Cobbles Along the San Juan River

By Gene Stevenson

Written October, 2015

Cobbles along Penguin Rock Beach. Photo by Paul Martini

Many people who hike alongside the banks of the San Juan River become enthralled by the incredible variety of river cobbles that they encounter. Almost always rounded and smooth, they range in color from black to white, with all the other colors represented too.  But where did they come from? How did they get here? These are simple questions that require a bit of explaining.

First, any rock type can potentially become a cobble.  “Cobble” only refers to size: they are larger than pebbles and smaller than boulders, having a diameter in the range of 64‑256 mm (2.5 ‑ 10 inches), or a size between that of a golf ball and that of a small volleyball.  San Juan River cobbles are always smooth to the touch, and well‑rounded.  The rounding and smoothing of cobbles is due, first to their relative hardness, and secondly to the mechanical process of banging and clacking into each other as the river gradually pushes them farther downstream – a process called “saltation”.

Along the banks of the San Juan River, cobble bars are a common occurrence.  Many a person walking on these rocks knows all too well how tough it is to move around on them, for they are large and loosely stacked, and readily gives way beneath your feet.  But cobbles can be found well away from the river, on top of the numerous benches and cliffs that define the river corridor.  How do these cobbles relate to those found at river’s edge?

Gene Stevenson pointing out cobbles during a field seminar for the Bluff Arts Festival in 2009. Photo: Theresa Breznau


Just about all the cobblestones along the San Juan River were derived from either the San Juan Mountains (the headwaters for the San Juan River), or its various tributaries that drain the numerous mountains of the Four Corners (La Plata’s, Sleeping Ute, Abajos, Carrizo’s, Chuska and Lukachukai’s).  And, for the most part, they all share a commonality in their origin in that they are either igneous or metamorphic, and not sedimentary (although small cobbles may represent local sedimentary rocks or the coarse grains that have dislodged from them).  “Igneous” rocks are those that are formed from a molten magma, while “metamorphic” rocks are those that have been altered due to extreme pressures and temperatures, but did not melt. Examples of igneous rocks include all the various granites found in the region and basalt, a black, dense volcanic rock type.  Examples of some metamorphic rocks include quartzites, slates and meta-conglomerates. 

So, by understanding the geology of these mountains around us, it is possible to determine which cobble came from where.  Some of the rock types are so diagnostic, that even the specific mountain creek or individual peak can be deduced. But by and large, almost all the river cobbles are products of the San Juan Mountains. Why?  Simply because it’s due to the tremendous volume of rock being eroded from the San Juan’s, AND that they are harder and more resistant to weathering than those derived from other sources.

Another attribute of San Juan River cobbles is their age. They are OLD!  That is to say, their point of origin (when they cooled from a molten magma, or were compressed and strained by earth’s forces) is quite ancient.  Through radioactive isotope dating of some of the minerals in these rocks, a date ranging from 1.45 to 1.78 BILLION years before present is frequently derived.  And that is a minimum date; in other words, they are AT LEAST that old.  Rocks that are of this age are placed into the Precambrian Era, or the time when Earth was in its formative stages, and before any complex multi‑cellular life formed.


Water derived from rain or melting snow gathers in the high country, runs downhill, and incorporates fine‑grained sediments in suspension as well as larger particles as stream bed aggregates.  At first, rocks are broken into smaller chunks by landslides, and other methods of mass wasting, and ultimately break into smaller sizes such that the forces of moving water and gravity push these larger masses down the mountain slopes, and into streams and creeks.  This process of erosion ultimately displaces rocks from the mountaintop to the sea.  And during this journey, the rocks get smaller, and rounder (if they are hard), or they disintegrate into sand or clay‑sized particles (if they are soft, or chemically unstable).

So, you see, it’s really quite simple to figure out where all the variety of river cobbles is coming from.  First, go hiking in the San Juan Mountains and the Abajo Mountains and some of the other mountains in the Four Corners area and sample all the different rock types, then compare your collection to that found along the river.  It’s that easy.  Then you can impress your friends by telling them not only the rock composition, but from which drainage system the particular rock was derived.

Photo: Theresa Breznau                    


Now that you know that most of the cobblestones are Precambrian age, and mostly derived from the San Juan Mountains, how do you think they were sprinkled across all the cliffs and benches?  Certainly, the process involved running water (rivers), but how do you explain river gravels perched hundreds of feet above the present‑day river valley?

Well, the answer is GLACIERS! The San Juan Mountains were high enough in Pleistocene time (only one and a half million years ago) that alpine glaciers covered most, if not all, of the mountain ranges and valleys that comprise the San Juan’s and possibly even the mountains of the Four Corners.  Some of the best examples of glacially‑carved valleys, cirques, arêtes, etc. are preserved in the San Juan Mountains. Visit Ouray, Colorado sometime (“the Switzerland of America”), or drive the “U‑shaped” Animas Valley north of Durango to witness some of the effects of glaciers. But the San Juan alpine glaciers melted and re‑froze at least four or five separate times during the Pleistocene epoch, and left a record of this episodic style.

During cold glacier‑building periods, little water ran from snow melt, and erosion was primarily the result of tons of ice and snow gouging and grinding up anything in its path. During warmer glacial‑melt periods, all of the ground‑up rock and soil was discharged by mega-huge volumes of melt water, forming glacial outwash terraces along the San Juan River corridor.  These flat‑top terraces are quite distinctive and readily discernable from Durango, Colorado to Farmington, New Mexico, and all the way here to southeastern Utah.

The terraces also define the incredible forces of huge volumes of moving water and its erosional consequences. The cobbles that are perched on the highest cliffs and benches are products of the earliest glacial melts, deposited when there wasn’t much of a canyon or corridor in the Bluff area. With each ensuing freeze‑ and thaw‑ the river valley deepened accordingly, such that, today, we have an inverted stratigraphic profile when it comes to cobble‑stratigraphy.  Thus, the older cobble terraces occur higher up the canyon walls, and get progressively younger as you approach the river (just the opposite of one of the basic “tenets” of stratigraphy: younger strata overlie older rocks and called “superposition”).

So now you know why there is so much river sand, gravel and cobble bars around Aneth & Montezuma Creek to Bluff, and on top of the sandstone bluffs; it was all due to glaciers melting in the San Juan Mountains, and flushing huge volumes of water and incredibly ancient rocks from the high ground. These days, typical springtime runoff flows rarely exceed 15,000 cubic feet per second (cfs). And the highest recorded flow was in October, 1911 when the river is estimated to have exceeded 150,000 cfs that washed away the bridges from Durango to Mexican Hat, but that still pales in comparison to what late Pleistocene outwash flows might have been. Based on what we know what flows have been recorded in the last 150 years, and by calculating the entire drainage area for the San Juan River, it is reasonable to assume that these outwash flows could have been several MILLION cfs! Or, for comparison, a river carrying as much water as the Mississippi River in high flood stage in modern times used to flow from the San Juan Mountains to the sea.

Radiometric Age Dates of Granite Intrusions

Ten Mile                      1.72 BY fine‑grained moderately gneissic with pink & black grains

Eolus                           1.46 BY medium‑grained pink & black grains

Baker’s Bridge             1.45 BY coarse‑grained pink, clear, and black grains

Electra Lake Gabbro   1.45 BY dark-colored coarsely crystalline

Precambrian Rocks

The late Precambrian igneous and metamorphic rocks from the Needles Mountains form the core of the San Juan Dome in southwestern Colorado. These rocks are in contact with volcanic rocks of Tertiary age and with Paleozoic sedimentary rocks. The Precambrian rocks are exposed in the glaciated valleys of the headwaters of the San Juan River near Wolf Creek Pass, and in the major tributaries like the Piedra, Pine, Vallecito, Animas and Florida Rivers. These complexes of very hard rocks comprise the bulk of what constitutes the boulders, cobbles and gravels deposited alongside the San Juan River.

The oldest gneisses and schists are exposed only in the drainages of the Animas River, and the Irving Formation and Vallecito Conglomerate are exposed only in the drainages of Vallecito Creek and the Pine River. The Uncompahgre Formation crops out along an arc 35 km (~22 miles) long in the northern and eastern Needles Mountains.

The Baker’s Bridge Granite intrudes the Irving Formation in the Animas Valley and is dated at about 1.450 BY and the overlying Twilight Gneiss is dated at about 1.780 BY.

The Irving Formation is both overlain and underlain by conglomeratic units where the entire sequence exhibits relatively undeformed degrees of metamorphism (Greenschist facies). The overlying Middle Mountain Conglomerate is meta-sedimentary drab-gray-green colored sandstone and siltstone sequence; the larger grains are not uniform in size but generally finer and more angular than those clasts seen in the underlying Vallecito Conglomerate; these rocks can only reach the San Juan River east of the Florida River.

Conversely, the Vallecito Conglomerate consists of much larger well-rounded pebble- to small-cobble-size clasts of colorful quartzite, slate and a very distinctive red-maroon to purple meta-shale (slate) and only crops out near the head of Vallecito Reservoir along Vallecito Creek east of Durango. Vallecito Creek is a tributary to the Pine River, but the confluence occurs upstream of the dam that was constructed in 1936 forming Vallecito Reservoir. And the Pine River is a major tributary into Navajo Reservoir that was dammed in 1962. So, if you find a cobble of Vallecito Conglomerate, it had to have rolled past both the Vallecito and Navajo dam sites prior to 1962; or to say a different way – there will not be any new supply of this rock type into the San Juan River until both Vallecito and Navajo dams ceases to exist as barriers.

The oldest gneiss/schist complex is dominated by amphibolite with minor gneiss and biotite and chlorite schist. These rocks are foliated and dark with unfoliated pods or lenses of white quartz and pink orthoclase. These rocks are in contact with the Baker’s Bridge Granite north of Durango near Rockwood. The Twilight Gneiss crops out in the Animas Valley from Electra Lake northward to Coalbank Pass and Twilight Peak and at Snowden Peak where it is in fault contact with the Uncompahgre Fm. Most of these cobbles would have rolled down the Animas or Florida Rivers and into the San Juan River.

Baker’s Bridge Granite: composed of pink microcline, white plagioclase, quartz (clear), hornblende (splintery black) and biotite (black specks of mica). This granite has coarse-grained phenocrysts, when compared to the finer-grained phaneritic Ten Mile Granite which has less hornblende as well. Both granite types are derived via the Animas River to the San Juan.

The Uncompahgre Formation is a thick bedded sequence that’s at least 2438 meters (8,000 ft) thick; white to lavender sandy and pebbly quartzite interlayered with gray slate, phyllite, and minor schist. It dominates the Grenadier Range in the western Needles and consists of a meta-sedimentary block that was thrusted southward onto older Precambrian crystalline basement rocks. It even preserves sedimentary structures like cross-bedding, graded beds and ripple marks from currents and waves. The Uncompahgre Formation exhibits low degree of metamorphism (Greenschist facies) where sand grains are fused to each other rather than grain-to-grain cement of silica or clay as typically seen in sedimentary sandstones. The Uncompahgre Formation supplies most of the cobbles found along the San Juan River since nearly all tributaries from the San Juan Mountains carry this rock formation.

Electra Lake Gabbro, Eolus Granite and Baker’s Bridge Granite dated by U-Pb analysis of zircons and Rb-Sr isochrons indicates that the Silver Plume Orogeny occurred about 1.450 BY. These rocks can only come down the Animas River.

Cenozoic Rocks

Igneous rocks of Cenozoic Age occur as both volcanic (extrusive) and as laccoliths (intrusive) and a special form of intrusive called “diatremes.” The diatremes are sprinkled across northeastern Arizona and into the northwest corner of New Mexico and southernmost Utah. The most spectacular remnants being Agathla Peak and Church Rock near Kayenta, Arizona, or Shiprock in New Mexico, or Alhambra, Boundary Butte or the Mule Ear in southern Utah. These are special kinds of rocks and we don’t find any cobbles or even sands derived from these features along the San Juan River, so we won’t discuss any further here.

Cenozoic volcanic rocks (extrusives and intrusives) brought gold-silver-zinc-lead-copper & other precious metal veins filling radiating fractures into Precambrian rocks from numerous calderas spread across the San Juan Mtns in southwestern Colorado. The volcanic rocks are mostly from the San Juan Volcanic Field in southwestern Colorado where thousands of feet of a mostly soft rock type called “tuff” was deposited. Tuff’s are chemically unstable and weather quickly, producing mostly very fine-grained sediments, like the fine-grained sands and muds along the river, but occasionally a volcanic remnant might be discovered. They are typically various shades of gray to tan with lots of holes in them, and light-weight and poorly rounded. However, black basaltic lava can be found and typically contains small circular holes, or air bubbles called “vesicles” when the lava was a hot molten magma.

The intrusive rocks formed as laccolithic intrusions and are approximately the same age as the San Juan Volcanics, but you might find cobbles derived from some of these intrusions along the gravel and cobble bars along the San Juan River or its tributaries. If you are collecting these types of rocks and exploring along the river near Aneth or Montezuma Creek, then more than likely, these rocks were derived from the Abajo Mountains and have rolled down Recapture or Montezuma Creek. They too are relatively easily eroded due to chemically unstable minerals, but are plentiful when found within 50 miles or less from their source. There are a number of laccoliths that form the mountains in the greater Four Corners area and include the following:

               La Plata Mtns

               Sleeping Ute Mtn

               Carrizo Mtn

               Abajo Mtn

               La Sal Mtns

               Henry Mtns

               Navajo Mtn

Pleistocene glaciation and interglacial warming allowed ice melts that deposited massive outwash of boulders, cobbles, gravel and sand forming outwash gravel & cobble terraces perched along the San Juan River.

River cobbles near Sand Island Campground. Photo: Theresa Breznau

Names of Major Glacial and Interglacial Events

From oldest to youngest

Nebraskan: 620,000 – 675,000 years ago

               Afton interglacial: 550,000 – 620,000 years ago

Kansan: 200,000 – 475,000 years ago

               Yarmouth interglacials (3): 425-375 ka; 300-330 ka; 200-240 ka

Illinoisan: 130,000 – 200,000 years ago

Sangamon interglacial: 115,000 – 130,000 years ago

Wisconsin: 15,000 – 72,000 years ago

               Holocene interglacial: Present day – *12,000 to 15,000 years ago

*The last glaciers began melting around 15,000 years ago and comprise the present day river levels & profiles. In other words, “Global Warming” at least as it relates to North America, Europe & Asia has been going on for all this time with several minor cold periods interspersed, but it remains to be seen as to whether global warming will continue, or that we just happen to live in another Interglacial period.



Amphibolite: igneous rocks composed of dark colored amphibole and plagioclase with little to no quartz

Andesite: extrusive equivalent of diorite; dark-color; phenocrysts; zoned plagioclase, biotite, hornblende, pyroxene

Basalt: very dark-colored extrusive igneous rock composed of iron & magnesium rich minerals; “lava” that commonly exhibits small cylindrical vesicles (gas bubbles)

Chalcedony: a translucent, vitreous, milky, smoky to waxy variety of chert; conchoidal fracturing common

Chert: a hard, semi-vitreous, microcrystalline rock consisting of interlocking crystals of quartz; typically has conchoidal fracture and ranges from white to black and all colors in-between due to various impurities

Dacite: extrusive fine-grained igneous volcanic rock; extrusive equivalent of granodiorite

Diorite: intrusive igneous rock; dark-colored with dark amphibole and sodic plagioclase

Gabbro: dark-colored intrusive igneous rock; it is the coarse-grained equivalent of basalt

Gneiss: a foliated rock formed by metamorphism in which bands or lenses of granular minerals alternate with flaky or elongate minerals

Granite: intrusive igneous rock with phaneritic phenocrysts of quartz, potassic-feldspar and biotite

Granodiorite: intrusive igneous rock containing quartz, plagioclase, and potassium feldspar with lesser amounts of biotite and hornblende; a rock intermediate in composition between granite and diorite

Monzonite: an intrusive igneous rock intermediate in composition between syenite and diorite; contains equal amounts of alkali feldspar and plagioclase, no quartz with dark greenish-black specks of the mineral augite, an iron-magnesium silicate

Phyllite: a metamorphosed rock, intermediate in grade between slate and mica schist

Quartzite: quartzose sandstone that has undergone low-grade metamorphism where grains are fused, or sutured together

Rhyolite: volcanic (extrusive) igneous rock, typically porphyritic and commonly shows flow structures with phenocrysts of quartz and alkali feldspars; the fine-grained equivalent of granite

Schist: a strongly foliated crystalline rock formed by high-grade metamorphism where grains are arranged in parallel to sub-parallel alignment

Slate: a fine-grained metamorphic rock that splits into thin slabs or plates; most slates are formed from shale

Syenite: a group of intrusive igneous rocks containing alkali feldspar, with lesser amounts of plagioclase, hornblende and quartz

Trachyte: a fine-grained, porphyritic rock; the extrusive equivalent to syenite

Tuff: consolidated or cemented volcanic ash


Feldspars:  a group of abundant rock-forming silicate minerals with general formula: M[Al(Al,Si)3O8], where M = K, Na, Ca, Ba, Rb, Sr or Fe; they are the most widespread of any mineral group and constitute 60% of the Earth’s crust

  • Orthoclase: a colorless, white, cream, pink to gray alkali feldspar; a common rock-forming mineral
  • Microcline: typically pink to red potassium rich mineral of the alkali feldspar group; a common mineral in granitic rocks and secondary to orthoclase
  • Plagioclase: a group of sodium or calcium-rich feldspars; among the most common rock-forming minerals

Quartz: crystalline silica: SiO2 – it is, next to feldspar, the commonest mineral on Earth; clear to transparent or colored by impurities

Amphibole: a group of dark, rock-forming ferromagnesian silicate minerals with complex formulae

  • Hornblende: the commonest mineral of the amphibole group; calcic, sodic magnesium & iron-rich silicate; Commonly black to dark green or brown; typically fibrous to columnar

Biotite: a widely distributed rock-forming mineral of the mica group; black to dark green, platy


Aphanitic: igneous rocks with grains too small to distinguish with unaided eye; microcrystalline; cryptocrystalline    

Phaneritic: texture of igneous rocks with grains large enough to be identified without magnification; megascopically crystalline

Phenocryst: a textural term for igneous rocks with relatively large, conspicuous crystals in a porphyritic rock

Porphyry: an igneous rock of any composition that contains conspicuous phenocrysts in a fine-grained groundmass

Foliation/Foliated: a term for a planar arrangement of textural features in any type of rock; planar parallelism in                metamorphic rocks

Euhedral: a grain bounded by perfect crystal faces; well-formed

Anhedral: a grain lacking well-developed crystal faces

Conchoidal fracture: a smoothly curving fracture surface common to rocks and minerals of homogeneous texture


Laccolith: an igneous intrusion with a convex-up roof and a flat floor

Orogeny: the process of the formation of mountains

Caldera: a large, basin-shaped volcanic depression, more or less circular in form; many times larger in diameter than craters and vents within

Radiometric: pertaining to the measurement of geologic time by the study of parent and/or daughter isotopic abundances and known disintegration rates of the radioactive parent isotopes; radiometric dating applies to all methods of age determination based on nuclear decay rates of naturally occurring radioactive isotopes

Saltation: a mode of transport (in this case by moving water currents in streams) in which cobbles are moved progressively downstream in a series of short intermittent leaps, jumps, hops, or bounces from a bottom surface and in the process of banging and clanging, the rocks are gradually rounded as angular pieces are chipped away

Roundness vs Sphericity: the degree of abrasion of a clastic particle as shown by the sharpness of its edges and corners; a perfectly rounded particle has a roundness value of 1.0 expressed as degree of “sphericity”; anything less than 1.0 is less spherical yet may be well-rounded

Caliche: a crust of soluble calcium salts commonly developed in alkaline soils in the southwestern U.S.; it can range from mm to meters in thickness, impermeable and hard

Superposition: the process by which successively younger sedimentary layers are deposited on lower and older layers; the order in which rocks are placed or accumulated in beds one above the other

Gene Stevenson is a geologist who has lived in Bluff for the past 31 years.