bed. It is the historical use of tuff to refer both to
primary pyroclastic deposits and to reworked volcanic ash that
led to adoption of the term ignimbrite to describe
primary pyroclastic deposits.
Some of the most ambiguous beds are found in the area north of
Los Alamos, where beds mapped as Cerro Toledo sediments in the
Guaje Mountain quadrangle are mapped as colluvium in the Puye
quadrangle. This accounts for the "boundary fault" in the map of
colluvium deposts east of Los Alamos.
Over time, colluvium is washed further downhill by streams and
floods to produce alluvial fans. These are found along much
of the inner rim of the Valles caldera, as well as other locations
of high relief.
Relief map of the Jemez with alluvial fans highlighted
One can examine an alluvial fan on the Valles
Caldera Trail, part of the Valles National Preserve. The
trail descends the inner rim of the caldera on its southeast to
the caldera floor, where the alluvial fan includes local debris
Alluvial fan boulder deposits within
Valles Caldera. 35
51.408N 106 26.110W
Alluvial fan deposits within Valles
51.432N 106 26.169W
Individual boulders are composed of the Tschicoma
Formation dacite of the caldera rim.
Tshicoma Dacite in landslide on Valles Caldera Trail
Debris flows continue to be a part of the modern Jemez landscape.
They are particularly active in areas that have suffered recent
fire damage, such as the area south of Redondo Peak (which was
burned during the 2011 Las Conchas fire).
Debris flow in drainage near El Cajete
trail, south of Redondo Peak. 35
50.198N 106 33,135W
Relief map of the Jemez with Totavi blocks of White Rock
Canyon highlighted in green and other landslides
highlighted in red.
The geological picture in many parts of the Jemez Mountains is of
very hard, dense lava flows lying on top of soft sediments. This
is a perfect recipe for what geologists call detachment events
— that is, landslides.
Toreva blocks in
White Rock Canyon
Some of the most spectacular landslides in the Jemez area are in
White Rock Canyon, where Cerros del Rio basalt overlies poorly
consolidated sediments of the Tesuque and Chamita Formations of
the Santa Fe Group. Because the basalt is so durable, the mass
wasting in White Rock Canyon consists of what geologists call
Toreva blocks. These are named after the location they were first
recognized (Toreva, Arizona.) An entire block of the canyon rim
comes loose as a unit and slides partway down the canyon wall,
more or less intact. A particular characteristic of these blocks
is that the block rotates slightly towards the surface from which
Diagram of Toreva block
The Toreva blocks in White Rock Canyon include some of the best
From the west canyon rim, south of Overlook Park, one has a
beautiful view of one
of the most impressive Toreva blocks to be found below the
canyon rim. The Google satellite image doesn't do it justice; try
switching to Map Terrain view, although this also doesn't quite do
it justice. Fortunately, it's highly photogenic.
Toreva block on west side of White Rock Canyon Looking
35 48.986N 106 11.422W
The broad meadow is a slice of basalt canyon rim that has broken
loose and slumped partway down the canyon wall. You can see that
the far end of the meadow is nearly at the level of the canyon
rim, and the nearer end has slumped a greater distance, almost as
if the block was hinged at the far end. You can see that there is
a trail down the slump block that affords some nice views of cross
sections of the basalt flows making up the canyon rim. I've hiked
this trail a couple of times but apparently I took no pictures.
The Toreva block gives us a nice view from high above of
hexagonal fracturing of the lava flow. We saw this picture
earlier, in the chapter on the Cerros del Rio.
Toreva block on west side of White Rock Canyon Looking
35 48.986N 106 11.422W
It is very common for a thick basalt flow, as cools and
contracts, to fracture into roughly hexagonal columns. You
can see the tops of such columns in this photograph.
There are large landslides on the west side of Borrego Mesa. Here
the hard cap is basalt of the Paliza Canyon Formation and the
underlying poorly consolidated sediments are mostly Zia Formation
of the Santa Fe Group. The landslides here do not take the form of
toreva blocks, but more closely resemble a rock avalanche.
Landslide west of Borrego Mesa. 35
38.671N 106 39.357W
The landslide flows include massive andesite landslide blocks.
Large landslide block. Near 35
41.063N 106 39.112W
Large landslide block. Near 35
41.063N 106 39.112W
Looking across the valley, one sees smaller but similar
landslides of Bandelier Tuff on Chinle Group sediments.
At center, a block of Bandelier Tuff has slumped across the Chinle
Group, which are the red beds underneath.
Landslides in Bandelier Tuff. Looking west from near 35
41.063N 106 39.112W
Most of the landslides appear to be at least 55,000 years old,
the age of the El Cajete Pumice. This is because beds of the
pumice are found in many locations on top of the landslides.
El Cajete Pumice bed on top of landslide. Near 35
40.951N 106 38.709W
As you can see from the footprints, I crossed part of this patch
before thinking to take a picture. Not all the landslides are
older than the El Cajete Pumice; a few small slides in this area
appear to lie on top of beds of the El Cajete.
The landslide deposits on Borrego Mesa include extensive boulder
Boulder field. Near 35
40.837N 106 38.771W
This is a rockslide deposit below a prominent knob of andesite,
of the same composition as the boulders here.
Vallecitos de los
The northern slopes of Vallecitos de los Indios are covered with
landslide deposits formed from Banco Bonito obsidian flows atop
mudstone of the Abo Formation. These are relatively accessible,
being crossed by the hiking trail to McCauley Springs. The size of
some of the clasts in the landslide is impressive.
Landslide deposits along the McCauley Springs trail.
Somewhere near 35.824N
The largest bounders here are devitrified obsidian boulders of
the Banco Bonito flow. The reddish soil shows its origins in the
mudstones of the Abo Formation.
Because of their nature, landslides are difficult to date. Those
in White Rock Canyon are less than 1.6 million years old, judging
from the way that deposits of Guaje Pumice on the rim match
deposits on the Toreva blocks. However, this is only a upper limit
on the age, which may be much younger. On the other hand, we have
already seen that the landslides at Borrego Mesa are mostly
overlain by El Cajete Pumice, suggesting they are over 50,000
A river runs
The presence of the Totavi Lentil shows that the Rio Grande
already existed 5 million years ago, when the Puye Formation began
to be laid down. The history of this river thus overlaps most of
the recent history of the Jemez volcanic field itself. Rather than
trying to "interbed" that story with the story of the Jemez
Mountains, I'm going to step back to 5 million years ago and tell
that story now as a coherent narrative of its own.
The location of the Lentil, well west of the Rio Grande and at a
higher elevation, suggests that the Rio Grande originally ran
further to the west and was slowly forced eastward by accumulating
sediments of the Puye Formation. The river was at least 225m
(4760') above its current level when it first appears in the
geologic record. The river may not yet have been fully integrated
along its entire modern valley. The full integration of the Rio
Chama and Rio Grande through the Espanola Basin is thought to have
occurred sometime between 2.8 and 4.0 million years ago.
Once it had eroded through the basalt cap, the Rio Grande in
White Rock Canyon rapidly cut deeply into the underlying soft
sediments of the Santa Fe Group, The river reached its maximum
depth 2.8 million years ago, at a level about 30 m (100') below
the modern level. It left behind a series of terraces to the west,
which are now deeply buried under the Pajarito Plateau and are
known only from test wells. These divide Santa Fe sediments
beneath from Puye Formation sediments above. The maximum depth is
recorded by a flow of olivine basalt very close to the modern
river level near Ancho
Canyon, which has a radiometric age of 2.47 million years.
This flow marked the beginning of a series of natural dams, formed
by volcanic activity, that forced the river level back up in upper
White Rock Canyon.
The highest of these dams was a flow of tholeiitic lava at the
current location of Water
Canyon that erupted around 2.46 million years ago. This
drove the level of the river back up to 285m (935') above its
current level. The resulting lake, Culebra Lake, covered much of
the Espanola Basin. It is now believed that much of the
preservation of the Puye Formation can be attributed to Culebra
Lake, which raised the base level for erosion of the Puye
Relief map of the Jemez with approximate extent of
Culebra Lake highlighted in blue.
The Rio Grande had cut back down to within 130m (430') of its
current level at the time of the Toledo event, 1.6 million years
ago. Exposures of the Otowi Member are present on the east side of
the river north
of Cochiti, but the resulting dam must have been
short-lived, and the river rapidly cut down to its former channel.
The failure of this natural dam may account for deposits of Otowi
Member-rich clasts far downriver, at Socorro and Las Cruces.
Another short-lived dam was formed shortly afterwards by an
eruption of basaltic andesite that overlies the Otowi Member.
There is a shallow paleocanyon east of the river in this area that
shows the river course was temporarily diverted.
The river was down to 120m (390') above its current level when
the Valles event took place, 1.22 million years ago. At this time,
the canyon was much narrower than it is today, with a maximum
width of perhaps 600m (2000') and no evidence of the large slump
blocks seen today.
The Tsherige Member completely filled White Rock Canyon between
Chaquehi and Frijoles Canyon, and filled an eastern meander
of the canyon further north.
Tsherige Member filling eastern
meander in paleochannel of Rio Grande. Looking east from 35
47.367N 106 12.630W.
Here the beds of the Bandelier Tuff overlie a thin layer of river
gravel, which in turn lies atop Cerros del Rio basalts. From here
the paleocanyon turned sharply west, roughly along the modern
Cerros del Rio basalts in the confluence
of Water Canyon and Potrillo Canyon. 35
47.445N 106 12.712W
At far left and far right are the basalt cliffs that mark the
sudden drop from the relatively shallow Potrillo Canyon into the
much deeper Water Canyon. Water Canyon descends from the Pajarito
Plateau at center right, and continues to its confluence with
White Rock Canyon at left. Here part of the east rim of White Rock
Canyon is visible, with Montoso Peak on the skyline.
The large outcropping of light pink Bandelier Tuff in the south
wall of Water Canyon marks the westward meander of the
paleocanyon. This outcropping has not slumped down the
canyon wall; it was this thick when deposited 1.21 million years
ago. Since older Cerros del Rio basalt and underlying Santa Fe
Group sediments form the rest of the canyon wall, this shows that
the Bandelier Tuff filled a deep westward meander of the river.
This meander also left an outcrop of Bandelier Tuff on the north
wall of the canyon.
Banderlier Tuff filling meander in White Rock
paleocanyon exposed on north wall of Water Canyon. 35
45.888N 106 15.661W
The paleocanyon was also filled in lower Frijoles Canyon.
Contact between Cerros del Rio and Bandelier Formations
in lower Frijoles Canyon. 35
45.888N 106 15.661W
The damming of the Rio Grande near Frijoles Canyon raised the
river level to the highest since its beginnings in the Miocene, at
325m (1070') above its current level. The Rio Grande was forced 2
km (1.25 miles) east of its former course, and while it likely
rapidly cut through the tuff, it then encountered 200m (660') of
solid basalt that would have taken significantly longer to cut
through. Thus a second great Culebra Lake was formed that extended
perhaps 70 km (45 miles) to the north.This likely formed the lake
bars on the White Rock Canyon rim near Overlook Park.
Gravel bank. 35
49.404N 106 11.057W
There are scattered beds of similar gravel for at least a mile
further down the canyon rim, and there is also a considerable
quantity of this gravel on a landslide block east of this point,
halfway down the canyon rim. The gravels beds appear to overlie
remnants of the Tsankawi Pumice and thus must be younger than 1.25
million years in age. The dam produced by the Tsherige Member is
estimated to have been 100m (180') higher than the canyon rim
where these cobbles are located.
The river was still probably around 150m (490') above its modern
level 0.62 million years ago. This is based on the location of ash
beds from the great Lava Creek eruption in Yellowstone at that
date, which spread ash across the United States. The rate has been
particular great in the last 100,000 years or so, at about 50 cm
(20") per thousand years. White Rock Canyon is now about 250m
(820') deep near the White Rock Overlook. The rate of incision
probably increased significantly sometime between 0.7 and 0.3
million years ago, when the San Luis Basin of southern Colorado
became integrated into the Rio Grande watershed, more that
doubling the watershed area above White Rock Canyon. This was also
about the time the upper and lower reaches of the Rio Grande
finallly became integrated in the El Paso area. This dramatically
dropped the base level of the upper Rio Grande, which had
continued to slowly deposit sediments in the northern Rio Grande
Rift up to that time. Thereafter rapid erosion set in, producing
most of the badlands seen along the northern Rio Grande today.
There have been several smaller lakes over the last 70,000 years,
mostly created by a large slump near Water Canyon that has
repeatedly slid into the river channel during periods of unusually
wet climate.The last such lake is thought to have formed about
12,400 years ago.
There are a number of prominent ledges eroded into the Bandelier
Tuff in Ancho Canyon.
Zeolitized ledges in Tsherige Member. 35
47.344N 106 15.836W
The ledges do not correspond to any flow boundaries or other
bedding features in the tuff, and they can be traced into other
canyons along the Pajarito Plateau. They are believed to show lake
levels in nearby White Rock Canyon. The lake water would have
saturated the tuff up to the lake level, and the few inches just
above the water level would have produced an ideal environment for
zeolites to form in the tuff. These are hydrous silicate
minerals that fill pore spaces and increase the durability of the
tuff. Radiometric analysis of the zeolite crystals gives an age of
about 1 million years for the ledges, but this age is rather
The Sierra Ladrones
Relief map of the Jemez with Sierra Ladrones Formation
outcroppings highlighted in red
Deposits interpreted as old river gravel are present throughout
the Cochiti area. These river gravels cannot be dated directly,
but some beds are overlain further north by formations known to be
about 2.5 million years old, showing that parts of the old river
gravel must be older than this. The older portions of these gravel
beds were mapped as "old gravels" in the classic Smith and Bailey
map of the Jemez Mountains, but they have since been assigned to
the Sierra Ladrones Formation of the upper Santa Fe Group.
Excellent outcrops of the Sierra Ladrones Formation can be found
at the small town of Pena Blanca. I drove on through the little
town of Pena Blanca and into an area
with e. This is a formation of the Santa Fe Group, which includes
all the rift-fill sediments of the Rio Grande Rift. The Sierra
Ladrones Formation is a relatively young member of the group.
This photograph shows the sandstone facies of the formation.
Sandstone facies, Sierra
Ladrones Formation. 35.543532N
Here a fault has thrown up a hill of the underlying axial gravel
facies of the formation.
Axial gravel facies, Sierra Ladrones Formation. 35.543532N
The San Francisco Fault passes through the notch at left. The
terrain on the near side of the notch is more of the sandstone
facies, while the hill on the other side is the axial gravel
facies. Axial gravel is laid down in the central channel of a
river, in this case the ancestral Rio Grande. These beds are
difficult to date precisely but cannot be older than about 6.5
million years or much younger than 1.6 million years, based on the
ages of the individual rock fragments deposited as gravel in the
formation and its relation with volcanic formations in the area.
Further down the road, a very pretty road
Road cut in piedmont faces of
Sierra Ladrones Formation. 35.503778N
The nicely bedded red sediments are the piedmont facies of the
Sierra Ladrones Formation. This is sediment brought in by the
Santa Fe River and Galisteo Creek from the east. East of this
spot, this facies contains abundant fossils of early horses (Equus)
and ancient relatives of elephants (Gomphotherium).
The gravel on top is relatively young alluvial gravel. The lower
portions of the gravel contain volcanic ash that has been traced
to the Lava Creek eruption of the Yellowstone caldera, 630,000
years ago, and presumably this gravel is about that age.
The gravel itself contains a mix of quartzite clasts, probably
Precambrian quartzite from the Tusas Mountains to the north, and
basalt clasts, likely from the Cerros del Rio southeast of White
Rock. This means this gravel was brought in by the Rio Grande.
Terrace grave atop Sierra
Ladrones Formation. 35.503778N
Relief map of the Jemez with terrace gravels highlighted
The ultimate fate of material eroded from the Jemez Mountains is
to be carried down the Rio Grande River to the Gulf of Mexico.
However, only clay and fine silt are likely make the trip
directly, suspended in the water. Sand and small pebbles roll and
bounce along the river bottom. Larger clasts work their way down
river channels in several steps, likely driven by infrequent but
powerful flood stages of the river.
The ability of a river to carry sediment depends on how swift the
river is. Turbulent motions in rapidly moving water keep the
smallest particles in suspension. Swift flow at the river bottom
keeps sand and pebbles rolling and bouncing. When the river
changes speed, the carrying capacity also changes. Thus, when a
river slows, it tends to drop sediments, a process called aggradation.
Since the swiftness of a river is largely determined by how level
its bed is, a river descending a steep slope quickly cuts down (incises)
a deep channel, while a river that is crossing nearly level ground
reaches an equilibrium in which aggradation balances erosion.
For an ancient river emptying into the sea, such as the
Mississippi, the effect is that the river cuts a very broad flood
plain of very low relief and elevation. The flood plain is
typically underlain by very fine sediment. Since sediment tends to
be deposited on the inside of curves in the river, and eroded from
the outside of curves, curves tend to grow and the river becomes a
meandering river, with a single channel showing intricate
loops. At the other extreme, very young rivers flowing across
steep ground tend to form several channels that split and
reconnect to form a braided river.
Curiously enough, some of the best examples of meandering streams
in the Jemez area are within the caldera itself, where the flat
caldera floor provides low relief for the East Fork Jemez River
and San Antonio Creek, which strongly meander. The Rio Grande is
young enough, geologically speaking, that it is intermediate
between a braided river and a meandering river, showing incipient
meandering in some stretches and braided features in others.
A river in an area of recent uplift, such as the Rio Grande in
New Mexico, may have relatively level stretches ending in knickpoints,
where the river loses elevation relatively rapidly as it crosses
resistant beds or other features than hinder rapid downcutting.
The knickpoint is said to establish the base level for the
river upstream, which becomes the level of a floodplain around the
river if the knickpoint lasts long enough. When the knickpoint is
cut through, or if some other process causes the river to begin
cutting downwards again, it is said to have abandoned its
former floodplain, and the remnants of that floodplain may remain
Northern New Mexico has experienced regional uplift for at least
the last several million years, and the larger rivers have
experienced repeated drops in their base levels. This has produced
multiple terrace levels along major rivers, such as
the Rio Grande. These terraces are often covered
with terrace gravels, deposited ny the main channel of the river
as it cut a flat surface onto the underlying bedrock.
There are impressive contacts between terrace gravels and the
underlying Abiquiu Formation west of Abiquiu. Here the terrace
gravels are derived from red source rocks, making a stark contrast
with the white Abiquiu Formation.
Terrace gravel on Abiquiu Formation. Looking northwest
12.751N 106 20.507W
The terrace gravel is the red bed about halfway up the hill side.
The white beds beneath and in back are Abiquiu Formation. The
terrace gravel thus is perched on the side of the hill, at what
was once the level of a side channel of the ancestral Rio Chama.
Terrace gravels associated with the ancestral Rio Chama extend
further south, and are well-exposed along Highway 84 between
Abiquiu and Medanales. Here the gravels form low hills west of the
Terrace gravel. 36
11.809N 106 14.396W
The terrace gravels are also well-exposed along Thirtyone Mile
Road west of Espanola.
Upper terrace gravel along Thirtyone Mile Road. 36
00.659N 106 07.021W
Nearby the underlying Santa Fe Group sediments are exposed.
Upper terrace gravel along Thirtyone Mile Road. 36
00.659N 106 07.011W
The large, well-rounded clasts show that this deposit is an axial
deposit, representing the major river channel in this area. The
road log for this area identifies this as the ancestral Santa Cruz
River. Note that the deposit in the second photograph has been
There are beds of terrace gravel along the main highway to Los
Alamos near the point where it crosses the Rio Grande.
Terrace gravel on top of Santa Fe Formation. 35.878N
These can be difficult to distinguish from the Totavi Lentil of
the Puye Formation, but their location close to the river, without
any overlying beds, hints at their true nature.
There are some particularly fine examples of terrace gravels near
Cochiti Dam. One example is so well sorted, with a very distinct
upper contact, that my first thought was that this was artificial,
placed by highway workers in the road cuts for erosion control or
some other purpose It was not until I saw the beds exposed in
arroyos well away from the road that I was convinced they were a
natural geologic feature.
Old gravel in Cochiti area. 35
36.406N 106 19.681W
This is one of the younger terrace gravels in this area, probably
less than 100,000 years old. It is at the fourth of five distinct
terrace levels that have been mapped in the area. The silty, tan
beds above the gravel are mapped as eolium, which is silt and fine
sand brought in largely by the wind.
Similar old gravels are found in the Abiquiu area and probably
represent gravel deposited by the ancestral Rio Chama.
Old gravel in Abiquiu area. 36
12.749N 106 16.916W\
This is apparently an old axial gravel of the Rio Chama,
deposited on Chama-El Rito Member, Tesuque Formation. The flat
ground above has ruins of an Ancestral Pueblo People city,
Some chapters back, we saw that there is a distinctive bed of
gravel between the Tesuque Formation and overlying terrace gravels
throughout the area west and north of Espanola. This marker bed is
particularly well exposed at Arroyo Largo.
Quartzite gravel marker bed. 36
01.457N 106 05.527W
This bed is very distinctive, consisting of well rounded clasts
of quartzite typical of the Tusas Mountains to the north. It mark
a geologically brief interval sometime in the last two million
years when the ancestral Rio Grande and its tributaries in the
Espanola area carried a heavy load of gravel from the mountains to
the north. One wonders if it correlates with the beds we've just
seen at Abiquiu and Cochiti and, if so, what event it records.
Relief map of the Jemez with alluvium highlighted in
Finally, we reach the modern flood plains, where alluvium is
being transported today or has been transported in the recent
past. River channels and their floodplains are among the more
obvious and recognizable of land forms.
Some of the most impressive alluvium deposits are associated with
the valles of the Valles caldera. These are broad valleys
winding among the ring domes that are devoid of timber but support
lush grass. We've seen many photographs of these areas in previous
The lack of timber arises from two causes. First, the valles are
floored with thick beds of clay in which trees have considerable
difficulty putting down roots. Second, the winter climate in the
valles can be brutal.
Valle Grande fogged in. 35
51.103N 106 27.312W
This is a photograph of Valle Grande, the largest of the valles,
on an October morning. The layer of fog is not unusual. The
caldera accumulates cold air, producing a temperature inversion
that often leads to the formation of fog or a low cloud layer.
During the winter months, when there can be long stretches of dry
and sunny weather, the snow cover evaporates off the valles floors
and leaves tree seedlings exposed to bitter night cold. This is
more than they can take.
Alluvium is also found along river channels, but here there is a
delicate balance between deposition and erosion. Where fire has
stripped cover and increased the power of water, erosion can
predominate, as in the areas burned by the 2011 Las Conchas fire.
Here the channel is being rapidly cut downwards to produce
Some real gullies. 35
50.198N 106 33.135W
They’re real, and they’re spectacular. This section was at least
ten feet deep, and it cut right across my path. I had to work
upstream a modest distance to find a spot where I could cross.
These channels represent concentrated flow. If the watershed of
such a channel were a square kilometer, and erosion in the
watershed averaged just a centimeter per thousand years, this
would correspond to a cube of debris more than two meters on a
side moving down the river each year. Most likely the rate is much
lower in ordinary years and much higher in years following a fire
or other disturbance. There is evidence that deposition of
alluvium in the canyon bottoms of the Pajarito Plateau experienced
a burst of alluvium deposition between 12,000 and 8000 years ago,
based on carbon dating of charcoal fragments. A borehole in Ancho
Canyon found a floodplain deposit 10,000 years old five meters
(16.5') below the present canyon floor.
Alluvium tends to be concentrated in river channels, but in arid
climates areas of low relief may become covered with sheetwash.
Sheetwash deposits on valley floor. 35
50.118N 106 32.746W
Sheetwash is deposited where flood waters run across a large
surface without being channeled. It is distinct from a floodplain,
formed by regular seasonal overflow of a river from its channel.
Sheetwash deposits occur where there is no nearby river channel.
Our story has now reached the point where it becomes our
story. In the next chapter, Homo sapiens arrives
on the scene.
Next page: Emerging from deep time
Copyright © 2015 Kent G. Budge. All rights reserved.