U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2007–5006

Pliocene and Quaternary Deposits in the Northern Part of the

San Juan Basin in Southwestern Colorado and Northwestern 

New Mexico
Pliocene and Quaternary Deposits in the 

Northern Part of the San Juan Basin in 

Southwestern Colorado and Northwestern 

New Mexico
Kplm Km

tC1

tC2

tA tB2

tB1

MH

EM MVC

FLC Kd

Kmv

hogback

tD

MR

Kd

WN HC ANIMAS

CITY MTN.

NEEDLE

Junction Creek MTNS.

PERINS PEAK low moraines

AC + SC

high moraines

CM2 CM1

LA PLATA 

MOUNTAINS 

3 5

6

4

2

1

The Animas River Valley near Durango is the north part of the San Juan Basin. Surficial deposits discussed in this report are 

chiefly in this valley. Left center skyline is north, toward the Needle Mountains, the most uplifted part of the San Juan Mountains 

and the source of the longest valley glaciers in the Rocky Mountains. Glaciers advanced southward to Durango at least six times, 

producing a series of end moraines there. Above the river at center and extending right (south) are flat surfaces at several levels. 

These terraces were cut by the river in soft shale and are armored by rounded clasts of hard bedrock lithologies from the Needle 

Mountains. Such deposits beneath terraces 360 ft (110 m) or less above the river begin at the south edge of a glacial moraine and 

thus are outwash gravel. Gravel capping terraces higher than 360 ft (110 m) above the river do not begin at a moraine, although 

some may have begun at moraines that were removed by erosion. Discontinuous remnants of hard gravel cap benches and a few 

high hills along the valley as far south as Farmington, New Mexico, and beyond.

Frontispiece
Kpc, Kf hogback tK

tH

tF

Kl

tD

tG

Tka MESA

MTNS. Kk

Kpc hogback 

San Juan Basin

Florida Mesa 9

8 7

Landmarks shown: EM—Engineer Mountain; HC—Hermosa Creek; WN—West Needle Mountain; AC—Animas City 

moraines; SC—Spring Creek moraines; FLC—Fort Lewis College; Stratified rock units (older to younger): Jm—Morrison Formation; Kd—Dakota Sandstone; Km—Mancos Shale; Kplm—Point Lookout Sandstone of Mesaverde Group; Kmv—Mesa Verde 

Group; Kl—Lewis Shale; Kpc—Pictured Cliffs Sandstone; Kf—Fruitland Formation; Kk—Kirtland Shale; Tka—Animas Formation.

Terrace alluvium: l—Flood plain or low terraces; 2—Upper Animas City outwash; 3—Lower Animas City outwash; 4—Upper 

Spring Creek outwash; 5—Lower Spring Creek outwash; 6—Upper Durango outwash; 7—Lower Durango outwash; 8—Alluvium; 

9—Florida Gravel; 10—Pediment gravel.
Pliocene and Quaternary Deposits in the 

Northern Part of the San Juan Basin in 

Southwestern Colorado and Northwestern 

New Mexico

By Glenn R. Scott and David W. Moore

Scientific Investigations Report 2007–5006

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Scott, G.R., and Moore, D.W., 2007, Pliocene and Quaternary deposits in the northern part of the San Juan Basin 

in southwestern Colorado and northwestern New Mexico: U.S. Geological Survey Scientific Investigations Report 

2007–5006, 13 p.
vii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Previous Work .......................................................................................................................................1

Surficial Deposits...........................................................................................................................................2

Bridgetimber Gravel .............................................................................................................................2

Landslide Deposit..................................................................................................................................5

Early Pleistocene (Blancan?) Outwash or Alluvium .......................................................................5

Florida Gravel.........................................................................................................................................6

Lava Creek B Ash..................................................................................................................................6

Pediment Alluvium of Middle Middle Pleistocene Age.................................................................6

Durango Till............................................................................................................................................7

Late Middle Pleistocene Outwash(?) ................................................................................................7

Lower Durango Outwash and Alluvium ............................................................................................7

Upper Durango Outwash.....................................................................................................................7

Lower Spring Creek Till........................................................................................................................8

Lower Spring Creek Outwash.............................................................................................................8

Upper Spring Creek Till ........................................................................................................................8

Upper Spring Creek Outwash .............................................................................................................8

Loess ....................................................................................................................................................8

Lower Animas City Till..........................................................................................................................8

Middle Animas City Till.........................................................................................................................9

Upper Animas City Outwash ...............................................................................................................9

Fan Alluvium...........................................................................................................................................9

Tsegi Alluvium........................................................................................................................................9

Naha Alluvium .....................................................................................................................................10

Validity of the Deposits ...............................................................................................................................10

Geomorphology............................................................................................................................................10

References Cited..........................................................................................................................................11

Figure

 1. Study area ........................................................................................................................................4

Table

1. Generalized stratigraphic section in the northwestern part of the San Juan Basin ........3
Pliocene and Quaternary Deposits in the Northern Part 

of the San Juan Basin in Southwestern Colorado and 

Northwestern New Mexico

By Glenn R. Scott and David W. Moore

Abstract

Unconsolidated late Cenozoic deposits in the northern 

part of the San Juan Basin range in age from late Pliocene to 

Holocene. Most of the deposits are alluvial gravel composed 

of resistant quartzite, sandstone, and igneous, metamorphic, 

and volcanic rocks derived from the uplifted central core 

of the San Juan Mountains 20–50 miles (32–80 kilometers) 

north of the basin. Alluvial deposits are most voluminous in 

the Animas Valley, but deposits of gravel of the same general 

age are present in the La Plata, Florida, Los Pinos, and Piedra 

River valleys as well. Alluvial gravel forms tabular deposits, 

generally about 20 feet (6 meters) thick, that are exposed 

beneath a sequence of terraces at many levels above the rivers. 

Gravel layers 360 feet (110 meters) or less above the Animas 

River are glacial outwash. The gravel layers begin at the south 

toes of end moraines and extend discontinuously downvalley 

at least 10–20 miles (16–32 kilometers). Farther south, distinction between outwash and nonglacial alluvium is problematical. Alluvial gravel beneath higher terraces does not grade to 

end moraines.

Glacial till forms a series of end moraines at the north 

edge of the town of Durango. The oldest moraines are farthest 

downvalley, are higher above the river, and have more mature 

surficial soils than do moraines farther north. The two youngest moraines, the Animas City moraines, are interpreted to be 

Pinedale in age. They have narrow, ridgelike crests and form 

nearly unbroken arcs across the valley floor. Small segments 

of still more weathered moraines, the Spring Creek moraines, 

are 170–230 feet (52–70 meters) above the river and are 

660–990 feet (200–300 meters) farther downvalley. The oldest 

moraines, the Durango moraines, are on the north end of the 

unnamed mesa on which Fort Lewis College is located. The 

base is about 180 feet (55 meters) above the river. These oldest 

moraines may be of Bull Lake age.

Alluvial fans, pediment gravel, and landslides are scattered at several levels in various valleys within the northern 

San Juan Basin. Except where the Lava Creek B volcanic ash 

(0.639 mega-annum) is interbedded in them, these crudely 

bedded accumulations of sandy or clayey material washed 

from side drainages and added little to our reconnaissance 

stratigraphic study. Scattered landslide deposits consist of 

unsorted, mixed soil and fragments of rock.

Loess and local silty and clayey sheetwash alluvium 

6–12 feet (2–4 meters) thick form a veneer on low terraces. On 

higher terraces, such as Red Mesa east of the La Plata River 

and Florida Mesa east of the Animas River, loess and sheetwash alluvium generally are about 20 feet (6 meters) thick 

but can be as thick as 40–50 feet (12–15 meters) in places on 

Florida Mesa.

When using the Lava Creek B volcanic ash as a time 

datum, apparently the timing of late Cenozoic continental 

deposition in the region was broadly similar to that in the 

Denver Basin–Front Range region and some other glaciated 

mountain ranges in the Rocky Mountains. A more accurate 

dating of Quaternary alluviation must await a thorough dating 

of the deposits. Thick, extensive outwash gravel indicates 

enhanced deposition during glacial epochs.

Introduction

 Pliocene and Quaternary deposits were studied near 

Durango, Colorado, in April and May of 1977, in company at 

different times with Donald A. Coates, John D. Strobell, and 

Juanita M. Scott. As a result of reconnaissance mapping, study 

of soils, geomorphic and stratigraphic positions of deposits, 

measurement of stratigraphic sections, pebble counts, and 

collection of volcanic ash beds (later identification by Glen A. 

Izett), a sequence of deposits was established. This sequence 

includes alluvial, loess, eolian sand, landslide, till, and outwash deposits ranging in age from Pliocene (Blancan) to latest 

Holocene. The terms Bull Lake and Pinedale are used here 

in a general time sense and are not intended to imply exact 

contemporaneity or correlation with Bull Lake and Pinedale 

deposits in the type areas in Wyoming.

Previous Work

 The Quaternary deposits in southwestern Colorado 

and northwestern New Mexico areas were studied by Atwood 

(1911), Atwood and Mather (1912, 1932), Dickinson (1965), 
Richmond (1965), Bandoian (1969), Pastuszak (1968), and 

Gillam (1982). Studies using modern techniques to determine 

the ages and correlations of the deposits had not been done 

prior to this study. Therefore, as a result of using the methods 

in this study, even though it was a reconnaissance investigation, the deposits here are arranged in a different sequence 

than proposed by previous workers. Some previously named 

deposits are lag gravel, and they have little or no significance 

as part of a geomorphic sequence. Those names are not used 

here. In addition, different ages are assigned herein to some 

deposits, and some deposits are placed in different map units.

Ages of deposits are not closely controlled by radiometric 

dating methods. The ages are inferred from height of deposits 

above rivers and from relative dating methods that involve 

soil and weathering development. The youngest glacial till is 

assumed to be middle Pinedale in age, and successively older 

tills follow the order of glaciations used by most workers in 

the Rocky Mountains.

Surficial Deposits

Most of the stream deposits in the northern San Juan 

Basin area are composed of glacial outwash. Eleven terrace surfaces have been recognized, ranging from 1,200 ft 

(366 m) to about 10 ft (3 m) above the present Animas River. 

Apparently the deposits beneath the oldest terrace surface 

and beneath the two youngest terrace surfaces are nonglacial alluvium. All of the others probably are outwash from 

glaciers near Durango during several glacial stages in the 

Quaternary. At least five terminal moraines and possibly one 

additional older end moraine were recognized. The youngest moraines form nearly continuous curving ridges that loop 

across the flat-floored broad valley of the Animas River near 

the northern edge of Durango. Upstream from the moraines 

is a typical U-shaped valley containing ground moraine and 

outwash. Because of stream abrasion, the less-resistant rock 

types become more scarce, and surface diameters of all clasts 

decrease downstream.

The wind has eroded bedrock and alluvium and deposited loess and eolian sand on upland areas. The loess overlies 

much of the northern part of the San Juan Basin, where it 

possibly forms several sheets that we estimate to range in age 

from about 300,000 to 50,000 years. The loess is some of the 

best grazing and cropland on the uplands. Eolian sand covers large parts of the San Juan Basin, where large dunes still 

are active. Several episodes of sand deposition are inferred to 

have occurred, ranging from about 25,000 years ago until the 

present.

Deposits that mantle slopes include sheetwash alluvium, 

a poorly sorted, crudely stratified material deposited by sheetflow of rainwater or snowmelt, and colluvium, deposited by 

downslope gravitational movement of material. These deposits 

generally are thin and of little significance except that they 

conceal bedrock.

Because of extremely deep incision by the Animas River 

near Durango (approximately four times as deep as the incision of the South Platte River near Denver, Colorado) and the 

incompetence of the Cretaceous Mancos Shale, many landslide deposits have formed, some of which are still active.

Table 1 shows the sequence of surficial deposits, heights 

of alluvial terrace surfaces above major streams, and known or 

estimated ages.

Bridgetimber Gravel

One of the oldest and geomorphically highest alluviums 

in the area is the Bridgetimber Gravel, which caps Bridge

Timber Mountain on the divide between the Animas and La 

Plata Rivers approximately 10 mi (16 km) southwest of Durango. The gravel is more than 100 ft (30 m) thick. The gravel 

is composed of boulders, cobbles, and pebbles, most of which 

are deeply weathered and set in a weathered matrix of sand, 

silt, and clay. In a fresh exposure, the matrix is chiefly composed of very dark colored, completely to partly dis­integrated 

clasts of volcanic and dike rocks, sandstone, and lesser vein 

quartz and granitic rocks; well-preserved clasts are harder rock 

types.

The gravel is on a well-formed erosion surface (pediment) on Bridge Timber Mountain approximately 2,100 ft 

(640 m) above the Animas River. This great height above the 

river led Atwood and Mather (1932) to suggest a Pliocene 

(?) age for the gravel. Later workers, including Barnes and 

others (1954), Wanek (1959), Baltz (1967), and Haynes and 

others, (1972), have assigned a Pliocene or Pleistocene age to 

the Bridgetimber Gravel and apparently correlative deposits, 

based partly on the original age assignment and partly on the 

height above the Animas River. Richmond (1965), on the other 

hand, tentatively assigned an age of Pleistocene (Washakie 

Point glaciation) to the gravel.

Based on the compositions of the clasts, we believe that 

the La Plata River deposited the Bridgetimber Gravel (fig. 1).

The composition of the gravel deposited by the Animas River 

differs from that deposited by the other rivers in the area. The 

Animas River gravel contains mostly less-resistant rocks but 

contains a few still-preserved hard rock clasts. In the Animas River, the rock clasts there are chiefly quartzite from 

the Uncompahgre Formation, various types of porphyry, and 

lesser amounts of quartz, chert, jasper, sandstone, hornblende 

gneiss, silicified fault gouge, and several different granitoid 

plutonic rocks. On Bridge Timber Mountain the gravel is 

composed of siliceous to intermediate volcanic and dike rocks 

(about 90 percent), quartzite, rare banded purplish-gray and 

green argillite(?), and rare sandstone clasts.

As previously stated, comparison of the clasts in the 

gravel on Bridge Timber Mountain with those deposited by 

the Animas and La Plata Rivers showed that the Bridgetimber 

Gravel in the type area was deposited by the La Plata River (a 

source in the La Plata Mountains also was suggested by Richmond [1965, p. C142]). The gravel in secs. 25 and 36, T. 34 

 Pliocene and Quaternary Deposits in the Northern Part of the San Juan Basin in Southwestern Colorado and 

 Northwestern New Mexico
Table 1. Generalized stratigraphic section in the northwestern part of the San Juan Basin.

Known

 or 

inferred age

Stratigraphic

unit

Height of alluvial or 

outwash terrace 

above major stream, 

(feet and meters) in 

Animas River

La Plata 

River 

(terraces 

rise 

downstream)

Front 

Range 

correlative (?) 

units

10,000 to 0 Naha Alluvium

Tsegi Alluvium

Eolian sand

Pinedale Upper Animas City

Middle Animas City till

Lower Animas City till

Lower Animas City outwash 

 (not discussed in text)

40(12) outwash

60(18)

40(12) Broadway 

 Alluvium

79,000

Loess

Bull Lake

122,000

Upper Spring Creek till

Upper Spring Creek outwash

Lower Spring Creek till

Lower Spring Creek outwash

150(45)

120(30)

80(24)

170(43–52)

140 in text

160(48)

Louviers

Alluvium

Sangamon 

interglacial

132,000

Late middle Sacagawea

Pleistocene Ridge?

(140,000–

302,000)(Wisconsin?)

Loess

Upper Durango Till 

Upper Durango outwash

Lower Durango Till

Lower Durango outwash

 Alluvium

230–340(70–104

320–360(97–110)

420–440 (128–134)

500(150)

Slocum 

 Alluvium

Middle middle

Pleistocene

interglacial

(639,000)

Pediment alluvium

Lava Creek B ash

Verdos 

 Alluvium

Early middle

Pleistocene

(610,000–770,000)

Florida Gravel (outwash?) 500–640(152–195)

Early Pleistocene

(1,300,000– 1,680,000)

Outwash or alluvium

High gravels of Animas

 River

840–1,000(256–305

1,150(351)

Rocky Flats

 Alluvium

Quaternary (many ages) Landslide deposits

Pliocene Blancan?)

(2.5–3 m.y.)

Gravel on Mesa Mountains

Bridgetimber Gravel 

 (deposit of the La Plata 

River) 

1,480 (451)

2,100(640) 1,150-1,350-411

Surficial Deposits 
Figure 1. Study area.

 Pliocene and Quaternary Deposits in the Northern Part of the San Juan Basin in Southwestern Colorado and 

 Northwestern New Mexico

0 3-1/2 7 MILES

0 3.5 7 KILOMETERS

Florida

River

36o45’

37o

37o15’

108o15’ 108o 107o45’

SAN JUAN BASIN

PINON

MESA

~

Animas

River

L

a

Plata

River

San

Juan River

Florida

River

Los Pinos River

Junction Creek

SAN JUAN COUNTY

RIO ARRIBA COUNTY

COLORADO

NEW MEXICO

MONTEZUMA COUNTY

LA PLATA COUNTY

La

Plata

River

Los Pinos River

Long Hollow

Animas River

LA PLATA MOUNTAINS

BASIN MOUNTAIN

DURANGO

WEST

DURANGO

EAST

BRIDGE TIMBER

MOUNTAIN

MESA

MOUNTAINS

HART MOUNTAIN

MOUNT NEBO

SLANE

KNOB

×

×

×

×

×

×

×

×

KNICKERBOCKER

PEAKS

SAN JUAN

COUNTY

LOMA LINDA

MOUNT NEBO

CEDAR HILL

LONG MOUNTAIN

BONDAD HILL

City Reservoir

Number 1

Fort Lewis

College

FLORIDA

MESA

Rules

Hill

ANIMAS

CITY

MOUNTAIN

Aztec

Farmington Bloomfield

Blanco

Turley

Archuleta

Cedar

Hill

Riverside

La Boca

La Posta Ignacio

La Plata

Kirtland

Bondad

Bocea

Oxford

Florida Falfa

Bayfield

Redmesa

Kline

Breen

Porter

Mancos

Hesperus

Durango

Animas

City

Carbon

Junction

Geology modified from Glenn R. Scott and 

David W. Moore, 2007

160

160

160

550

550
N., R. 11 W., is about 1,150–1,350 ft (305–411 m) above the 

La Plata. The divide between the La Plata River and the 

Animas River that existed during deposition of the Bridgetimber Gravel was destroyed by headward cutting of trib­utaries 

of the Animas River. Listed in table 1 are high gravels of the 

Animas River. Some of those high gravels are discussed in following sections. Other apparently correlative ancient deposits 

of the Animas River are on Knickerbocker Peaks, in the NW1⁄4 

sec. 16, T. 30 N., R. 10 W., altitude 6,788 ft (2,070 m), Hart 

Mountain, in secs. 30 and 31, T. 31 N., R. 9 W., altitude 6,943 

ft (2,116 m), and Slane Knob, in the SE1⁄4 sec. 10, T. 30 N., R. 

10 W., altitude 6,858 ft (2,090 m), southeast of Aztec (fig. 1), 

where the deposits are about 1,150, 1,150, and 1,220 ft (350, 

350, and 372 m), respectively, above the Animas River.

A presumed correlative deposit on Mount Nebo was 

deposited by the Animas River, about 1,140 ft (348 m) above 

the Animas. A strongly developed brown soil is present in the 

upper part of the alluvium on Mount Nebo, altitude 13,205 ft 

(4,024 m), in the SW1⁄4 sec. 2, T. 39 N., R. 6 W. A reddishbrown B horizon is well preserved there, overlying a strong K 

horizon. The K horizon shows a 1/8-inch-thick (3 mm) laminar layer (stage 4 of Gile [1966]); it probably contains about 

75–80 percent calcium carbonate (Michael N. Machette, oral 

commun., 1977). Rinds of calcium carbonate on the bottoms 

of stones are more than one inch (2.5 cm) thick.

We consider the Bridgetimber Gravel to be Pliocene 

(Blancan?, 2.5–3.0 m.y.) in age and possibly the same age as 

the Nussbaum Alluvium in eastern Colorado and the lower 

part of the Blanco Formation in Texas. The Pliocene age is assigned because of its great height above the La Plata and Animas Rivers (it is the highest fluvial pediment deposit), because 

it has reversed remanent magnetism in outcrops in northern 

New Mexico (J.D. Strobell, Jr., written commun., 1979), and 

because the rock clasts are so extremely weathered in contrast 

to clasts in fluvial deposits of early Pleistocene age.

Gravel on Mesa Mountains 14–20 mi (22–32 km) south 

of Durango and about 7,630 ft (2,325 m) in altitude is perhaps 

one of the older remaining surficial deposits in the area, but 

the exact age and origin of the gravel are uncertain. Relative to 

the Animas River the deposits are lower than the Bridgetimber 

Gravel, and thus they possibly are younger. They are present 

as discontinuous, patchy veneers 1–5 ft (0.3–1.5 m) thick, at a 

lower altitude, and their basal contact with underlying bedrock 

rises and falls in unison with the rolling topographic surface. 

The deposits are interpreted to be lag gravel on a rolling erosional upland surface, with one exception.

The exception is a small (0.2 by 0.6 km) deposit of gravel 

with rounded clasts in NW1⁄4, sec. 35, T. 33 N., R. 9 W. It 

forms a hill about 100 ft (30 m) high above 7,400 ft (2,255 m) 

on the rolling upland surface of Mesa Mountains. It is horizontally bedded near its base and is as thick as 16 ft (4.8 m). 

The gravel is composed of cobbles of quartzite and syenite 

porphyry and some gneiss and scarce granite. The composition is similar to that of gravel of the Animas River after softer 

sandstone clasts were removed by abrasion owing to river 

transport. A strong soil is developed in surficial clayey 

material.

Clues to its origin are horizontal bedding, its composition 

of the most resistant rock types in the Animas River drainage basin, its altitude high on the isolated Mesa Mountains 

in the San Juan Basin, and the presence of lag gravel scattered widely on the rolling upland surface. It appears to be a 

remnant terrace deposit of the Animas River about 1,480 ft 

(451 m) above the present river. An extensive fluvial gravel 

deposit probably formerly capped and protected the Mesa 

Mountains in Pliocene time. 

Landslide Deposit

Landslide deposits of many ages, types, and sizes were 

studied. Most are slumped bedrock, but commonly the deposits also include some surficial deposits. Landslides commonly 

were caused by oversteepening of slopes in the Mancos or 

Lewis Shales or other clayey Upper Cretaceous rocks. Many 

deposits are more than 1 mi2 (1.6 km2) in area. One of these is 

partly buried by pre-Bull Lake till and outwash gravel (Moore 

and Gillam, 1984, p. 201). The slide deposits, which are apparently older than the glacial till, slid into an ancestral valley 

of the Animas River when the river flowed through the area 

now occupied by Fort Lewis College. On the scarp now facing westward toward Durango, the deposit is about 15–50 ft 

(4–15 m) thick. It is composed chiefly of fragments of Point 

Lookout Sandstone and older, shaly Mesozoic rocks, but it 

also contains rounded clasts of granite. Some of the angular boulders are larger than 6 ft (2 m) in diameter. The slide 

deposits apparently originated on a hill to the west that was 

removed by erosion.

Early Pleistocene (Blancan?) Outwash or 

Alluvium

The second oldest alluvium in the study area is best 

preserved as a main stream deposit in center sec. 7 and NE1⁄4 

sec. 18, T. 32 N., R. 9 W., La Plata County, Colorado, on 

the east side of the Animas River between the Florida River 

and New Mexico where it overlies the San Jose Formation. 

The deposit is coarse gravel containing well-rounded clasts 

of quartzite from the Uncompahgre Formation, hornblende 

gneiss, porphyritic dike rocks and volcanic rocks, granitic 

rocks, quartz, chert, siliceous rocks from Paleozoic limestone 

beds, and sandstone from the San Jose Formation. A strongly 

developed calcic layer with laminar structure, several inches 

thick, overlies the gravel. The gravel is on an erosion surface 

about 840 ft (256 m) above the Animas River. A cluster of 

potentially correlative (or possibly slightly older) deposits 

is on the west side of the Animas River about 900–1,000 ft 

(275–305 m) above the river in secs. 9, 16, and 24, T. 32 N., 

R. 10 W. Deposits were not found, and apparently are not preserved, along the other streams in the area. An inferred early 

Pleistocene (Blancan?) age is based solely on the position of 

Surficial Deposits 
the gravel in the geomorphic landform sequence. The gravel 

probably correlates with the Rocky Flats Alluvium in eastern Colorado, and possibly with the upper part of the Blanco 

Formation of Texas.

Florida Gravel

The Florida Gravel at its type locality caps Florida Mesa 

between the Animas River and the Florida River. Only the 

highest part of the mesa surface is capped by the type Florida 

Gravel as designated by Atwood and Mather (1932, plate 

1); the lowest part of the mesa surface is capped by younger 

gravel that Richmond referred to as Sacagawea Ridge outwash. The highest gravel is 600–620 ft (183–190 m) above the 

Animas River, and it generally is about 500 ft (153 m) above 

other streams. The higher deposit (type Florida Gravel) is 

coarse gravel containing well-rounded clasts, including boulders larger than 3 ft (0.9 m) in diameter. The clasts along the 

Animas River, in order of decreasing abundance (in percentage 

of 100 samples), are porphyry (23), hornblende gneiss (20), 

quartzite (19), sandstone (17), aplite (5), siltstone (5), granite 

(3), granite gneiss (2), chert (2), limestone (2), vein or pegmatite quartz (1), and pegmatite (1). Clasts along the La Plata 

River are about 90 percent siliceous to intermediate volcanic 

and dike rocks; the rest are quartzite, banded purplish-gray 

or green argillite, and sandstone. More than 50 percent of the 

clasts are intensely weathered.

Florida Gravel equivalents crop out along the Animas, 

Florida, La Plata, Los Pinos, and San Juan Rivers and extend 

southwest as remnants along the San Juan River at least as far 

as Kirtland, New Mexico. The composition of the clasts varies 

depending on which stream deposited the gravel, but commonly quartzite and silicic volcanic rocks are best preserved. 

The gravel generally is 10–30 ft (3–9 m) thick, but locally it is 

thicker. The Florida Gravel was deposited as pediment covers 

and as fills along the main streams. Sorting and rounding are 

good in the terraced fill deposits. The gravel commonly is 

covered by reddish-brown or yellowish-brown loess.

Lava Creek B Ash

 At several places the gravel is overlain by water-lain 

silt and volcanic ash. One sample of the ash was identified 

as Lava Creek B (G.A. Izett and R.E. Wilcox, 1982), which 

was erupted from a caldera at Yellowstone National Park 

approximately 639,000 years ago. A 2-ft-thick (0.6 m) bed of 

white to very pale brown ash was found above the gravel on 

a pediment west of Long Hollow in the SW1⁄4SE1⁄4SW1⁄4 sec. 

11U, T. 34 N., R. 11 W., La Plata County. The ash also was 

found in several nearly contiguous places along the edges of 

the high remnants of Florida Gravel north and south of U.S. 

Highway 64, southeast of Durango. These outcrops, in E1⁄2 sec. 

3, NW1⁄4 sec. 11, and NW1⁄4 NE1⁄4 sec. 12, T. 34 N., R. 9 W., 

La Plata County, are interpreted to be remnants of a formerly 

widespread sheet of ash in this local area. Here also, the ash is 

about 2 ft (0.6 m) thick, although not preserved continuously, 

and it overlies the Florida Gravel, well-sorted, 21-ft-thick 

(6.4 m), main-stem gravel, apparently outwash of early middle 

Pleistocene age. Locally, the ash is overlain by about 30 ft 

(9 m) of poorly sorted pediment or fan gravel that includes 

pebbly, cobbly sand in the upper part. This gravel also is 

younger than the Florida Gravel and possibly is interglacial in 

age. In 1906, Woolsey (see also Larsen and Cross, 1956, 

p. 209–210; and Vanderwilt, 1947, p. 254) described volcanic 

ash that was being extracted from a pit in this same area—possibly from a pit near the center of the NE1⁄4 sec. 3, T. 34 N., 

R. 9 W.

Woolsey (1906) found ash at another site on a ridge about 

420 ft (128 m) above Junction Creek northwest of Durango. 

There the ash is about 10 ft (3 m) thick. Woolsey mentioned 

another ash bed “at the east end of the dry valley north of 

Animas City Mountain, on the shoulder of the southwardfacing spur.” We were unable to find this ash bed. Three miles 

(5 km) north of Ignacio, Moore found a 3-ft-thick (1 m) ash 

bed near the crest of a gravel-capped hill on the west side of 

Los Pinos River, NE1⁄4 NW1⁄4 SE1⁄4 sec. 29, T. 34 N., R. 7 W., 

Durango l:250,000 quadrangle.

A strongly developed Brown soil was examined at several 

places in the upper part of the Florida Gravel. The dark-reddish-brown B horizon is 1.5–3 ft (0.45–0.9 m) thick, is very 

clayey, and has a strong medium columnar structure and a 

hard, dry consistence. The K horizon is 3 to more than 4 ft 

(0.9–1.2 m) thick and it consists of yellowish-gray, stony, 

clayey sand to gravel having laminar calcium carbonate (stage 

4 of Gile, 1966) and rinds of calcium carbonate 0.5 inch 

(1.2 cm) thick on the bottoms of stones. A podzolic soil about 

2 mi (3.2 km) south of Hesperus in NW1⁄4 SW1⁄4 sec. 25, T. 35 

N., R. 11 W., La Plata County, Colorado, contains the following horizons: A1, 6 inch (15 cm); A2, 8 inch (20 cm); B1, 

14 inch (36 cm); B2, 20 inch (51 cm); and weathered C (parent material—early middle Pleistocene outwash).

The age of the Florida Gravel, based on the age of the 

overlying Lava Creek B ash, is early middle Pleistocene. It 

was interpreted by Richmond (1965, p. C139) to have been 

deposited during his Sacagawea Ridge glaciation.

Pediment Alluvium of Middle Middle 

Pleistocene Age

Overlying the Florida Gravel and the Lava Creek ash 

locally is approximately 30 ft (9 m) of yellowish-gray to 

purplish-gray, poorly sorted, crudely stratified, pebbly, cobbly 

gravel and sand that contains boulders as large as 2 ft (0.6 m) 

in diameter. Most of the clasts are sedimentary rock types that 

are exposed nearby. The purplish-gray color is derived from 

the McDermott Member of the Animas Formation. Locally, 

the sand is well cemented. This deposit is well preserved on a 

southward-facing valley wall in the SE1⁄4 sec. 3, T. 34 N., R. 9 

W., La Plata County, Colorado (Loma Linda 71⁄2-minute 

 Pliocene and Quaternary Deposits in the Northern Part of the San Juan Basin in Southwestern Colorado and 

 Northwestern New Mexico
quadrangle). The gravel is correlated with the Verdos Alluvium in the Denver, Colorado, area.

Atwood and Mather (1932) believed that a “Canyon cycle 

of erosion” ended the “Florida cycle of erosion.” Actually, the 

depth of erosion following deposition of the Florida Gravel 

was only about 140 ft (42 m), which is not even as much as 

the interval between some of the other alluvial units. Therefore, we do not believe that this erosional interval should be 

distinguished by a separate name. Erosion of canyons in the 

area apparently began at a time we estimated as the end of the 

Miocene.

Durango Till

At the north end of the unnamed high mesa that is the 

home of Fort Lewis College are deposits of till that possibly 

represent two intervals of late middle Pleistocene Durango 

glaciation. At least one of the moraines, probably the youngest, is composed of the type Durango Till of Atwood and 

Mather (1932). The till was attributed to the Sacagawea Ridge 

glaciation by Richmond (1965). The oldest of the presumed 

two moraines trends northeastward and is just west and southwest of the City Reservoir No. 1. This moraine is inferred to 

be either an earlier moraine of Durango Till or the moraine 

of an earlier temporary stillstand of the glacier that deposited 

the younger moraine of Durango Till. A good exposure of the 

till in the older moraine was not found, but a shallow trench 

exposed a soil that is equally as well developed—if not more 

strongly developed—than the soil on the younger moraine. 

This older till is compact, bouldery, and nonsorted. The composition is similar to that of the younger till; however, most 

of the clasts are completely weathered. Clasts of dike rocks, 

siltstone, sandstone, quartzite, and some plutonic rocks are 

relatively unweathered.

The youngest of the presumed two tills forms a northeast-trending ridge just north of City Reservoir No. 1 on the 

Durango East 71⁄2-minute quadrangle. The ridge is rather 

smooth and has gentle slopes. Boulders project above its 

surface, perhaps one-fourth to one-third of their diameters. 

The till is reddish brown or mottled, compact, bouldery, sandy, 

and nonsorted. Clasts are hornblende gneiss, quartzite, granite, quartz monzonite, granodiorite (some gneissoid), a few 

volcanic or dike rocks, limestone (especially siliceous [chert?] 

fragments), sandstone, and shale. Many clasts are intensely 

weathered. The upper part of the till contains a well-developed 

soil having a B horizon as thick as 54 inches (137 cm). The 

horizon is reddish brown, clayey, blocky, and very plastic. 

A Cca horizon thicker than 36 inches (91 cm) underlies the 

B horizon and contains white calcium carbonate that is very 

firmly cemented. The calcium carbonate forms rinds as thick 

as 0.25 inch (6 mm) on the undersides of clasts. The till is at 

least 25 ft (7.6 m) and possibly as much as 160 ft (49 m) thick.

Late Middle Pleistocene Outwash(?)

Two terrace surfaces along the Animas River are 500 ft 

(152 m) and 420–440 ft (128–134 m) above the river. The 

lithology of these deposits is almost identical with that of the 

Durango outwash. Soils are more strongly developed than 

those on the Wisconsinan-age deposits.

Lower Durango Outwash and Alluvium

Remnants of an outwash plain of a late middle Pleistocene Durango glaciation underlie a terrace 310–400 ft 

(95–122 m) above the Animas River south of Durango. This 

terrace was mapped as Durango outwash by Atwood and 

Mather (1932). Southward into New Mexico, the terrace 

surface is 300–360 ft (92–110 m) above the Animas and San 

Juan Rivers.

Gravel beneath the terrace ranges in thickness from 12 to 

28 ft (3.6–8.5 m). Boulders are as large as 8 ft (2.4 m) in diameter near the terminal moraine east of Durango but decrease in 

size downstream so that near Farmington, New Mexico, few 

are larger than 1 ft (0.3 m). The clasts are chiefly quartzite 

from the Uncompahgre Formation, porphyritic volcanic and 

dike rocks, granite, quartz monzonite, granodiorite, quartz 

diorite, sandstone, hornblende gneiss, quartzite, quartz, conglomerate, siliceous fossiliferous fine-grained Paleozoic rocks, 

and local limestone and dolomite, some of which contain chert 

nodules. Sedimentary clasts decrease in relative abundance 

downstream. Just south of Durango, the upper 10 ft (3 m) of 

the gravel contains boulders 6–8 ft (1.8–2.4 m) in maximum 

dimension, whereas the average size is only a foot or so. We 

suggest that the large boulders were deposited during a catastrophic glacial meltwater flood.

A soil in the upper part of the outwash contains a moderate-reddish-brown B horizon 3.5 ft (1.1 m) thick that has a 

strong, medium blocky structure and is sticky and plastic when 

wet and hard consistence when dry. The sandy, platy Cca (or 

K) horizon contains pebbles having calcium carbonate rinds 

about 1 inch (2.5 cm) thick.

Upper Durango Outwash

Remnants of an outwash plain of a late Durango glaciation were traced southward from Durango as terrace remnants 

230–340 ft (70–104 m) above the Animas River. The outwash 

consists of bouldery, cobbly, stratified, fairly well sorted 

alluvium. The outwash is more than 10 ft (3 m) thick, and it 

commonly is overlain by more than 10 ft (3 m) of sandy fan 

alluvium, especially along the south side of the Animas River 

northeast of Farmington, N. Mex. The lithology of the outwash is similar to that of the lower Durango outwash. The soil 

also is similar, but it is not as strongly developed.

Surficial Deposits 
Lower Spring Creek Till

Till, apparently of an early Spring Creek glaciation (early 

Bull Lake stade of Richmond, 1965), is present in three small, 

gently rounded remnants of a terminal moraine 2–3 miles 

(3–5 km) east of Durango along the Florida road. The till is 

greatly dissected and thinner than usual (less than 25 ft). The 

deposits are bouldery, sandy, nonsorted, and nonstratified. A 

strongly developed soil in the upper part contains a B horizon 

20 inches (51 cm) thick that is moderate reddish brown, very 

clayey, and blocky. An underlying Cca horizon is 2 ft (0.6 m) 

thick.

Lower Spring Creek Outwash

A discontinuous terrace underlain by lower Spring Creek 

outwash is present along the Animas and other rivers in the 

study area. The terrace surface ranges from 130 to 170 ft 

(43–52 m) above the rivers. The outwash, which is 20–30 ft 

(6–9 m) thick, is poorly sorted and is composed of boulders as 

large as 12.5 ft (3.8 m), cobbles, pebbles, and a sand matrix; it 

is fairly well washed and contains little silt or clay. The large 

clasts, chiefly locally derived sedimentary rocks, suggest that 

the upper part of the outwash was deposited by a catastrophic 

meltwater flood. A soil in the upper part contains an A horizon 

about 12 inches (30 cm) thick, and a light-brown to reddishbrown B horizon 20–30 inches (51–76 cm) thick that has a 

fine, strong, blocky structure and a sticky, plastic, hard consistence. In the underlying Cca horizon about 3 to 4.5-ft- (0.9 to 

1.4 m) thick rinds of calcium carbonate 0.5 inch (1.3 cm) thick 

coat the undersides of cobbles. Along the Animas River the 

outwash is composed of quartzite, hornblende gneiss, granitic 

rocks, gabbroic rocks, porphyry, and resistant sedimentary 

rocks.

Upper Spring Creek Till

Till, apparently deposited by a late Spring Creek glacier 

(late Bull Lake stade of Richmond, 1965), forms a terminal 

moraine in the northern part of Durango. The moraine is 

preserved as five remnants of a former looplike moraine with 

as much as 150 ft (45 m) of relief. The remnants are gently 

rounded, and few boulders project more than one-third of their 

diameters above the surface of the moraine. The till is bouldery, clayey, nonsorted, and nonstratified. A soil in the upper 

part contains a brownish-gray, silty, humus-rich A horizon 

10–12 inches (25–30 cm) thick that has a weak platy structure. 

The underlying B horizon is 20–24 inches (50–60 cm) thick, is 

moderate reddish brown, clayey, and very plastic when moist 

and hard when dry. An underlying Cca horizon is concentrated 

through about 4 ft (1.2 m) of the parent till; it contains rinds 

of calcium carbonate 1/3 to 3/16 inch (8–10 cm) thick on the 

undersides of cobbles. The till is composed of hornblende 

gneiss, quartzite, granite, quartz monzonite, granodiorite 

(some foliated), a few volcanic or dike rocks, limestone, ironstone, sandstone, and shale.

Upper Spring Creek Outwash

A fairly continuous terrace underlain by upper Spring 

Creek outwash is present along the Animas and other rivers. 

The terrace surface is 90–120 ft (27–36 m) above the streams, 

and the gravel is 20–30 ft (6–9 m) thick. The outwash is 

moderate reddish brown and is fairly well sorted in the lower 

part where it contains boulders as large as 24 inches (61 cm) in 

diameter. The lower 15 ft of the deposit, 2 mi (3.2 km) downstream from the moraine and on the west side of the Animas 

River just downstream from the mouth of Junction Creek, is 

composed chiefly of porphyry, sandstone, and siltstone clasts. 

The porphyry was transported by Junction Creek from the La 

Plata volcanic center. The upper 15 ft (4.5 m) of the deposit 

at the same place is poorly sorted; it contains many boulders 6–12 ft (1.8–2.4 m) in diameter, indicating that it may 

have been deposited by a catastrophic flood. Well-rounded 

clasts in the upper part are chiefly sedimentary and igneous 

rocks transported by the Animas River. They are composed 

of granite, quartz diorite, granodiorite, quartz monzonite, 

hornblende gneiss, quartzite, sandstone, and dolomite. A soil 

in the upper part contains a reddish-brown A horizon 6 inches 

(15 cm) thick, a reddish-brown, very clayey B horizon 2–4 ft 

(0.6–1.2 m) thick, and a 3.5-ft-thick Cca horizon. Calcium 

carbonate forms crusts 1 mm thick on the undersides of clasts. 

Locally, as in the SE1⁄4 sec. 24, T. 34 N., R. 10 W., the soil is a 

podzolic soil and it contains no Cca horizon; there, the B horizon is 4 ft (1.2 m) thick, is grayish red, and extremely clayey. 

At a gravel pit in the Aztec quadrangle a conglomerate bed 4 ft 

(1.2 m) thick, cemented by calcium carbonate, was exposed 

about 4.6 ft (1.2–1.8 m) above the base of the gravel.

Loess

Loess of at least two ages was observed. The older loess 

is interpreted to be early or late middle Pleistocene in age; the 

younger loess probably is late Pleistocene in age.

The older loess is generally silty clay that is strongly 

oxidized to a moderate reddish-brown color. An A horizon has 

a strong prismatic structure and is 18 inches (45 cm) thick. A 

B horizon also has a strong prismatic structure, very clayey 

consistence, and is about 40 inches (100 cm) thick. A Cca 

horizon contains calcium carbonate coatings, veinlets, and 

hard nodules and locally is about 5 ft (1.5 m) thick.

The younger loess is yellowish-brown or grayish-orange, 

clayey silt that locally is more than 20 ft (6 cm) thick.

Lower Animas City Till

A terminal moraine formed by an early Pinedale glacier 

(early stade of Richmond, 1965) is a nearly continuous loop 

 Pliocene and Quaternary Deposits in the Northern Part of the San Juan Basin in Southwestern Colorado and 

 Northwestern New Mexico
across the Animas River at the north end of Durango. The moraine is fairly sharp crested and steep sided. Most of the surface boulders project about one-half of their diameters above 

the till surface. The till is brown, bouldery, sandy, nonsorted, 

and nonstratified. It contains boulders larger than 8 ft (2.4 m) 

in diameter. A soil in the upper part contains a brownish-gray, 

sandy, clayey, hard, blocky A horizon 1–8 inches (2.5–20 cm) 

thick, underlain by a moderate-reddish-brown, pebbly, cobbly 

B horizon. The B horizon is 14–24 inches (35–60 cm) thick, 

is very clayey, has strong, medium columnar structure, and is 

sticky and plastic when moist and hard when dry. An underlying light-brown, sandy, stony Cca horizon is 30–36 inches 

(75–90 cm) thick.

Middle Animas City Till

A terminal moraine deposited by a middle Pinedale glacier is just north of the moraine of the early Pinedale glacier. 

The term middle (middle stade of Richmond, 1965) is based 

on an assumption that a still younger Pinedale moraine is 

present farther up the valley of the Animas River. Richmond 

(1965) mentioned seeing a terminal moraine of a late Pinedale 

glacier, but we did not search for a moraine. Johnson (1990) 

recognized an end moraine segment near Baker’s Bridge, 

12 mi (20 km) north of Durango. Whether this is a terminal 

moraine or a recessional moraine is not clear. 

The middle Animas City moraine is sharp crested, steep 

sided, and somewhat hummocky in spite of its narrowness. 

Most of the surface boulders project one-half to two-thirds of 

their diameters above the surface of the moraine. The till is 

light brown, compact, sandy, bouldery, nonsorted, and nonstratified. A soil in the upper part contains a brownish-gray, 

sandy, clayey, blocky, hard A horizon 8–12 inches (20–10 cm) 

thick, underlain by a moderate-reddish-brown, clayey B horizon. The B horizon is 12–18 inches (30–45 cm) thick, and it 

has a moderately strong, medium columnar structure. It has a 

sticky plastic consistence when moist and it is very hard when 

dry. An underlying light-brown Cca horizon, 22–30 inches 

(55–75 cm) thick, is sandy. Clasts have rinds of calcium carbonate one-eighth to one-fourth inch (3–6 cm) thick on their 

undersides.

Upper Animas City Outwash

A terrace composed of upper Animas City outwash 

is nearly continuous along the Animas River south of the 

terminal moraines. The outwash is bouldery, cobbly alluvium 

containing a loose, sandy matrix. Only one terrace, about 50 ft 

(15 m) above the river, is present immediately south of the two 

moraines. At several places in the Loma Linda quadrangle, 

however, are two terraces, 40 ft and 60 ft (12 and 18 m) above 

the river. Probably each glacier produced an outwash plain, but 

meltwater from the middle Pinedale glacier removed most of 

the outwash of the early Pinedale glacier. Partial erosion of the 

60-ft (18-m) terrace is well illustrated in the center of sec. 8U, 

southern T. 34 N., R. 9 W., in the Loma Linda quadrangle.

A soil in the upper part of the outwash contains a moderate-brown, sandy, humus-rich, silty A horizon about 3 inches 

(7 cm) thick. An underlying, moderate brown or grayish-red, 

clayey B horizon 8–20 inches (20–50 cm) thick generally has 

a fine moderate prismatic or a medium strong blocky structure. It is sticky and has a plastic consistence when moist and 

is hard when dry. Locally, the B horizon is sandy, platy, and 

only slightly hard. An underlying grayish-red Cca horizon is 

about 14 inches (35 cm) to more than 5.5 ft (1.7 m) thick and 

is very weak (probably stage 1 or 2 of Gile, 1966). The rind of 

calcium carbonate on the bottoms of clasts is only 

0.4–0.7 inch (1–2 mm) thick.

The outwash along the Animas River is composed of 

porphyry, granitoid rocks, hornblende gneiss, and fragments 

of quartzite from the Uncompahgre Formation, sandstone, and 

limestone. Some boulders are as large as 5 ft (1.5 m) 

in diameter.

Fan Alluvium

Alluvium in fans was observed at many places, but the 

thickest accumulation is along the southeast side of the Animas River in the Aztec quadrangle, New Mexico. Most of the 

fan alluvium ranges in age from Bull Lake to Holocene. Generally, it is granular or pebbly sand but locally is gravel. The 

thickness of the fan deposits probably averages about 15 ft 

(5 m) but locally is much more. Evidence that there are several 

ages of fans is provided by the soils. The fan over­lying the 

terrace of early Bull Lake age has a B horizon 10–12 inches 

(25–30 cm) thick and a Cca horizon 3–6 ft (1.9–1.8 m) thick; 

the fan overlying the terrace of late Bull Lake age has a B 

horizon 0.4–0.8 ft (12–24 cm) thick and a Cca horizon 0.9 ft 

(27 cm) thick; the fan overlying the Pinedale terrace has a 

B horizon 0.8-1.0 ft (24–30 cm) thick and a Cca horizon 

1.5-1.9 ft (45–57 cm) thick. Although thicker, it has weaker 

development than the Cca horizons on the other fan deposits.

Tsegi Alluvium

A deposit of alluvium in most of the valleys is correlated 

with the Tsegi Alluvium of Hack (1941). The alluvium generally underlies a terrace about 10–20 ft (3–6 m) above stream 

level or arroyo bottom. The Tsegi is brownish-gray sand and 

silt layers that commonly are 1⁄4 inch to 2 ft (0.6–0.6 m) thick. 

Gravel layers a few inches thick generally are present in the 

lower part. The deposit is rich in humus. A soil in the upper 

part is weakly developed. It has only a slight color or clay 

increase in the B horizon and rarely has a Cca horizon containing spots of calcium carbonate 1/8 inch (0.3 cm) in diameter. 

The thickness is as much as 20 ft (6 m).

Surficial Deposits 
Naha Alluvium

The youngest alluvium that forms a terrace is correlated 

with the Naha Alluvium of Hack (1941). The terrace surface 

generally is less than 10 ft (3 m) above arroyo floors. The 

Naha is grayish-brown, well-bedded to cross-stratified, friable 

to slightly compact sand. It contains no zonal soil, but the 

upper few inches is humic-rich. The thickness generally is less 

than 10 ft (3 m).

Validity of the Deposits

When we started this geologic investigation we had concerns regarding the validity of stratigraphic names, the locations of certain named deposits, the accuracy of the mapped 

contacts of all units, and the ages of many.

The Bayfield Gravel was named by Atwood and Mather 

(1932) who stated, “Pebbles and cobblestones of bluish 

quartzite and milky-white quartz, as much as 6 inches in diameter, rest on the highest part of Animas City Mountain, 31⁄2 mi 

north of Durango, at altitudes up to 8,170 ft. This is the type 

locality for the formation.” However, they named the formation for the town of Bayfield, about 14 mi east of Durango, 

where they found “scattered pebbles, cobblestones and boulders” on several flat-topped hills, rather than for Animas City 

Mountain, the type locality. They correlated deposits at other 

sites with their Bayfield Gravel. Richmond (1965) mapped 

“Bayfield gravel” on the summits of Rules Hill and nearby 

hills 6 mi (10 km) northwest of Bayfield.

Glenn R. Scott and John D. Strobell examined the surface 

of both Animas City Mountain and the hills northeast of Bayfield and found only sparse, scattered, rounded clasts in both 

places. D.W. Moore and Jay Stravers later examined Rules 

Hill and nearby hill summits northwest of Bayfield. There is 

no gravel on Rules Hill. On a hill many yards (0.5 km) southeast of Rules Hill are scattered a few hundred pebbles of volcanic rhyolite porphyry. They are presently weathering out of 

the underlying bedrock, an arkosic conglomeratic sandstone, 

part of the Upper Cretaceous and Paleocene Animas Formation. The beds strike N. 70o E. and dip 40o S., conformable with 

Upper Cretaceous beds that form the bedrock hogback there. 

The scattered pebbles on the hill are residual and modern. 

They have not been transported by a river in Quaternary time, 

as required if they are correlated with the Bayfield Gravel. 

They form no significant deposit of alluvium; therefore, we 

believe that the term Bayfield Gravel is completely unnecessary.

Richmond (1965, p. C142) reexamined the Oxford Gravel 

of Atwood and Mather (1932) and found it to be colluvium. 

Richmond stated, “As such it is not stratigraphically significant to subdivision of the Quaternary of the area.” We too 

studied the Oxford and found in some places it is lag gravel 

that originated from older, topographically higher gravel 

deposits. We agree with Richmond that the deposit lacks 

geomorphic significance; therefore, we believe that the name 

Oxford Gravel is completely unnecessary.

Dickinson (1965) examined the Cerro Till of Atwood 

(1916) at its type locality on Cerro Summit in Colorado. 

Because there was no till, but only mass-wasting debris at 

the type locality, Dickinson abandoned the name Cerro Till. 

Some workers (for instance Richmond [1986]) suggested that 

some deposits elsewhere that were formerly mapped as Cerro 

Till may actually be till; if so, for those deposits a new name 

should be applied and their lithology, geomorphic position, 

age, and all other required information for a newly named 

deposit should be published.

Concerning the Bridgetimber Gravel, Richmond (1965, 

p. C141) stated, “The gravel on the lower [southern] surface of 

Bridge Timber Mountain is 20-40 ft thick.” We found only lag 

gravel on the lower and southern part of Bridge Timber Mountain. Only the Bridgetimber Gravel that caps the upper and 

northern part of the mountain has geomorphic significance.

The Los Pinos Formation (Atwood and Mather, 1932) 

apparently was named by E.S. Larsen for exposures in the 

canyon of Los Pinos Creek, near the town of San Miguel in 

the extreme northern part of New Mexico about 10–12 mi (16–

18 km) southwest of Antonito, Colorado. Atwood and Mather 

assigned a Pliocene(?) age to the Los Pinos and equated it 

with the Bayfield and Bridgetimber Gravels. Lipman (1975) 

assigned an age of Pliocene to Oligocene to the Los Pinos and 

stated that it underlies and interfingers with basalt flows of the 

Hinsdale Formation that vary in age from about 5 to 23 m.y. 

(Miocene and Pliocene). If the Los Pinos is indeed of that age, 

it would now be considered early Miocene.

Geomorphology

The most comprehensive discussion of the geomorphology of the San Juan Mountains and surrounding region is that 

of Atwood and Mather (1932). Unfortunately, many if not 

most of their conclusions about glaciation, peneplanation, canyon cutting, and even deposition are invalid, largely because 

of the invalidity of many of the deposits upon which the 

postulated events were based, as we have stated earlier in this 

report. In addition, several important events were omitted from 

Atwood and Mather’s geomorphic history. Omissions include 

erosion of the late Eocene surface and several stages of canyon 

cutting and fluvial deposition.

The earliest event that exerted a major influence on the 

physiography of the area was the cutting of a late Eocene 

erosion surface. This extensive surface developed on Precambrian rocks was widespread. The surface was later covered 

by alluvial gravel called by several names at different places: 

Telluride Conglomerate, Blanco Basin Formation, El Rito 

Formation, San Jose Formation, and Ritito Formation. These 

probable Eocene and younger deposits were then covered by 

volcanic or volcaniclastic deposits that generally were of local 

derivation and of Oligocene and Miocene age. They include 

10 Pliocene and Quaternary Deposits in the Northern Part of the San Juan Basin in Southwestern Colorado and 

 Northwestern New Mexico
from oldest to youngest, among other deposits, the Conejos 

Formation, then a series of ash-flow tuffs, then the Los Pinos 

Formation, and finally the Hinsdale Formation. Atwood and 

Mather (1932) also inferred the development of a widespread, 

so-called San Juan peneplain that supposedly formed during 

the transition between the Pliocene and Pleistocene. Steven 

(1968) reviewed the evidence concerning the concept of a San 

Juan peneplain and decided that such a surface never existed. 

He concluded that, “The detailed volcanic and geomorphic 

history of the central San Juan Mountains indicates no peneplain cycle of erosion between the end of major volcanism and 

the present time.”

We find that the large-scale eruptive and aggradational 

event that resulted in the deposition of volcanic and volcaniclastic rocks had ceased by the end of Miocene time. Canyon 

cutting began in the Pliocene and still continues. The first 

fluvial material deposited then was the Bridgetimber Gravel. It 

probably was deposited 2.5–3.0 m.y. ago and is about 1,150 ft 

to 2,000 ft (350 to 640 m) above the modern rivers. At about 

the same time or earlier, gravel on Mesa Mountains may 

have been deposited by the Animas River. The next younger 

alluvial deposit is yet unnamed; it is 845–900 ft (258–274 m) 

or 1,000 ft (300 m) above the Animas River. It probably is 

early Pleistocene (Blancan?) in age. No evidence was found to 

relate these last deposits to glaciation. The next younger alluvial deposit is the Florida Gravel, which is interpreted to be 

glacial outwash in spite of a lack of direct evidence to confirm 

a glacial association. The Florida Gravel is about 500–640 ft 

(152–195) above the Animas River; it is early middle Pleistocene in age and slightly older than 639,000 years. Very soon 

after deposition of the Florida Gravel, an airfall volcanic ash 

(the Lava Creek B ash from Yellowstone National Park) was 

deposited across this area and much of the Great Plains. It was 

concentrated by water in locally thick deposits. Local nonglacial alluvium was almost immediately deposited on the ash.

Two late middle Pleistocene terraces are at 500 ft (152 m) 

and 400 ft (122 m) above the Animas River. They possibly 

represent one or two glaciations between the Florida and 

Durango events. The next younger episode of alluviation is 

probably also of late middle Pleistocene age. It is correlated 

with the Durango Till, which is on the high surface occupied 

by Fort Lewis College east of Durango. Two stages of glaciation are inferred to have occurred in Durango Till time. The 

corresponding outwash surfaces are about 360 ft (110 m) 

and 340 ft (104 m) above the Animas River. The gradual and 

stepped decrease in altitude of the alluvial deposits above a 

given reach of the river through time indicates that canyons 

were cut progressively deeper throughout Pliocene and Quaternary time.

In late middle Pleistocene (Spring Creek or Bull Lake) 

time, glaciers advanced as far south as Durango twice and 

deposited outwash gravel on the valley floor. The gravel now 

underlies terraces 130–170 ft (40–52 m) and 120 ft (30 m) 

above modern streams. Similarly, in Pinedale time glaciers 

reached Durango twice and deposited outwash gravel that now 

underlies terraces 60 ft (18 m) and 40 ft (12 m) above streams. 

Loess derived from fine-grained outwash was deposited on 

valley deposits in middle Pleistocene time and again in late 

Pleistocene time. Eolian sand was deposited from near the end 

of the Pleistocene until early Holocene time. Clayey fan alluvium, derived chiefly from Upper Cretaceous marine shale, 

was washed onto valley floors and terrace gravel throughout 

the Quaternary Period.

Icecap glaciation was inferred by Atwood and Mather 

(1932), by Richmond (1965), and by Leonard (1979). We 

follow an accepted definition of an icecap: “A dome-shaped or 

platelike cover of perennial ice and snow, covering the summit area of a mountain mass so that no peaks emerge through 

it” (Bates and Jackson, 1980, p. 307). We would not call the 

apparent glacial maximum in the San Juan Mountains an “icecap” because the highest glacial scouring is many tens or even 

a few hundred meters below most high peak summits. In other 

words, it was an ice field.

Apparently the chief reason that Atwood and Mather 

suggested that this was icecap glaciation was their belief in 

the glacial origin of the so-called Cerro Till; it is a widespread 

landslide deposit, not a till sheet. On a map showing the extent 

of glaciers, Atwood and Mather extended the margins of many 

glaciers beyond the confines of the valleys. We plotted the 

upper limits of many of the glaciers and found that in every 

area studied, the glacier boundaries were more restricted than 

shown by Atwood and Mather (1932).

An independent approach by Porter and others (1983) 

used paleoclimatic factors to model a late Wisconsinan ice 

field in the San Juan Mountains. Their ice field was about 

1,000 m thick, 10–25 km south of the Animas River–Uncompahgre River divide. Ice flowed over many divides, but 

whether most high peaks were above the ice surface is a moot 

question. Our field examination suggests that the ice scoured 

some divides (for example Weminuche Pass) (lat 37o43′ N., 

long 107o43′ W.) between the Animas River and the headwaters of Lime Creek. Nevertheless, we do not see justification for use of the term icecap glaciation. Gillam and others 

(1984, p. 154) also questioned Atwood and Mather’s concept 

of the so-called icecap, “Although they called this mass of ice 

an icecap, they recognized that the ice was mostly confined to 

valleys and rarely crossed drainage divides.”

References Cited

Atwood, W.W., 1911, Physiographic studies in the San Juan 

district of Colorado: Journal of Geology, v. 19, p. 449–453.

Atwood, W.W., 1916, Eocene glacial deposits in southwestern Colorado: U.S. Geological Survey Professional Paper 

95–B, p. 13–26.

Atwood, W.W., and Mather, K.F., 1912, The evidence of three 

distinct glacial epochs in the Pleistocene history of the San 

Juan Mountains, Colorado: Journal of Geology, v. 20, 

p. 385–409.

References Cited 11
Atwood, W.W., and Mather, K.F., 1932, Physiography and 

Quaternary geology of the San Juan Mountains, Colorado: 

U.S. Geological Survey Professional Paper 166, 176 p.

Baltz, E.H., 1967, Stratigraphy and regional tectonic implications of part of Upper Cretaceous and Tertiary rocks, 

east-central San Juan Basin, New Mexico: U.S. Geological 

Survey Professional Paper 552, 101 p.

Bandoian, C.A., 1969, Geomorphology of the Animas River 

valley, San Juan County, New Mexico: Albuquerque, 

University of New Mexico, Master’s thesis, 88 p.

Barnes, Harley, Baltz, E.H., and Hayes, P.T., 1954, Geology 

and fuel resources of the Red Mesa area, La Plata and 

Montezuma Counties, Colorado: U.S. Geological Survey 

Oil and Gas Investigations Map OM 149, scale 1:62,500.

Bates, R.L., and Jackson, J.A., 1980, Glossary of geology (2d 

ed.): Falls Church, Va., American Geological Institute, 

751 p.

Dickinson, R.G., 1965, Landslide origin of the type Cerro Till, 

southwestern Colorado: U.S. Geological Survey Professional Paper 525–C, p. C147–151.

Gile, L.H., 1966, Cambic and certain noncambic horizons in 

desert soils of southern New Mexico: Soil Science Society 

of America Proceedings, v. 30, no. 6, p. 773–781.

Gillam, M.L., 1982, Quaternary alluvial deposits and soil 

formation, lower Animas River area, Colorado and New 

Mexico: Geological Society of America, Cordilleran Section, 78th annual meeting, abstracts with programs, p. 166.

Gillam, M.L., Moore, D.W., and Scott, G.R., 1984, Quaternary 

deposits and soils in the Durango area, southwestern Colorado, in Brew, D.C., ed., Rocky Mountain Section Field Trip 

Guidebook: Geological Society of America, Rocky Mountain Section Field Trip Guidebook 37th Annual Meeting; 

Durango, p. 149–182.

Hack, J.T., 1941, Dunes of the western Navajo country: 

Geographical Review, v. 31, no. 2, p. 240–263.

Haynes, D.D., Vogel, J.D., and Wyant, D.G., 1972, Geology, 

structure, and uranium deposits of the Cortez quadrangle, 

Colorado and Utah: Department of the Interior, U.S. Geological Survey, Miscellaneous Investigations Series, Map 

I–629, scale: 1:250,000

Izett, G.A., and Wilcox, R.E.,1982, Map showing localities 

and inferred distributions of the Huckleberry Ridge, Mesa 

Falls, and Lava Creek ash beds in the western United States 

and southern Canada: U.S. Geological Survey Miscellaneous Investigations Map I–1325, scale 1:4,000,000.

Johnson, M.D., 1990, Fabric and origin of diamictons in end 

moraines, Animas River Valley, Colorado, U.S.A.: Arctic 

and Alpine Research, v. 22, no. 1, p. 14–25.

Larsen, E.S., Jr., and Cross, Whitman, 1956, Geology and 

petrology of the San Juan region southwestern Colorado: 

U.S. Geological Survey Professional Paper 258, 303 p.

Leonard, E.M., 1979, Topography and glaciation in the San 

Juan Mountains and Front Range, Colorado: Geological 

Society of America abstracts with programs, v. 11, no. 6, 

p. 277.

Lipman, P.W., 1975, Evolution of the Platoro caldera complex 

and related volcanic rocks southeastern San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 

852, 128 p. (see p. 77).

Moore, D.W., and Gillam, M.L., 1984, Road log—Quaternary 

deposits and soils in the Durango area, southwestern Colorado, in Brew, D.C., ed., Rocky Mountain Section Field Trip 

Guidebook: Geological Society of America, 37th annual 

meeting, Durango, Colo., p. 183–209.

Pastuszak, R.A., 1968, Geomorphology of part of the La Plata 

and San Juan Rivers, San Juan County, New Mexico: 

Albuquerque, University of New Mexico, Master’s

 thesis, 84 p.

Porter, S.C., Pierce, K.T., and Hamilton, T.D., 1983, Late 

Wisconsin mountain glaciation in the western United States, 

in Porter, S.C., ed., Late Quaternary environments of the 

United States, volume 1—The late Pleistocene: 

Minneapolis, University of Minnesota Press, p. 71–110

Richmond, G.M., 1965, Quaternary stratigraphy of the 

Durango area, San Juan Mountains, Colorado: U.S. 

Geological Survey Professional Paper 525C, p. C137–143.

12  Pliocene and Quaternary Deposits in the Northern Part of the San Juan Basin in Southwestern Colorado and 

 Northwestern New Mexico
Richmond, G.M., 1986, Stratigraphy and correlation of 

glacial deposits of the Rocky Mountains, the Colorado 

Plateau, and the ranges of the Great Basin: p. 99–122, 

in Richmond, G.M., and Fullerton, D.S., Quaternary 

glaciation in the United States of America: Quaternary 

Science Reviews, v. 5.

Steven, T.A., 1968, Critical review of the San Juan peneplain, 

southwestern Colorado: U.S. Geological Survey 

Professional Paper 594–I, p. I1–I19.

Vanderwilt, J.W., 1947, Mineral resources of Colorado: State 

of Colorado Mineral Resources Board, 547 p.

Wanek, A.A., 1959, Geology and fuel resources of the Mesa 

Verde area, Montezuma and La Plata Counties, Colorado: 

U.S. Geological Survey Bulletin 1072 M, p. IV, 667–721.

Woolsey, L.H., 1906, Volcanic ash near Durango, Colorado: 

U.S. Geological Survey Bulletin 285, p. 476–479. 

References Cited 13
Scott and Moore— Pliocene and Quaternary Deposits in the Northern Part of the San Juan Basin in Scientific Investigations Report 2007–5006

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Southwestern Colorado and Northwestern New Mexico