Geology of the Gila Cliff Dwellings
Forest Service, U.S. Department of Agriculture

Gila Cliff Dwelling National Monument and the surrounding Gila Wilderness are located in the southern Rocky Mountains volcanic province, one of several major volcanic terrains of Middle Tertiary age [2 to 65 million years ago]. This province extends from the southern Rocky Mountains in Colorado to the Sierra Madre Occidental in Mexico. Many episodes of volcanic eruption, faulting, and erosion have alternated through millions of years to form the landscape we see today.

The cliff dwellings stand within, and near the eastern margin, of a large volcanic structure called the Gila Cliff Dwellings caldera. Collapse calderas such as this are formed by the rapid eruption of enormous amounts of pumice and ash which spread for tens of miles across the surrounding area. The removal of such a great volume of magma from a subsurface magma chamber, in only days or weeks, removes support from the chamber roof. This has caused it to collapse into the magma chamber, leaving a depression hundreds of feet deep and approximately ten miles across. These events took place about 28 million years ago and were followed by more eruptions that filled the caldera so the depression is no longer present. Faulting and erosion have further modified the land to form the present landscape.

The geologic materials present at the mouth of Cliff Dweller Canyon include three bedrock formations plus the flood plain sediments being deposited today along the West Fork of the Gila River. Each year, seasonal floods deposit sand and gravel over the banks of the river and occasionally large floods uproot trees and destroy bridges. Meanwhile, the river continues its main work of wearing away the hillsides and mountains and transporting the eroded rock debris downstream and eventually to the ocean.

The white outcrops one can see in the lower cliff on the north side of the parking lot are called Bloodgood Canyon Tuff (BCT), named for Bloodgood Canyon on the other side of the ridge at the head of Cliff Dweller Canyon. The BCT is 28 million years old and is the formation that erupted and filled th Gila Cliff Dwellings caldera after it collapsed. Only the upper 50 feet or less of the tuff is seen at the parking lot. But during exploration for hot, geothermal water, holes drilled downstream near Doc Campbell’s Post have shown the tuff is about 600 feet thick. Upstream along the West Fork of the Gila River, where the tuff outcrops rise higher and higher above the surface, it forms cliffs up to 1100 feet high and the bottom of the stuff still is not visible.

The Bloodgood Canyon Tuff was produced by a special kind of extremely explosive volcanic eruption during which a huge column of pumice and ash collapsed. These pyroclastic materaisl piled up at the base of the eruption column and then rushed out over the surrounding countryside at speeds up to 100 kilometers or 60 miles per hour. The pumice and ash formed a sheet up to hundreds of feet thick, which thins away as one approaches the eruptive vents. Such ash-flow sheets – called ignimbrites – make up very low profile, domed-shaped volcanoes. With their typically collapsed caldera vent areas, they are very different from the classical, high, symmetrical cone-shaped volcano.
Geologists and volcanologists also describe tuffs, such as the BCT, as ash-flow tuffs, pyroclastic flows, ignimbrites, and welded tuffs to distinguish them from the somewhat less catastrophic tuffs formed as fallout from ash clouds dispersed in the upper atmosphere. Pyroclastic flows, or ash-flow tuffs, are a mixture of hot pumice and ash particles enclosed in a cloud of fiery, hot gases. This not only increases their mobility but also conserves much of the original heat of the eruption. If temperatures remain above about 600 to 750 degrees Celsius, the result is a welding together of the pyroclastic fragments after they are deposited. Depending on temperature and pressure of the overlying material, welding can be quite varied throughout a deposit. There can be a complete range from non-welded tuff to completely welded tuff.

The BCT at the north side of the parking lot is moderately welded as determined by the partially flattened pumice fragments giving the tuff a weakly layered or foliated appearance. This rock contains rounded, glassy crystals of quartz and rectangular glassy to dull-white crystals of feldspar of the variety called sanidine. The crystals are mainly less than 1/10th of an inch (2 to 3 mm) across. When the sun reflects off these rocks, some of the sanidine crystals show a satiny sheen or even a blue color if the light is just right. This gives rise to the name “moonstone.” As a result, the BCT is commonly referred to as “moonstone tuff.” However, not all moonstone tuff is BCT.

Quartz and feldspar crystals and rare black biotite (mica) crystals make up only about 10 to 15 percent of the tuff. The rest consists of microscopic shards of glass and more or less flattened chunks of white pumice generally less than an inch or two (1 to 3 cm) long. Called rhyolite, this is a volcanic rock containing more than about 70 percent silica (quartz is made of silica). Rhyolite is commonly a light-colored rock because it contains very little iron and other constituents that form dark minerals. Exceptions are obsidians, glassy rhyolites containing iron-bearing minerals that are so widely distributed throughout the glass, it has dark and even dense, black colors.

Above the BCT at the parking lot, one can see dark gray to reddish-brown basaltic to andesitic lava flows, about 100 feet thick, overlain by light-brown, bedded sedimentary rocks consisting of sandstone and conglomerate. This conglomerate is commonly referred to as Gila Conglomerate. It consists of grains, pebbles, and boulders eroded from the surrounding volcanic mountains, then transported and deposited by streams. The Gila Conglomerate is described as a volcaniclastic deposit because the sedimentary materials have all come from the erosion of volcanic rocks.

Looking at the cliffs on the south side of the river at the mouth of Cliff Dweller Canyon, one can see the same sequence of Gila Conglomerate overlaying the basaltic lava flows. However, the BCT is out of sight below the river level, but can be seen on the south side of the river a short distance upstream.

The lower part of the trail to the cliff dwellings is in the basaltic lava flow. We do not have a chemical analysis of the flows here, which is why we hedge on their composition. We know similar flows in this area range from andesite to basalt, containing only about 50 to 58 percent silica, in contract to the 75 percent silica in the BCT, which we call rhyolite. Also, unlike rhyolite, the andesitic flows contain an abundance of dark, iron-bearing minerals.

The basaltic and andesitic lava flows here have not been dated. We do know they are younger than the 28-million-year-old BCT. A good date on the lava flows would give up a maximum age of the lower part of the Gila Conglomerate. However, similar lava flows in this same position in the rock sequence nearby are about 25 to 26 million years old, so these flows are probably about the same age.

From the cliff dwellings, the trail continues back to the parking lot. As you navigate the trail, notice the overlying Gila Gonglomerate on the lava flow. As you descend, there are good exposures of the basaltic lava showing the concentration of vesicles and amygdules at the top of the flow. Also note the irregular bedding and mixture of coarse and find materials in the conglomerate, characteristic of deposits formed by streams exiting mountains and leaving their sediment loads in alluvial fans.

Along the trail, one can identify a major feature characteristic of basaltic and andesitic lava flows, namely vesicles and amygdules (pronounced ah-MIG-dules). Vesicles are small holes present in the lava flows. They are formed when gases dissolved in the lava tried to escape under the low pressure conditions at the surface. Gas tends to rise toward the surface of a lava flow. It is concentrated there, and the vesicles show where one flow stopped and another began. Vesicles, filled with minerals crystallized from the gaseous solutions trapped in the vesicles, are called amygdules. Calcite, quartz, and zeolites are the mineral most commonly present in amydules. Some of the zeolites present in amygdules can be recognized by their radiating crystal form.

I’m going to detour here a minute into an area of impressive interest. Regarding zeolites, we learn from John McPhee, in his book Basin and Range, “Certain zeolites (there are about thirty kinds) have become the predominant catalysts in use in oil refineries, doing a job otherwise assigned to platimum.” McPhee quotes his source, Princeton geology professor, Kenneth Deffeyes. Zeolites, Deffeyes says, “contain aluminum, silicon, calcium, sodium, and an incredible amount of imprisoned water. ‘Zeolite’ means ‘the stone that boils.’ If you take one small zeolite crystal, of scarcely more than a pinhead’s diameter, and heat it until the water has come out, the crystal will have an internal surface area equivalent to a bedspread.”

Back at the Gila Cliff Dwellings. As you wander the trail and observe the dwellings, you might as questions that commonly occur to visitors:

When was Cliff Dweller Canyon formed and how long before the dwellins were built were the caves formed?

How much deeper is Cliff Dweller Canyon now than when the cliff dwellers lived there?

If we assume the caves began to form when the canyon was only as deep as the level at the bottom of the caves – following suggestions the caves were started by lateral cutting of the stream into the side of the canyon – then we know the canyon has been cut about 175 feet deeper since then. To determine when the caves formed, we need to know the average rate of down cutting or erosion. If we had a date on the age of the basaltic flows at the base of the conglomerate and another dated lava flow or ash bed higher in the conglomerate, we could calculate an average rate of down cutting in Cliff Dweller Canyon.

Lacking that information, we have to use an average rate of down cutting from somewhere else, such as the Grand Canyon of the Colorado River, where it is know it has taken 10 million years to excavate the canyon to a depth of about 1800 meters or 5670 feet. Doing a little arithmetic shows this is an average rate of down cutting of about 0.2 mm or 0.0068 inches per year. At this rate, it probably took 266,700 years to deepen Cliff Dweller Canyon from the level of the caves to the present level. This means the caves began forming sometime in the late Pleistocene Period [approximately 300,000 to 500,000 years ago]. The canyon is probably only a few inches to a foot deeper than when the caves were occupied 700 years ago in the late 1200's.

Once started by stream action, the caves continued to be enlarged by weathering processes that continue today. Exfoliation, a major process contributing to continued enlargement of the caves, can be seen clearly where multiple cracks have formed concentric to the openings.

From the cliff dwellings, the trail continues back to the parking lot by an upper route providing a good view of the Gila Conglomerate overlaying the lava flows. As you descend, you’ll find good exposures of the basaltic lava showing the concentration of vesicles and amygdules at the top of the flow. You’ll also find irregular bedding and mixture of coarse and fine minerals in the conglomerate, characteristic of deposits formed by streams exiting mountains and leaving their sediment loads in alluvial fans.