WOLCOTT

SYNCLINE

HORN

RANCH

ANTICLINE

UTE

CREEK

SYNCLINE

 20

 20

 50

 15

 30

 22

 12

 18

 40

 35

 30

 46

 42

 35

 33

 17 20

 22

 20

 40

 55

45

L6681b

A'

M

M

Kp

Qac

Qcg

Qtl

Kn

Qac

Qac Qac

Qac

Kn

Kn

Kp

Kb

Kb

Kb

Kn

Kn

Qac

dPsb

Kd

Phms

Qc

he

hee

Qac

he Qac

hee

Qac

af

Qc

Qac

Qty

Qc

Jm

Qac

he

he

Phm

Qalc

Qalc

Qalc

Qalc

Qalc

Qalc

Qalc

Qalc

Qalc

Qfo

 20

 18

 15

 14 16

 16 18

 20

 9

 12

 9

 5

 12

 8 8

 6 13 15 7

 13

 22

 25

 20

 20

 18

 15

 25

 25

 45

 45

 40

 80 45 35

 27

 45

 40 60

 60

 30

 75

 20

 50

 50

 47

 30

 45

 25

 35

 85

 35

 42

 41

 30

 36

 43

 43

 50

 37

 60

 70

 35

 45

 50

 38

 42

 55

 20

 18

 10

 25 9

 30

 35

 38 32

 30

 24

 25

 32

 30

 30

 21

 28 22

 20

 24

 16

 25

 25

 15

 9

 9

 12

 30 15 10

 10 15

 16 9

 20

 10 27

 10

 10

 8

 30

 20

 15

 15

 12

 25

 20

 16 20

 10

 15

 6

 10

 10

 15

 11

 15

 15

 10 14

 12

 10

 19

 13

 9

 10

 12

 23 15

 10

 13 12 16

 6

 8

 11 9 10

 9

 15

 16

 13

 10

 14

 14

 5

 9

 16

 9

 15

 20 13

 7

 25

 12

 16

 13

 12

 25

 10

 14

 20

 17

 22

 17

 20

 15

 6

 25

 15

 20

 80

 15

 30

 62

 25

 25

 23

 15

 20

 17

 15

 27

 22

 30

 25

 38

 28

 30

 28

 25

 42

 20

 24

 44

 43 41

 30

 45

 36

 30

 30

 42

 58

 75

 60

 45

 45

 40

 60

 40

 30

 50

 30

 44

 50

 80 36

 40

 28

 52

 82

 85 75

 20

 50

 75

 40

 40

 30

 55

 38

 60

 20 35

 40

 70 45

 55

 35

 65 75

 55

 48

 63 45 45

 85 55 60

 30

 55

 70

 80

 45

 65

 76

 70

 55

 80

 15

 50

 68

 10

 40 42

 38

 68

 68 60

 60

 63 75 63

 52

 60

 55

 60

 60 80

 85

 65 70

 85

 87

 50

 80

 80

 65

 58 77

 62

 70 66

 70

 13

 18

 13

 15

 45 43

 10

 14

 10

 7

 85

 68

 50

 30

 40 43

 75

 80

 45

 50

 52

 45

 30

 33

 57

 25

 16

 23

 45 30 50

 32

 20 14

 40

 37

 35

 20

 28

 45 10

 45

 35 22

 20

 55 35 75

 10

 17

 16 55

 5

 5 5

 10

Qac

Qac

Kn

Kn

Kp

Kn

Kn

Kp

Qac

Kn

Kb

Kb

Kd

Qac

Qac

Jm

Qac

Kb

Qc

Qac

Qc

Kn

Kp

Kp

Kp

Qac

Qal

Qtl

Kp

Kp

Kp

Kp

Kp

Kp

Kp

Kp

Qac

Qac

Qac

Qac

Kp

Qtl

Qtl

QTdi

Kp

Qac

QTg

Qtl

Qal

Kn

Kn

Kn

Qac

Kb

Qac

Kn

Kn

Kn

Kp

Kd

Jm

Phm

he

Qac

Qac

Qty

Qty

Qty

Kb

af

Qalc

Kb

Qac

Qac

Qc

Qc

Qc

Qc

Qc

Qac

dc

dPsb

Phm

Qc

Qac

he

Qc

Phm

dPsb

dc

Qfy

Qac

he

he

Qac

Qc

Qc

Qc

Qc

hee

he

hee

he

hee

Qac

he

Tb

Phm

dPsb

dc

Jm

he

Qc

Qc

Qc

hee

hee

he

Qac

Qac

he

Tb

Qac

he

he

hee

hee

Qdu

he

Qdu

hee

Phm

hee

he

hee

Qc

Qac

Qac

Qc

Qc

Qc

Qc

Qty

Qac

he

Qac

Qty

Qac

he

Kd

he

Qc

Qc

Qfy

Qc

Kd

dc

Qc Qc

Qc

Qc

Qtl

Qalc

QTdi

QTdi

QTdi

Qfo

Qac

Qc

Qfo

Qfo

Qdu Qdu

Qtl

Kn

Qfo

Qto

Tg

Tg

Tg

Tg

Tg

Tg

Tb

Tg

Kd

Qac

Tg

Tg

Qac

Qac

Kb

Kb

Qlsb

Qlsb

Qlsn

Qlsb

Qlsb

Qlsb

Kb

Qlsd

Qlsb

Qcg

Qcg

Kn

Qcg Qcg

Qlsg

Qac

Qcg

Kb

Qcg

Kn

Qac

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qac

Qlsd

Kp

Qlsd

Qcd

Qcd

Qcd

Qlsg

Qlsd

Kps

QTg

QTg

QTg

QTg

QTg

Kps

Kps

lf

lf

Kn

Qlsg

QTg

Qcg

Qcg

Qcg

Qcd

Qcd

Qcd

Qcd

Qcd

Qlsd

Qlsd

Qlsd

Qlsd

Qlsd

Je dc

Qcd

Qlsb

Kd

Qlsd

Jm

Je

Phms

Phm

Qcg

Qlsv

Tb

Tb

Tb

Tb

Qac

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qc

Qcg

Je

Phms

he

he

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Qcg

Phm

Qcg

Qac

Tb

he

dPsb

Phms

dc

Je

Jm

Qcg

Phm

Qto

Qcg

Qc

Phm

dc

Jm

Kd

dPsb

Je

Jm

af

Qcg

Qcg

Qlsb

Qcg

dPsb

Jm

af Qlsr

Je

Qlsn

Qcd

af

Qcg

Qcg

Qcg

Qcg

Qcg

Qlsv

Qlsg

Qc

Qcg

af

Qac

Je

Qalc

af

Qlsd

Qcg

Qcg

Qac

Qcg

Qlsb

Qlsb

Jm

Qlsd

Qlsd

Qlsd

Kb

Qcg

T

T

Qcg

Qcg

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

Surficial deposits shown on this map are generally at least 1 m thick, thinner deposits are not shown. 

Thin, discontinuous colluvial deposits, residual material on bedrock, and some artificial fills were not 

mapped. Many contacts of surficial deposits are approximate because the contacts are poorly exposed or 

gradational (for instance; the contacts between landslides and colluvium, or gravelly colluvium and other 

units).

Divisions of Pleistocene time correspond to those of Richmond and Fullerton (1986): Holocene, 0 to 

11,560 yr ago; late Pleistocene, 11,390 to 127,000 yr ago; middle Pleistocene, 127,000 to 778,000 yr 

ago; and early Pleistocene 778,000 to 1,806,000 yr ago (D.S. Fullerton, U.S. Geological Survey, 

written commun. 1997).

Age assignments for surficial deposits are based chiefly on stratigraphic relationships, the degree of 

erosional modification of original surface morphology, height above stream level, and degree of soil 

development. Age assignments for some units are inferred chiefly on the basis of regional rates of 

stream incision of about 13 m/100 k.y. (k.y. = thousand years) and 190 m/m.y. (m.y. = million years). 

The first incision rate is based on a value for stream incision since the deposition of the 660 k.y. Lava 

Creek B volcanic ash (Izett and others, 1992), where it is present about 80 m above the White River near 

Meeker, Colorado (Whitney and others, 1983). The second incision rate is based on about 1,830 m of 

downcutting by the Colorado River since the deposition of 9.7±0.05 m.y. basalt on Grand Mesa (Marvin 

and others, 1966).

Grain-size terminology for the surficial deposits is based on visual estimates and follow the modified 

Wentworth grade scale (American Geological Institute, 1982). In descriptions of surficial deposits, the 

term “clasts” refers to the fraction greater than 2 mm in diameter, whereas the term “matrix” refers to 

particles less than 2 mm in size.

ARTIFICIAL FILLS

Artificial fill (latest Holocene)—Compacted and uncompacted fill material composed 

mostly of varying amounts of silt, sand, and rock fragments. Unit includes fills beneath 

Interstate 70 and the tracks of the Denver and Rio Grande Western Railroad. Thickness ranges from about 1 to 15 m

Landfill (latest Holocene)—Compacted and uncompacted fill material composed mostly of 

varying amounts of silt, sand, and rock fragments and human refuse. Unit includes 

two deposits north of Wolcott and the Eagle River; southern deposit is reclaimed and 

compacted landfill, northern deposit is compacted and uncompacted, active landfill, 

and its outline is approximate. Thickness ranges from about 3 to 10 m

ALLUVIAL DEPOSITS

Stream-channel and flood-plain deposits along Eagle River (Holocene)—The upper 1-

to 2 m of the unit is commonly a light-yellowish-brown, massive, silty fine sand to medium sand that locally contains minor amounts of pebbles and cobbles in lenses generally less than 20 cm thick. The lower part of the unit, poorly exposed and of unknown 

thickness, consists of well-sorted, rounded and well-rounded, pebble-cobble gravel derived from a variety of sedimentary, igneous, and metamorphic rocks. Clasts consist 

predominately of fine-grained black basalt, white quartzite, red sandstone, light-colored 

granite and granodiorite, and dark-colored schist and gneiss. Some clasts have a thin, 

white carbonate coat. Matrix in lower, gravelly part consists of very pale brown, sandy 

silt and silty sand. Unit may include low terraces along the flood plain. Unit subject to 

periodic flooding. Thickness estimated to be 5–10 m

Alluvium along tributaries of Eagle River (Holocene and latest Pleistocene)—Includes 

stream-channel, flood-plain, and some sheetwash deposits. Very pale brown to very 

pale gray, massive, sand, silt, and clay with matrix-supported pebble- to cobble-size 

clasts of locally derived Cretaceous shale and sandstone. Locally bedded and laminated and contains clast-supported, pebble to small cobble gravel lenses. Mapped only 

along some tributaries of Eagle River, north of Eagle River. Thickness estimated to be 

2–7 m

Terrace alluvium along tributaries of Eagle River (late and middle Pleistocene)—Alluvium deposited by Alkali and Muddy Creeks that underlies at least two different terrace 

levels that are undifferentiated on the map. Exposed mainly as gravelly rubble with 

yellowish-brown, silty, fine and medium sand. Consists of rounded and well-rounded 

pebble-boulder gravel that is capped by and mixed with subangular to subrounded pebble-boulder clasts of basalt. Rounded and well-rounded gravel clasts consist primarily 

of white and tan quartzite and red sandstone, with sparse clasts of white granite and 

granodiorite and dark gneiss and schist. Clast composition is similar to Quaternary to 

Tertiary alluvium (QTg) mapped nearby and to Tertiary alluvium (Tg) mapped to southwest. Mapped only near junction of Alkali and Muddy Creeks in north-central part of 

area. Terraces present at about 6 m and about 27–30 m above present Alkali Creek. 

Maximum preserved thickness about 3 m

Younger terrace alluvium of Eagle River (late Pleistocene)—Alluvium deposited by Eagle 

River that underlies terraces about 6–8 m above river. Unit consists of well-sorted, imbricated, rounded and well-rounded pebble-cobble gravel. Clasts consist predominately 

of white and pink coarse-grained granite, white and gray fine-grained granite and granodiorite, dark gneiss and schist, white, pink, and tan quartzite, fine- to medium-grained 

red sandstone, and basalt. Some clasts have a thin, white carbonate coat. Matrix consists of pale-brown, silty sand and sand. Gravel is commonly overlain by about 1 m of 

overbank materials consisting of massive, light-yellowish-brown, silty fine sand and fine 

sand. Wedges of undivided alluvium and colluvium (Qac) overlie and cover terrace 

margins. Unit is probably equivalent in part to outwash of the Pinedale glaciation, 

which is about 12–35 k.y. old (Richmond, 1986, chart 1A). Unit is a potential gravel 

resource. Thickness commonly 5–10 m

Older terrace alluvium of Eagle River (middle Pleistocene)—Alluvium deposited by Eagle River that underlies small terrace remnants about 25–30 m above Eagle River. Unit 

consists of well-sorted, rounded and well-rounded, pebble-cobble gravel. Clasts consist 

mostly of white and pink, coarse-grained granite, gray, fine-grained granite and granodiorite, dark gneiss and schist, white, pink, and tan quartzite, fine-and medium-grained 

red sandstone, and light- to dark-gray basalt. Matrix consists of pale-brown, silty sand. 

Gravel is commonly overlain by about 1 m of overbank materials consisting of massive, light-yellowish-brown, silty fine sand. Unit found at two localities in the map area: 

(1) underlying a terrace remnant just west of junction of Squaw Creek and Eagle River, 

southeast part of map area, and (2) a few patches of terrace remnants are present on 

spur ridges north of Eagle River in west-central part of map area. A wedge of undivided alluvium and colluvium (Qac) overlaps and covers much of the terrace near Squaw 

Creek. Unit probably equivalent in part to outwash of the Bull Lake glaciation, which 

is about 140–150 k.y. old (Pierce and others, 1976). Terrace remnants too small to be 

potential gravel resource. Preserved thickness commonly 1–3 m

Alluvium (early Pleistocene to Pliocene?)—Gravelly alluvium containing clasts likely deposited by an ancestral Eagle River or its tributaries. Present as several gravelly patches in northeast part of map area where it is characterized by poorly exposed gravelly 

rubble and yellowish-brown, silty, fine and medium sand matrix material. Consists of 

rounded and well-rounded, pebble-boulder gravel capped by and mixed with subangular to subrounded pebble-boulder clasts of basalt. Rounded and well-rounded gravel 

clasts consist primarily of white and tan quartzite, red sandstone, and sparse clasts of 

granite and granodiorite and dark gneiss and schist; clast composition similar to terrace deposits along Alkali and Muddy Creeks (Qtl) and to Tertiary alluvium (Tg) in 

southwestern part of map area. Locally may include gravelly colluvium (Qcg). Remnants of unit about 150–300 m above modern Eagle River. Maximum preserved 

thickness about 5 m

Alluvium (early Miocene or Oligocene)—Gravelly alluvium containing clasts likely deposited by an ancestral Eagle River or it's tributaries. Unit present as several remnant, gravelly patches in southwest part of map area where unit locally appears to underlie basalt 

(Tb). Poorly exposed, mainly as numerous patches of gravelly rubble in southwestern 

part of map. Consists of rounded to well-rounded pebble-cobble gravel (sparse boulders) of weather-resistant rock types. Clasts are primarily white, tan, and pink quartzite; 

red, fine- to medium-grained sandstone; and very sparse clasts of granite and gneiss. 

Commonly covered by or littered with debris from overlying basalt. Locally may include gravelly colluvium (Qcg). Remnants of unit about 300–670 m above modern 

Eagle River; large elevation range of unit may reflect post-depositional collapse of this 

region related to dissolution or movement of the underlying Eagle Valley Evaporite 

(hee). Similar in appearance and clast composition to gravelly alluvium north of Eagle 

River (QTg and Qtl). Maximum preserved thickness about 5 m

ALLUVIAL AND COLLUVIAL DEPOSITS

Younger fan alluvium and colluvium (Holocene and latest Pleistocene)—Unit forms alluvial and colluvial fans along Eagle River. One small, low-gradient fan is present along 

east side of Eagle River in east-central part of map area. Several steeper-gradient, coalesced, alluvial and colluvial fans present along south side of Eagle river in west-central part of map area were collectively mapped as a single fan complex. Consists of 

pebble- to boulder-size, angular to subrounded, locally derived clasts that are mostly 

matrix supported in massive fine sand, silt, and clay. Unit may be subject to channel 

modification and debris flows during intense or prolonged rainstorms. Maximum 

thickness estimated to be about 7–10 m

Undivided alluvium and colluvium (Holocene to middle Pleistocene)—Sediments deposited in tributary valleys of Eagle River and as broad alluvial and colluvial aprons that 

overlap and cover terrace deposits along the Eagle River. Includes stream-channel, 

flood-plain, fan, sheetwash and colluvial deposits that interfinger. Character and composition variable and dependent on local source rocks, topography, and relative position within drainage. In northern part of quadrangle, where Cretaceous shales underlie most valley areas, deposits are chiefly light-gray to grayish-brown silty clay and 

clayey silt with some matrix-supported, angular to sub-rounded shale and sandstone 

clasts; deposited principally as interfingered colluvium and sheetwash. These northern 

deposits are commonly overlain by gravel clasts that appear to be lag gravels of older, 

high-level, Quaternary-Tertiary alluvium (QTg) present nearby. In southern part of 

quadrangle unit is more variable: along Squaw Creek, gravelly alluvium interfingers 

with small fan and colluvial deposits; most other southern deposits are fine sand, silt, 

and clay derived principally from the Eagle Valley Formation and Eagle Valley Evaporite, and many of these deposits are evaporite-rich. Gullies within unit may be subject 

to flash floods and, near canyon mouths, may be subject to debris flows related to unusually intense rainstorms. Fine-grained deposits of unit may be susceptible to hydrocompaction- and piping-related subsidence (particularly evaporite-bearing deposits), 

and expansive clays may be present in deposits derived from Benton and Pierre Shales 

(Kb and Kp). Maximum thickness estimated to be about 10 m

Older fan alluvium and colluvium (middle Pleistocene)—Light-brown, poorly sorted silt, 

sand, and clay with matrix-supported, subangular to rounded, pebble-cobble gravel of 

locally derived sandstone and other rock types. Forms low-gradient fan deposits at 

base of some landslide deposits in northeastern part of map area. The older fan deposits cut and are younger than these landslide deposits. Present adjacent to intermittent tributaries of Eagle River at elevations 25–45 m above active channels of Ute and 

Cache Creeks. Some undifferentiated alluvium and colluvium (Qac) in northwestern 

part of map area may be remnants of similar older fan deposits. Maximum thickness 

estimated to be about 5 m

COLLUVIAL DEPOSITS

Colluvium (Holocene and late Pleistocene)—Clast and matrix supported, mostly angular 

to sub-angular, pebble- to boulder-size clasts in sand, silt, and clay matrix derived from 

weathered bedrock. Clast lithology, size of clasts, and character of clast support variable and dependent mostly on source rocks, slope gradient, and relative position on 

slope. Easily eroded source rocks tend to form finer grained, matrix-supported deposits with fewer and smaller clasts; resistant source rocks, such as those of the Dakota 

Formation (Kd), tend to form blocky, matrix-supported, coarser-grained deposits with 

more and larger clasts. Colluvium derived dominatly from Dakota Formation (Kd) and 

from gravelly alluvium (Qtl, QTg, and Tg) are distinctive and are differentiated as such 

in most places on the map (Qcd and Qcg). Locally may include talus, landslide deposits, sheetwash deposits, and debris-flow deposits that are too small or indistinct to map 

separately. Areas of colluvium may be susceptible to debris flows, rock falls, and small 

slumps and landslides; fine-grained deposits may be prone to settlement from piping 

and hydrocompaction, particularly where derived from evaporitic rocks. Maximum 

thickness as much as 15 m, but generally much thinner

Colluvium undifferentiated—Dominantly fine-grained deposits of sand, silt, and clay that 

include matrix-supported, angular, pebble- to cobble-size clasts of underlying bedrock

Colluvium of Dakota Sandstone—Angular, pebble- to large boulder-size clasts of white 

to tan sandstone and orthoquartzite that are mostly clast supported in matrix of silty 

sand. Locally contains blocks as much as 3 m in length

Gravelly colluvium—Derived from Quaternary-Tertiary, gravelly alluvium (Qtl, QTg, and 

Tg). Forms distinctive gravelly slopes near depositional sites of gravelly, source alluvium. Distinguished from alluvial deposits principally by position on slope; locally may 

include some stable alluvium

Landslide deposits (Holocene to middle Pleistocene)—Includes a variety of materials: 

from relatively intact masses of bedrock to unconsolidated surficial material and rock 

fragments that have moved down slopes. Deposits mostly formed as rock falls and 

topples, translational and rotational rock and debris slides, and rock and debris flows. 

These differing types of mass movement deposits are broadly lumped under the term 

"landslide" after Varnes (1978). Many of the landslide deposits mapped are complex in 

that they include two or more of the general types mentioned. Scarps and remnants of 

scarps within many of these deposits suggest some parts of the deposits moved recurrently. Scarps and pressure (buckle) ridges shown on the map were mostly identified 

on aerial photographs. In the map area, landslide deposits were subdivided broadly 

based on age of movement, material, and dominant type of slope movement

Recent landslide deposit (latest Holocene)—Heterogeneous mixture of unconsolidated 

surficial material and rock fragments showing a fresh (unvegetated) scarp headwall. 

Only one deposit mapped, which is prominent along Highway 6 on south side of Eagle River in west-central part of map area. Present along edge of older and larger 

landslide (Qlsd) where cut by Highway 6. Other recent small slumps and slides are 

present, but they are too small to map near the base of the north flank of Bellyache 

Ridge where old landslide deposits have been destabilized by road cuts of Highway 6 

and Interstate 70. Maximum thickness about 10 m

Landslide debris of alluvial gravels (Holocene to middle late Pleistocene?)—Unconsolidated debris consisting primarily of rounded and well-rounded gravel clasts of Quaternary-Tertiary gravels (QTg) mixed with locally derived bedrock and surficial material. 

Present as two deposits in north-central part of map area. Base of these gravelly landslides may extend beneath gravel into underlying shale of the Niobrara Formation (Kn). 

Maximum thickness about 5–10 m

Landslide debris of volcanic rocks (Holocene to middle Pleistocene?)—Unconsolidated debris consisting primarily of angular to subangular, pebble- to large boulder-size 

clasts of basalt, minor gravel, and unconsolidated surficial material. Only two deposits 

mapped in basalt (Tb) field in southwestern part of map area. Other small basalt slides 

and rubble are present along the south flank of Bellyache Ridge and suggest some potential for future slides and rockfalls along this flank. Maximum thickness about 10 m

Landslide of Niobrara Formation (Holocene to middle Pleistocene?)—Niobrara Formation (Kn) shale and limestone beds in extensive slide masses that formed on northerly 

facing, bedding-plane dip slopes. Niobrara rocks appear relatively intact, but they are 

folded or warped above one or more northerly dipping zones of landslide failure planes 

in northerly dipping shale of the Niobrara. Principal bedding-plane zone of failure commonly occurred in shale just above basal limestone of the Niobrara Formation. Probably formed principally by slow, deep creep that probably was most active during wet periods of the late and middle Pleistocene. Slide masses may be susceptible to creep; 

however, active creep unknown. Maximum thickness estimated to be about 20–25 m

Landslide of Benton Shale (Holocene to middle Pleistocene?)—Relatively intact, but 

deformed Benton Shale (Kb), in extensive slide masses that formed on northerly-facing, bedding-plane dip slopes. Similar in all fundamental characteristics, origin, and 

age to landslides of Niobrara Formation (Qlsn). Principle bedding-plane zone of failure commonly occurred in shale just above basal, sandy transition zone with underlying Dakota Formation (Kd). Slide masses may be susceptible to creep; however, active 

creep unknown. Soil and other surficial materials derived from Benton slide masses 

may contain expansive clay. Maximum thickness estimated to be about 20–25 m

Landslide debris of Dakota Sandstone (Holocene to middle Pleistocene?)—Unconsolidated debris primarily derived from the Dakota Sandstone (Kd) mixed with other 

surficial material. Includes both clast- and matrix-supported material with sandstone 

clasts as much as 5 m in length. Large slide masses present mostly along north flank 

of Bellyache Ridge and on slope east of Ute Creek Many deposits show some scarps 

near their bases and internally that suggest reactivation of parts of the slide masses after major slide activity ceased. Probably formed mainly during moist periods of the 

late and middle Pleistocene. Deposits prone to slumping where destabilized by road 

cuts. The westernmost slide mass of Bellyache Ridge shows several small, unmapped 

slumps where cut by Interstate 70, and the recent landslide (Qlsr) formed where the 

western edge of this slide mass is cut by Highway 6. Thickness variable; westernmost 

landslide of Bellyache Ridge may exceed 80 m

Debris-flow deposits, undifferentiated (late and middle(?) Pleistocene)—Consists mostly of debris-flow deposits and minor amounts of stream alluvium; may include earthflow deposits. Includes both matrix- and clast-supported, subangular to rounded, pebble- to boulder-size clasts of locally derived bedrock in a poorly sorted silty and sandy 

matrix. Composition, form, and character of deposit variable and dependent primarily 

on source material and age of deposit. The two deposits in the Squaw Creek drainage, southeastern part of map area, are the most eroded and dissected, contain the 

most alluvium, and are the only debris-flow deposits that contain granitic and gneissic 

clasts; these deposits probably are late to middle Pleistocene in age. The debris-flow 

deposit directly south of the Eagle River consists primarily of Dakota Sandstone (Kd) 

clasts, shows little dissection, and on aerial photographs shows some preservation 

where the flow formed natural levees; it appears to be the youngest such deposit in 

the quadrangle and probably is latest Pleistocene in age. The form and dissection of 

the debris-flow deposit north of the Eagle River suggests it is intermediate in age with 

respect to the deposit directly south of the Eagle River and those in Squaw Creek 

drainage. Thickness variable; locally may exceed 15 m

Diamicton (middle Pleistocene to Pliocene?)—Isolated and highly eroded, colluvial deposits that locally cap ridge crests in the northeast part of the map area. Consists of 

angular to subangular pebble- to large boulder-size clasts mostly of Dakota Sandstone 

(Kd) and some Morrison Formation (Jm) clasts in matrix- to clast-supported, poorlysorted, sandy and silty matrix; some clasts as much as 5 m in length. Present only in 

northeastern part of map area where occurs on erosionally beveled ridge crests. Compositionally similar to younger landslides of Dakota Sandstone (Qlsd), but lacks humocky surface morphology. Origin is uncertain. Adjacent Qlsd deposits in northeastern corner of map area are, in part, derived from diamicton deposits. Holocene and 

Pleistocene erosion and downcutting of 250 m or more by local streams isolated erosional remnants of diamicton deposits from Dakota Sandstone (Kd) source areas to 

east. Deposits locally appear to overlap and cover remnants of high-level QuaternaryTertiary, gravelly alluvium (QTg). Diamicton may represent old debris-flow or other 

landslide deposits derived from highlands adjacent to alluvial lowlands. Maximum preserved thickness estimated to be about 10 m

BEDROCK UNITS

Basaltic flows (early Miocene)—Dark- to light-gray and orangish-red, vesicular to non-vesicular basaltic flows. Contains phenocrysts of olivine and pyroxene as much as 5 mm 

in length in a devitrified and altered, microgranular to pilotaxitic groundmass of plagioclase, pyroxene, olivine, and iron oxides. Olivine commonly altered to iddingsite. Locally, vesicles are filled by calcite amygdules. X-ray fluorescence analyses of two samples gave chemical compositions of basaltic trachyandesite (J.R. Budahn, U.S. Geological Survey; written commun., 1997). Forms large field of basaltic flows occupying 

much of southwest part of map area and small, isolated remnant along eastern part of 

southern map border. Consists of two or more flow units that were not mapped separately. Weathers to rounded knobs and rubble. Best exposed on crest of Bellyache 

Ridge. Erratic flow-foliation orientations combined with an elevation difference of 

nearly 600 m between the topographically highest flow exposures along crest of Bellyache ridge and the lowest flow exposures farther south, suggest collapse caused by 

withdrawal or dissolution of the underlying Eagle Valley Evaporite as suggested by Hubert (1954). Cinder-bearing basaltic rocks are present in Warren Gulch, but no obvious cinder cone or other vent sources were identified. 40Ar/39Ar age spectrum analyses were obtained for two samples; sample L6681b, collected in the map area directly 

north of Warren Gulch (NW1⁄4, sec. 31, T. 4 S., R. 83 W.), and sample L6844, located about 1.5 km south of the map area in Trail Gulch (SE1⁄4, SW1⁄4, sec 18, T. 5 S., 

R. 83 W.). Sample L6844 yielded a plateau age of 22.3 ± 0.1 Ma with 67% of the 

39Ar released on the plateau. Inverse isotope correlation of the age spectrum data 

yielded an age of 22.2 ± 0.2 Ma, with an initial 40Ar/36Ar ratio of 341 ± 15, and 

mean standard weighted deviation (MSWD) = 0.967. Although the inverse isotope 

correlation analysis suggests the presence of a small amount of extraneous argon in 

the sample, the agreement of the correlation age with the plateau age, within the limits of analytical precision, suggests that this basaltic flow was emplaced about 22.3 ± 

0.1 m.y. ago. Sample L6681b yielded a disturbed age spectrum that did not have a 

plateau age. Inverse isotope correlation of the age spectrum data yielded an age of 

22.8 ± 0.3 Ma, with an initial 40Ar/36Ar ratio of 329 ± 28, and MSWD = 1.303. 

The age of this sample is not as certain as that of sample L6844, because it did not 

have an age plateau. Nonetheless, its correlation age is similar to the apparent ages 

of L6844 and suggests that it was emplaced 22–23 m.y. ago (M.J. Kunk, U.S. Geological Survey; written commun., 1997)

Pierre Shale (Upper Cretaceous)—Medium- to dark-gray, olive-gray, and brownish-gray 

slightly to non-calcareous shale and silty shale. Contains numerous calcareous concretions present as irregular masses and lenses. Locally contains thin, impure, and discontinuous fine-grained sandstone and siltstone layers.. Top of formation not exposed 

in map area, only about 50 m of poorly exposed shale is preserved above the upper 

sandstone marker bed in northeast corner of map area. Directly north of the map 

area, about 245 m of silty shale overlies the upper marker bed and is unconformably 

overlain by Tertiary basalt (Wanek, 1953). Mollusk fossils are locally present throughout formation but most abundant in middle to upper part of unit. Marine fossils, characteristic of the Pierre Shale in the map area and areas nearby, have been described 

by Cobban and Reeside (1952), Stauffer (1953), and Wanek (1953). Basal contact of 

Pierre Shale with underlying Niobrara Formation is poorly exposed and gradational. 

Contact approximately located where medium- to dark-gray shale assigned to upper 

Niobrara Formation is overlain by dark-gray, silty, slightly calcareous to non-calcareous, concretion-bearing shale assigned to Lower Pierre Shale. About 825 m thick in 

northeast part of map area but top not exposed. Directly north of map area, Wanek 

(1953) measured a total thickness of about 1,070 m beneath the Tertiary unconformity present there

Sandstone marker beds in Pierre Shale—Two prominent ledge-forming sandstone 

marker beds (Kps) are present about 700 m above base of Pierre Shale (Kp). Marker 

beds are wavy-bedded, shaly, silty, micaceous, fine-grained to very fine grained, saltand-pepper sandstone that contains rusty-weathering nodules, fucoid casts and impressions, and some thin, discontinuous, silty shale interbeds. Upper sandstone marker 

bed is about 45 m thick and lower sandstone marker is about 10 m thick. Marker 

beds separated by about 70 m of dark-gray, silty, wavy-bedded shale that contains 

thin, lenticular sandstone interbeds

Niobrara Formation (Upper Cretaceous)—Light- to dark-gray and brownish-gray, calcareous shale and very light gray, dense, conchoidal-fracturing, sparsely fossiliferous 

limestone. Upper 70 m is mostly medium-gray and brownish-gray, silty, calcareous 

shale. Middle 185 m is dark-gray shale that commonly weathers to paper shale with 

white to light-gray surface coatings. Lower 75 m is interbedded light-gray, fissile, calcareous shale, shaly limestone, and dense, blocky- to massive-weathering limestone. 

Limestone forms ledges, and base of unit marked by prominent ledge-forming, lightgray limestone with some interbedded, light-gray, limey shale. Contains marine mollusks and, in upper part, cycloid fish plates and bone fragments (Stauffer, 1953; Wanek, 1953). Conformably overlies black shale assigned to the top of the Benton Formation. Unit may be susceptible to translational sliding and creep on dip slopes. Total thickness about 335–365 m

Benton Shale (Upper Cretaceous)—Dominantly black and dark-gray, slightly calcareous, 

platy shale with thin bentonite stringers and thin, buff, carbonaceous, fine-grained 

sandstone lenses. Upper part consists of 6–10 m of black shale that underlies Niobrara Formation. Beneath upper shale is about 3 m of thinly and unevenly laminated, 

sandy, gray, brown-weathering, fetid, crystalline limestone that contains abundant marine mollusk shells and some small fish teeth (Stauffer, 1953; Wanek, 1953; Hubert, 

1954). Remainder of unit is mostly black shale. Lower 10–15 m contains numerous 

1–15-cm-thick beds and lenses of buff sandstone and siliceous, dull-greenish-gray to 

black siliceous shale; fucoid casts locally present in lower shale. Basal contact appears 

gradational and conformable with underlying Dakota sandstone. Weathers to rounded 

slopes and valleys and generally poorly exposed. Best exposed, particularly upper 

part, in cliffs directly north and east of Wolcott along Eagle River. Unit may be susceptible to translational sliding and creep on dip slopes. Total thickness 115–125 m

Dakota Sandstone (Upper and Lower? Cretaceous)—Cliff-forming, crossbedded, tan, 

white, and light-greenish-gray, medium- to coarse-grained, silica-cemented, well-sorted 

sandstone and orthoquartzite interbedded with black, yellowish-gray, and sparse, palepurplish-red shale. Contains scattered lenses of chert and quartz-pebble conglomerate; commonly a thin conglomerate bed is present at or near base of unit. Where well 

exposed, divisible into three parts (not distinguished on the map) that vary in thickness. Upper part consists of tan, well-sorted, well-cemented, medium- to coarsegrained, blocky- to massive-bedded sandstone and orthoquartzite. Middle part is interbedded, lenticular, dark-gray to black and yellowish-gray shale and lenticular, tan, palegreenish- to yellowish-gray, and white sandstone. Lower part is crossbedded, thin- to 

medium-bedded, light-gray to tan sandstone interbedded with thin lenticular, gray shale 

and sparse lenses of pebble conglomerate; conglomerate consists of black chert and 

quartz clasts in iron-stained matrix of medium-grained to very coarse grained, silicacemented sand. In the vicinity of Wolcott, the stratigraphic position of the boundary 

between Upper and Lower Cretaceous rocks is not known. Existing data suggests that 

the boundary probably is present within the Dakota Sandstone (Wanek, 1953; W.A. 

Cobban, U.S. Geological Survey, oral commun., 1998). Dakota Sandstone is best exposed in cliffs along north side of Eagle River north and west of Wolcott. Basal contact with underlying Morrison Formation (Jm) is sharp but rarely exposed, and is associated with pebble conglomerate; no angular unconformity is apparent. Forms northerly dipping slope along northern flank of central and eastern parts of Bellyache 

Ridge. Highly resistant to weathering and forms abundant blocky landslide deposits 

and other colluvial deposits in many localities in map area. On the map, Colluvial deposits composed primarily of Dakota Sandstone clasts are labeled Qcd and landslide 

deposits labeled Qlsd; diamictite deposits (QTdi) are also composed mostly of Dakota 

Sandstone clasts. Commonly jointed, and Dakota Sandstone cliffs are very susceptible 

to rock falls. Total thickness about 60–85 m

Morrison Formation (Upper Jurassic)—Interbedded, calcareous sandstone, siltsone, mudstone, shale, and limestone. Upper 90–120 m is dominantly interbedded and lenticular red, green, and purplish-gray siltstone, mudstone, and shale interbedded with 

greenish-gray calcareous sandstone and gray limestone. Lower 30 m consists of four 

or more 1–3-m-thick, white, lenticular, calcareous, fine-grained, crossbedded sandstone beds interbedded with pale-greenish-gray, sandy siltstone and shale; oolitic, lightgray limestone beds are locally present in lower part. White cross-bedded sandstone 

beds in lower part are similar to white sandstone at top of Entrada Sandstone (Je). 

Basal contact with underlying Entrada Sandstone (Je) mapped beneath lowermost 

greenish-gray siltstone or shale of sandy lower part. Poorly exposed throughout most 

of map area; commonly weathers to rounded slopes covered by colluvium and rock-fall 

deposits derived from Dakota Sandstone. Best exposed along ridge west of Red Canyon near eastern border of map area. Total thickness 120–155 m

Entrada Sandstone (Upper Jurassic)—Primarily salmon-pink, fine- to medium-grained, 

well-sorted, well-rounded, crossbedded, calcareous, cliff-forming sandstone. Upper 

1.5–3 m consists of white, crossbedded, calcareous sandstone similar to sandstone in 

lower part of Morrison Formation. Large-scale, low- to moderate-angle, tangential 

crossbedding is common and, combined with frosted sand grains, suggests eolian deposition in dune fields. Weathers to form distinctive, salmon-colored, blocky to rounded, cliff faces in which crossbedding is often obscured. Entrada sandstone is best exposed in cliffs directly north of Eagle River in western part of map area. Contact with 

underlying red shale and siltstone of Chinle Formation (dc) is sharp and unconformable; locally shows slight angular discordance. Total thickness 17–25 m

Chinle Formation (Upper Triassic)—Brownish-, purplish-, and orangish-red, calcareous 

shale, siltstone, mudstone, and very fine grained sandstone. Contains distinctive limestone-pebble conglomerate in beds ranging from 15 cm to 2 m thick and some thin, 

mottled red and gray limestone beds. Limestone-pebble conglomerate consists of limestone and red sandstone pebbles in light-gray or reddish-brown, sandy limestone matrix. Paleosols identifiable in good exposures have been described by Dubiel (1992). 

Gartra Sandstone member (not mapped separately) forms base of unit and consists of 

3–8 m of purplish-red and reddish-brown, fine-grained to very coarse grained and pebbly, silica-cemented sandstone that locally is conglomeratic; commonly weathers to 

blocky or massive sandstone ledges. In and adjacent to the map area, Stauffer (1953), 

Wanek (1953), and Hubert (1954) all mapped the Shinarump Formation at the base of 

the Chinle Formation, apparently based on previous nomenclature (Lovering and 

Johnson, 1933; Thomas and others, 1945) and on the presence of petrified wood 

fragments. Their descriptions, thickness estimates, and stratigraphic placements of the 

Shinarump at the base of the Chinle suggest that the Gartra Sandstone, identified at 

base of Chinle in this report and by Dubiel (1992), comprises most of their Shinarump 

Formation. Petrified wood fragments are present in float at a few localities, but are 

sparse in outcrop. Basal contact with underlying State Bridge Formation (dPsb) is 

sharp and easily located where the distinctive Gartra Sandstone is well exposed; the 

Gartra rests unconformably on reddish-brown and orangish-red, very fine grained sandstone and siltstone of the underlying State Bridge Formation (dPsb). Where the Gartra Sandstone is poorly exposed or absent, contact appears gradational and is difficult 

to locate precisely. In map area, unit thickness ranges from about 80 to 110 m; unit 

thins to east and northeast. Hubert (1954) reported a total thickness of as much as 

253 m directly south of map area and noted rapid thinning of Chinle to northeast

State Bridge Formation (Lower Triassic and Permian)—Orangish-red to reddish-brown, 

even-bedded, calcareous siltstone and very fine grained sandstone. Near top of formation is a 3–6-m-thick, fine-grained, crossbedded, orangish-red sandstone that commonly forms ledges. Lower part locally contains a 0.5–1-m-thick bed of medium-gray, 

finely crystalline limestone or light-gray dolomite that may be correlative with the 

South Canyon Creek Dolomite Member (Hubert, 1954), which is mapped farther west 

(Murray, 1958; Bass and Northrop, 1963; Freeman, 1971). North of map area, 

Sheridan (1950) subdivided the State Bridge Formation into three members, a subdivision that defines the thin South Canyon Creek Dolomite Member as the middle member between much thicker upper and lower members. In western part of the map 

area Hubert (1954) measured a 50-m-thick lower unit, overlain by a 0.6-m-thick middle unit of gray limestone (South Canyon Creek Member?), which is, in turn, overlain 

by an 88-m-thick upper unit. The State Bridge Formation was not subdivided in map 

area because the thin, middle limestone-dolomite does not appear to be present consistently and the upper and lower parts are not sufficiently distinct from one another. 

The basal contact of the State Bridge Formation is sharp and easily located in most of 

map area where the underlying Schoolhouse Member of the Maroon Formation 

(Phms) is present. Where the Schoolhouse Member is present, the contact was mapped at the base of reddish-brown to orangish-red State Bridge sandstone and siltstone 

and directly above distinctive, light-colored, fine to coarse-grained sandstone of the 

Schoolhouse Member. Where Schoolhouse Member is absent, in easternmost part of 

the map area, contact was mapped at base of 0.6–1-m-thick, light-gray dolomite that 

pinches out abruptly northward (South Canyon Creek Dolomite?). Farther north, contact was placed where finer grained, orangish-red beds of State Bridge Formation 

overlie interbedded, reddish-brown and light-grayish-green, coarser grained sandstone 

assigned to Maroon Formation (Phm). The State Bridge Formation thins abruptly in 

eastern part of the map area, north of the Eagle River. If the thin, light-gray dolomite 

present there is equivalent to South Canyon Creek Dolomite Member, then the lower 

part of the State Bridge Formation is absent as well as the Schoolhouse Member of 

the Maroon Formation (Phms) and perhaps part of the main body of the Maroon Formation (Phm)—suggesting the presence of an unconformity in the middle to lower 

part of the State Bridge Formation or unidentified faults. Total thickness ranges from 

about 145 m in western part of map area to about 100 m in eastern part

Maroon Formation (Lower Permian and Pennsylvanian)—Predominantly red beds of 

sandstone and sandy siltstone with minor shale and sparse lenticular beds of pebble 

conglomerate. Light-colored sandstone of Schoolhouse Member (Phms) at top of formation mapped separately where present. Usage of the nomenclature “Schoolhouse 

Member of the Maroon Formation,” after Johnson and others (1990)

Schoolhouse Member of Maroon Formation—White- to buff-weathering, grayishwhite to pale-grayish-green, feldspathic and micaceous, medium- to coarse-grained, 

and locally pebbly sandstone with some interbeds of grayish-red and pale-reddishbrown sandstone and siltstone. Well exposed in cliffs north and south of Eagle River 

in western part of map area. Thins to east and northeast. Can be traced discontinuously from just east of basalt field (Tb) eastward around nose of Bellyache Ridge to 

where Schoolhouse Member pinches out abruptly northward, about 0.8 km south of 

Eagle River. Member is entirely absent north of Eagle River in eastern part of map 

area. Where exposed along east flank of Bellyache Ridge, consists of white, mediumgrained, noncalcareous sandstone that grades downward through limey, white sandstone into white, crystalline limestone. Directly north of Eagle River a thin bed of 

light-gray dolomite, assigned to base of the State Bridge Formation (dPsb), occupies 

about same stratigraphic position as the absent Schoolhouse Member. Maximum 

thickness about 14 m

Main body of Maroon Formation—Mostly interbedded, brick-red, reddish-brown, and 

some orangish-red, micaceous, fine to medium-grained, arkosic sandstone, siltstone, 

and shale. Shale is sparse and present mainly as thin beds and partings at silty tops of 

blocky sandstone beds. Locally contains coarse-grained to pebbly sandstone lenses, 

thin, mottled gray and red, argillaceous limestone interbeds, and light-greenish-gray to 

white sandstone interbeds. Green reduction spots and mottles locally common in red 

beds. Cliff-forming and blocky weathering. Sedimentary structures include ripple 

cross-laminations, low-angle parallel and tangential crossbedding, and, locally, syneresis cracks. Contact with underlying Eagle Valley Formation (he) is gradational and 

placed at prominent color change between bright-red rocks of the Maroon Formation 

and pale-reddish-brown rocks of upper Eagle Valley Formation. Color change also coincides closely with uppermost medium- to dark-gray, micritic limestone in underlying 

Eagle Valley Formation. Hubert (1954) used same criteria to define base of Maroon, 

but mapped underlying rocks as Minturn Formation. Hubert measured 323 m of Maroon Formation below Schoolhouse Member in the western part of map area, in cliffs 

directly south of the Eagle River. Main body of the Maroon formation thins rapidly to 

east. Total thickness ranges from about 335 m in western part of map area to 160 m 

in eastern part where Schoolhouse Member is absent

Eagle Valley Formation (Middle Pennsylvanian)—Reddish-brown, reddish-gray, gray, 

light-green, and tan shale, siltstone, and fine-grained to very fine grained sandstone interbedded with distinctive, dark- to light-gray micritic to finely crystalline limestone. 

Limestone beds range from about 5 cm to 2 m thick and are present throughout formation, but particularly common in middle and lower part of unit; seventeen individual 

limestone beds were identified in a partial section measured in the south-central part 

of the map area, on south flank of Bellyache Ridge near axis of Wolcott syncline 

(C.D. Blome, U.S. Geological Survey, oral commun., 1997). Locally contains evaporite interbeds; most are near base and are part of a 50–90-m-thick basal transition into 

thick evaporite of the underlying Eagle Valley Evaporite (hee). Tweto and others 

(1978) first mapped the Eagle Valley Formation in this region and defined it as finegrained clastic rocks transitional between coarse-grained clastic rocks of the Minturn 

and Maroon Formations and evaporitic rocks of the Eagle Valley Evaporite. Precise 

correlation among these Pennsylvanian and Permian Formations remains unresolved. 

Hubert (1954) mapped rocks assigned to the Eagle Valley Formation and Eagle Valley 

Evaporite in the Wolcott quadrangle as Minturn Formation and speculated that the upper limestone of his Minturn correlated with the Jacque Mountain Limestone, which is 

defined as the top of the Minturn Formation farther east (Tweto and Lovering, 1977). 

Schenk (1992), however, presented evidence that the Jacque Mountain Limestone is 

present near the bottom of the sequence mapped as Eagle Valley Formation in this report. Rocks assigned to the Eagle Valley Formation in this report and in Tweto and 

others (1978) differ distinctly from the Maroon and Minturn Formations and from the 

Eagle Valley Evaporite. In the map area, the Eagle Valley Formation everywhere underlies the Maroon Formation (Phm) and overlies the Eagle Valley Evaporite (hee). 

In the south-central part of the area, along the south flank of Bellyache Ridge, a typical section of Eagle Valley Formation was found to be present between the Maroon 

Formation (Phm) and Eagle Valley Evaporite (hee), where reconnaissance mapping 

by Tweto and others (1978) had shown the Eagle Valley Formation as absent. Unit is 

well exposed along south flank of Bellyache Ridge and in cliffs south of Eagle River directly west of map area. Basal contact with underlying Eagle Valley Evaporite (hee) is 

conformable, gradational over 50–90 m, and commonly deformed by salt diapirism of 

evaporite interbeds and evaporite in the underlying Eagle Valley Evaporite (hee). 

Contact difficult to locate precisely or consistently; placed approximately at base of 

dominantly clastic sequence and above thick, massive evaporite. Evaporite interbeds 

may be susceptible to ground subsidence from solution voids and to diapiric swelling. 

Soils and other surficial materials derived from evaporite interbeds may also be susceptible to subsidence from hydrocompaction and piping of evaporitic material. Evaporite-bearing solutions derived from bedrock, soils, or surficial deposits may be corrosive to materials such as conventional concrete and metal pipes. Total thickness 

about 700 m

Eagle Valley Evaporite (Middle Pennsylvanian)—Light- to dark-gray and white evaporitic 

sequence consisting mostly of gypsum with interbeds of tan-weathering, light- to darkgray shale and argillaceous limestone, tan, very fine grained sandstone, and sparse, red 

silty sandstone. Exposures commonly poor and consist mostly of dirty-gray gypsum 

commonly weathered to rounded slopes and covered by thin residuum of gypsum-rich 

silt that shows a popcorn-like texture from wetting and drying. Fresh exposures are 

light to dark gray. Evaporitic rocks and interbedded limestone are commonly brecciated and breccia may be collapse-related, resulting from dissolution of evaporitic rocks. 

Unit is also commonly contorted and faulted by salt diapirism, flowage, and volume 

changes related to hydration and dehydration reactions of gypsum and anhydrite, respectively. Much of the deformation was likely episodic and related to burial depths, 

uplift and erosion, and water saturation. Some deformation in the unit probably is related to Laramide and younger tectonic events, but distinction of deformation related 

to regional tectonics from local, salt-related deformation, or from tectonic-induced salt 

deformation is difficult and speculative in map area. Locally contains dissolution voids 

as much as a few meters in diameter and several meters deep. Unit is susceptible to 

ground subsidence from solution voids and may be susceptible to diapiric swelling. 

Soils and surficial deposits overlying or derived from unit may also be susceptible to 

settlement from hydrocompaction and piping of evaporitic materials. Evaporite-bearing solutions derived from bedrock, soils, or surficial units may be corrosive to materials such as conventional concrete and metal pipes. Base of unit not exposed in map 

area, and poor exposures and contortion of unit precludes accurate thickness estimates, but exposures directly east of map area suggest unit is at least 500 m thick

Qalc

Qtl

QTg

af

lf

Tg

Qfy

Qac

Qfo

Qc

Qcd

QTdi

Qdu

Qlsb

Qlsn

Qlsv

Qlsr

Qcg

Qlsd

Qlsg

Contact—Dashed where approximately located; dotted where concealed

High-angle fault—Dashed where approximately located; dotted where concealed; ball and 

bar on apparent downthrown side. Labeled T, where concealed and inferred to be a 

tear fault, based on apparent, inconsistent lateral offsets

Reverse fault—Dashed where approximately located; dotted where concealed; hachures 

on upthrown side of fault

Block-glide fault—Approximately located; dotted where concealed; double hachures on 

block-glide slab above fault

Folds—Showing approximate trace of axial surface and direction of plunge; dotted where 

concealed

Anticline

Syncline

Minor folds—Showing trend of axial surface and, locally, direction and amount of plunge

Anticline

Syncline

Strike and dip of beds

Inclined

Vertical

Overturned

Strike and dip of inclined flow foliation in volcanic rocks

Red marker bed in Eagle Valley Formation (he)—Shown locally in southeastern part of 

map area to help depict structure

Gravel pit—Showing approximate outline of mined area

Mass movement structures and features

Landslide scarp—Hachures on downslope side

Pressure (buckle) ridge in landslide

Strike and dip of contorted beds in landslide

Minor fold in shale beds in landslide

Fault—In cross section; arrows show directions of relative movement in plane of section; 

T, movement towards observer; A, movement away from observer

Bedding formline—Shown in subsurface of cross sections

Projected contact and fault formlines—Shown above topographic surface in cross 

sections 

DISCUSSION OF STRUCTURAL FEATURES

Structural features in the map area include faults and folds of differing ages and origin (fig. 1). 

Most faults and folds appear to be related to deep-seated, regional tectonism. Some folds and a 

large sag in the southwestern part of the map area, however, probably are local features related to 

gravity block-glide landsliding, and to collapse or upheaval of rock units due to dissolution and diapiric movement of evaporitic rocks of the Eagle Valley Evaporite (hee). The north and central 

parts of the quadrangle contain gently north-dipping Pennsylvanian to Late Cretaceous strata that 

are deformed by three north- to northeast-trending folds. From west to east they are the Horn 

Ranch Anticline and the Wolcott and Ute Creek Synclines. Rock units of the Middle Pennsylvanian Eagle Valley Formation and the Eagle Valley Evaporite are exposed in the southern part of the 

quadrangle; these strata are complexly deformed by tight, closely-spaced folds that are cut by normal and reverse faults. In the southwestern part of the quadrangle, early Miocene basaltic flows unconformably overlie previously folded rocks of the lower part of the Eagle Valley Formation and 

the Eagle Valley Evaporite. These basaltic flows apparently have collapsed as much as 430 m due 

to dissolution and removal of underlying evaporitic rocks. In the central part of the quadrangle, a 

crescent-shaped anticline and syncline pair and smaller cross folds formed in a large, block-glide 

landslide that consists mainly of the Dakota Sandstone. These structural features are labeled on 

figure 1 and described, from oldest to youngest, in greater detail below.

SOUTHERN FOLD AND FAULT ZONE

Complexly deformed strata of the Eagle Valley Formation and Eagle Valley Evaporite in the 

southern part of the map area comprise a structural domain referred to here as the “southern zone 

of folds and faults” (fig. 1). Folds in this zone are tight and have variable to sinuous trends, and in 

places are cut by steep to moderately dipping normal and reverse faults. Fold axes trend eastsoutheast to east-west in the western and central part of the zone, but they are bent and refolded 

into a northerly configuration in the eastern part of the zone where they wrap around the east limb 

of the Wolcott syncline. Folds in the eastern part of the zone are cut by north- to northeast-striking reverse faults. Early Miocene basaltic rocks are unaffected by the folds and many of the faults 

that deform the underlying Pennsylvanian rocks. This indicates that the folding and faulting in the 

southern zone is Pennsylvanian or younger but older than the early Miocene basaltic rocks that 

truncate these structures.

Folds and many of the faults in the southern zone are the oldest structures identified in the map 

area, because they are deformed by younger structures like the Wolcott syncline. Folds in the 

southern zone may have initially developed during (1) Pennsylvanian to early Permian deformation 

related to formation of the ancestral Rocky Mountains, (2) Laramide deformation (Late Cretaceous 

to Eocene) that initiated the formation of the present Rocky Mountains, or (3) flowage and diapiric 

activity of evaporitic rocks in the Eagle Valley Evaporite at sometime during Pennsylvanian to pre 

late Laramide time. The folds in the southern zone are principally confined to the lower part of 

the Eagle Valley Formation and the underlying Eagle Valley Evaporite, and they appear to have 

had consistent west-northwest trends prior to refolding by the northerly trending folds. The pattern of consistent initial trends of these folds probably argues against an origin from diapiric movement of the evaporite, which might be expected to produce folds with more chaotic trends and less 

continuity. Both Pennsylvanian to early Permian and Laramide deformation are known to have 

had components of north-northeast/south-southwest-oriented compression in the region (Hongzhuan and others, 1996; Warner, 1956; Burbank and Lovering, 1933), either of which could account for the origin of these features. Near the contact of the Eagle Valley Formation and the Eagle Valley Evaporite, smaller scale folds were locally observed that are cut or detached along lowangle faults. These relations may suggest that the relatively incompetent rocks of the Eagle Valley 

Formation and Eagle Valley Evaporite served in part as de’collement horizons during compressional deformation related to Pennsylvanian to Permian and (or) Laramide tectonic episodes.

NORTHERLY TRENDING FOLDS

Mesozoic and Paleozoic strata in the north and central parts of the quadrangle are deformed by 

three north- to northeast-trending folds; the Ute Creek syncline, Wolcott syncline, and Horn Ranch 

anticline. In the eastern part of the map area, the Ute Creek syncline has a very gentle dipping 

west limb, but a very steep to overturned east limb that contains at least two northeast-trending reverse faults. The northern part of the Ute Creek syncline trends northeast, but its southern part 

bends to a north-northwest trend. The Wolcott syncline is a broad, northeast-trending fold in the 

central part of the quadrangle. The Horn Ranch anticline is a broad, northeast-trending anticline in 

the western part of the quadrangle that shares its western limb with the Wolcott syncline. The Ute 

Creek syncline affects rocks at least as young as the Late Cretaceous Pierre Shale, and the Wolcott 

syncline and Horn Ranch anticline deform rocks as young as the early to late(?) Cretaceous Dakota 

Sandstone. Therefore, these folds are all Cretaceous or younger in age and probably mostly related 

to Laramide (Late Cretaceous to Eocene) episodes of deformation; Locally, however, these folds 

may be related to, or modified by, Cretaceous or younger salt tectonism.

The northerly trending folds and reverse faults and an inferred northwest-trending tear fault 

(fig.1) all appear to be related in age and collectively suggest west-northwest/east-southeast-oriented compression during their formation. I have inferred the presence of a buried, northwest-trending tear fault directly south and southeast of the Ute Creek syncline in the Eagle River Valley. Tear 

faults commonly form nearly perpendicular to related folds and reverse faults and they serve as 

partitions between domains of rock in which a common magnitude of shortening has been achieved in different ways (Davis, 1984). In my interpretation, the inferred tear fault provided a partition (zone of lateral slip) that allowed different sets of northerly trending folds and reverse faults, 

north and south of the tear, to accommodate shortening related to west-northwest/east-southeastoriented compression. In the Red Canyon area, about 22 km to the north, Beckett (1955) found 

similar relations among northeast-trending folds that deform northwest-trending folds, which are all 

cut and truncated by a northwest-trending tear, or strike-slip fault. Folds and faults in the Red Canyon area are Laramide in age (Beckett, 1955). Similar to the Red Canyon area, north- to northeast-trending folds and reverse faults and the tear fault in the map area may be related to the latest 

episodes of Laramide deformation, and the northwesterly trending folds in the southern zone 

might similarly be related to the earliest phase of Laramide deformation.

The Horn Ranch anticline may have also formed during the west-northwest/east-southeast, 

Laramide compression discussed above, however, the Horn Ranch anticline appears to connect 

with an unnamed, easterly trending anticline that deforms early Miocene basaltic rocks near Gypsum and Eagle directly west of the map area. This unnamed anticline appears to reflect diapiric 

upwelling of the Eagle Valley Evaporite (hee) along the Eagle River Valley (M.R. Hudson, U.S. 

Geological Survey, unpubl. mapping, 1997; Scott and others, 1998). The Horn Ranch Anticline 

also exists in the Eagle River Valley and may also reflect salt tectonism from diapiric rise of the Eagle Valley Evaporite (hee) at depth, which would account for this anticline as a product of salt tectonism and not Laramide compressional deformation. Laramide compressional deformation and 

Neogene salt tectonism appear to be equally valid interpretations for the origin of this broad anticline

COLLAPSED BASALTIC FLOWS

In the southwest part of the quadrangle, early Miocene basaltic flows overlie highly folded Middle Pennsylvanian rocks (fig. 1). Mapped relations suggest the basaltic rocks collapsed into a sag 

feature that formed in the underlying Middle Pennsylvanian strata. Collapse of the basaltic flows 

appears to have resulted from dissolution and removal of evaporitic rocks from the underlying Eagle Valley Evaporite (hee) (Hubert, 1954). On Bellyache Ridge, these basaltic rocks are at an altitude of about 2,750 m and dip about 20–25 degrees southwest toward the central part of the collapse or sag feature. In this area, Middle Pennsylvanian to Jurassic strata that underlie the basalt 

dip in the opposite direction, to the northeast, at about 20–30 degrees. In the central part of the 

collapse feature, the base of the basaltic flows is at an altitude as low as about 2,320 m, suggesting 

that the basalt collapsed about 430 m relative to its exposure along the crest of Bellyache Ridge as 

was suggested by Hubert (1954). The orientations of flow foliation of flows in the central part of 

the collapse feature are erratic and the rocks are commonly rubbly and poorly exposed, but it appears that the folds in the Eagle Valley Formation (he) and Eagle Valley Evaporite (hee) were erosionally beveled and partly covered with Early Miocene or Oligocene gravel prior to emplacement 

and later collapse of the basaltic flows.

Structural relations between the northeast-dipping Pennsylvanian-Jurassic rocks and southwestdipping basaltic rocks along the northeastern margin of the collapse feature suggest that the Pennsylvanian-Jurassic rocks dipped more steeply (by about 20–25 degrees) to the northeast when the basaltic flows were originally deposited. Restoration of the southwest-dipping basaltic flows to their original 

sub-horizontal orientation would require that the underlying Pennsylvanian-Jurassic rocks also be restored to a pre-collapse orientation where existing northeast dips would increase from about 25 to 45 

degrees or greater. The most likely explanation for this collapse feature is Neogene removal of a 

large volume of evaporitic rocks of the underlying Eagle Valley Evaporite by groundwater dissolution 

and transport. Neogene removal of evaporite apparently was most active beneath the central part of 

the collapse feature, but evaporite removal was probably also active beneath the northeastern margin 

of the collapse, which would account for subsequent down-dropping and related rotation of both the 

sedimentary and basaltic rocks. Similar large areas of collapsed early Miocene basaltic flows have 

been recognized in the vicinity of Carbondale, Eagle, Gypsum, and State Bridge; existing information 

suggests that these collapse features resulted from evaporite removal after about 8 Ma over a period 

of a few to several million years, and suggests that evaporite removal is still active in some areas (Kirkham and others, 1997; Kunk and others, 1997; Scott and others, 1998). Collapse features and 

processes characterisic of the Carbondale region, are described and interpreted in Kirkham and Widmann (1997) and Kirkham (1997). Some of the same features and processes appear to characterize 

collapse in the map area and elsewhere in the Eagle-Gypsum region (Scott and others, 1998).

BLOCK-GLIDE FOLDS

In the central part of the quadrangle, a crescent-shaped anticline-syncline pair and associated 

smaller-scale, west-trending folds are in a large landslide block composed mostly of the Dakota Formation and parts of the underlying Morrison Formation and overlying Benton Shale. I interpret 

these folds to represent internal deformation related to block-glide landsliding. The block slid essentially intact down-slope to the northeast on a glide plane consisting of lithologically unstable shale, 

mudstone, and siltstone in the upper part of the underlying Morrison Formation. The slip surface at 

the base of the slide is primarily a bedding-plane fault that is poorly exposed or covered throughout 

the map area. The southeast margin of the block appears to have slipped along a steeper edge of 

the plane, which cut the Dakota Sandstone and separated Dakota Sandstone involved in the blockglide from Dakota Sandstone that was not involved. Folds are present in similar block-glide landslides composed of the Dakota in several localities along the Colorado Front Range (Braddock, 

1978). Braddock has shown that those folds are essentially fault-bend folds that reflect the ramp-flat 

geometry of the underlying slip surface. The crescent-shaped axes of the anticline-syncline pair in 

the map area suggest that these folds may reflect an underlying crescent-shaped ramp along the basal, block-glide fault, a segment of which is depicted in cross section B-B’. The west-trending cross 

folds, which are present between the crescent-shaped folds, may be mainly buckle folds that indicate 

most of the movement and buckling of the block was down-dip to the north-northeast and at an oblique to nearly perpendicular angle to western segments of the crescent-shaped, ramp-related folds.

GEOLOGIC HAZARDS

LANDSLIDES

As used in this report, “landslide” is a general term that includes a wide variety of mass-movement deposits and processes involving the slow to rapid downslope transport of surficial materials 

and bedrock blocks by gravity. This definition includes various types of flows, slumps, slides, and 

combinations thereof. In the map area, landslides and landslide deposits are very common along 

the north flank of Bellyache Ridge and in the northern part of the quadrangle; they commonly occur on north-facing slopes in Cretaceous-age shale and sandstone whose bedding layers also dip 

north at moderate inclinations. Landslide deposits cover about 10% of the map area. Landslide 

types that are present in the map area include; earth flows, earth slumps, debris flows, debris 

slumps, rock-block slides, translational slides, rock falls, and complex landslides (Varnes, 1978; 

Cruden and Varnes, 1996). Many of the extensive landslide deposits in the map area were first 

mapped by Colton and others (1975) in their overview of Colorado landslides. 

Landslide deposits were recognized by a combination of field observations and air photo analysis. Basically, landslide deposits were identified by (1) hummocky topography, including closed depressions, (2) the deflection of stream channels at the toes of deposits, (3) headwall scarps and depressions, (4) vegetation differences between landslide and adjacent stable areas, (5) anomalous 

strikes and dips of bedrock blocks, and (6) intact masses of material downslope of their sources. 

Low sun angles in the early morning or late afternoon highlighted the subdued topography of 

many old landslide deposits and facilitated their identification in the field. 

For the most part, landslides in the map area were subdivided based on source material, which 

appears to exert a fundamental control on the types of landslide deposits developed (table 1). The 

most common and largest landslide deposits are derived from the Cretaceous Formations. Specifically the Dakota Sandstone (Kd), Benton shale (Kb), and Niobrara Formation (Kn).

Shaly rocks of the Benton and Niobrara fail similarily and commonly form large masses of relatively intact, but folded or rumpled shaly beds that have detached and slipped downhill above moderately dipping bedding surfaces of the underlying stable bedrock. The humocky surfaces that develop on these deposits are most spectacular on aerial photographs, where the entire extent of the 

rumpled surfaces of individual landslide masses can be viewed. Particularly striking in this regard 

are the Benton Shale landslide masses (Qlsb) in the Milk Creek drainage in the northwest part of 

quadrangle. The type of slope failure and movement suggested by the surface character of these 

deposits is deep creep (Varnes, 1978). Deep creep slide masses characteristically move imperceptibly slow, but over long periods of time they may move tens to hundreds of meters, causing deformation of shaly bedding layers within the slide masses. In the map area, much of the movement is probably Late Pleistocene or older in age; however, imperceptible, slow creep, might locally still be active in some deposits. Human activities or natural events, such as road cuts, stream incision along the toes of landslides, and increased precipitation, may aid in reactivating creep in 

parts of these deposits. In the Milk Creek drainage these deposits locally appear to be as much as 

30 m thick, whereas in the eastern part of the north Flank of Bellyache Ridge the deposits probably do not exceed about 15 m.

The Dakota Sandstone commonly forms landslide deposits (Qlsd) composed of broken and 

chaotic debris mixed with surficial materials and locally these deposits contain sandstone blocks as 

much as a few meters in length. These deposits formed by slope movement processes ranging 

from debris and rock slides to debris flows as defined by Varnes (1978). Individually mapped slide 

masses commonly are complex in that they include components of slides, flows, and even rock 

falls and topples near cliffs and cliffy headwall scarps. Most of the large slide masses derived from 

the Dakota Sandstone appear to be relatively old and relatively stable, probably principally late 

Pleistocene and older, but unconsolidated material in these slides has the potential for becoming 

destabilized by human activity such as road cuts and by natural or human-induced increases in water saturation. For example, a road cut (Highway 6, now the frontage road of I-70) destabilized the 

edge of an old landslide (Qlsd) and that edge reactivated to form the recent landslide deposit 

(Qlsr), which episodically reactivates during moist periods. Several smaller, recent slumps and 

slides (not mapped) along Interstate 70 east of the Qlsr deposit are also examples of recent slope 

failures that have resulted from destabilization of these relatively old, unconsolidated, landslide deposits. Other unconsolidated deposits on slopes, such as colluvium (Qc, Qcd, Qcg) and diamicton 

(QTdi), also have some potential to become destabilized and form small slump or slide masses.

Near the crest of Bellyache Ridge, the Dakota Sandstone (Kd) and the uppermost part of the 

Morrison Formation (Jm) have also failed as a large, essentially intact, block-glide mass that slid 

northward down slope. The toe of this large block-glide appears to have been folded above its underlying block-glide fault as it slid (see map, cross section B-B’, and discussion of structural features). Braddock (1978) has discussed and described similar large block-glide masses that involve 

the Dakota Sandstone and Morrison Formation in several locations along the Colorado Front 

Range. The Bellyache Ridge block glide formed before formation of the landslide deposits (Qlsd) 

that cover its northern edge; therefore, movement of the block-glide mass probably occurred in the 

early Pleistocene or earlier.

The potential for rock-fall hazards exist along nearly all of the steep cliffy slopes in the map 

area, particularly those containing sandstone ledges. The Dakota Sandstone cliffs appear to be particularly susceptible to rock falls as is evident from the abundant large sandstone boulders on slopes 

and at the base of slopes beneath cliffs of the Dakota Sandstone.

FLOODING

The flood plain of the Eagle River is very narrow throughout most of the map area, as defined by the stream channel and flood-plain deposits (Qalc) along the Eagle River. The Eagle 

River floodplain has potential for flood hazard during prolonged or high precipitation and 

runoff events (table 1). Other areas of possible flood hazard during such events include lowlying areas along Squaw Creek and it’s tributaries and low-lying areas along Alkali, Muddy, 

and Red Canyon Creeks. In addition to these locations, low-lying areas near the mouths of 

Rube, Milk, and Alkali Creeks may also be prone to rare floods and debris-flow events caused 

by intense thunderstorms in their headward regions. Numerous relatively low-lying locations 

in the map area underlain by undivided alluvium and colluvium (Qac), particularly fine-grained 

deposits in the northern part of the quadrangle, may be susceptible to local flooding accompanied by rapid erosion and gullying during cloudburst or prolonged thunderstorm conditions.

GROUND SUBSIDENCE

Nearby the map area and in localities elsewhere in Eagle and Garfield counties, significant property losses have resulted from ground subsidence related to solution voids in evaporitic rocks, and 

from subsidence related to hydrocompaction of soils and low-density, surficial materials containing 

depositional voids or soluble minerals such as evaporitic minerals (J.M., Soule, Colorado Geological 

Survey, oral commun., 1998) (table 1). The Eagle Valley Evaporite (hee) and the Eagle Valley Formation (he) are respectively the principal evaporite-rich and evaporite-bearing rock units in the 

map area. These two units are widely exposed in the southern part of the map area.

In Eagle and Garfield Counties, evaporite-related collapse features range in scale from local, relatively small and widely scattered sinkholes, to relatively large regional collapse centers (Kirkham 

and others, 1997; Scott and others, 1998). The sinkholes commonly are a few meters to tens of 

meters in length and width, and show surface collapse of about one to several meters; many of 

these have formed relatively recently, a few within the last several years. In contrast, the regional 

collapse centers are a few to many kilometers in length and width and appear to have collapsed as 

much as a thousand meters. The regional collapse centers apparently have collapsed slowly, and 

perhaps continually, during the last few to several million years. Much of the evidence for the presence of these collapse centers comes from deformation and sagging of 24 to 8 Ma basaltic flows 

where they overlie evaporitic rocks. The basaltic flows (Tb) in the southwestern part of the map 

area overlie the Eagle Valley Evaporite (hee) and Eagle Valley Formation (he), and these basaltic 

rocks appear to have collapsed or sagged (Hubert, 1954) as much as 430 m during some period of 

time after about 22 m.y. ago when they were initially deposited. This area of regional collapse in 

the map area is considered part of the recently identified, but incompletely defined, Eagle collapse 

center (Kirkham and others, 1997; Scott and others, 1998).

Areas underlain by evaporitic rocks have some potential for diapiric upwelling (rising ground) related to differential loading, or perhaps, hydration of evaporitic material. Diapiric upwelling, however, also appears to be principally a slowly evolving, regional-scale process, most notable in some river 

valleys where rapid downcutting by rivers in the last several million years has decreased the overlying 

load above evaporitic rocks at depth and thereby triggered the anticlinal upwelling, or rise, of the 

evaporitic rocks beneath the river valley (Kirkham and others, 1997; Kirkham and Widmann, 1997). 

The Horn Ranch anticline, in the west-central part of the map area, may reflect diapiric upwelling of 

evaporitic rocks at depth beneath the steep canyon of the Eagle River (Scott and others, 1998).

Areas underlain by and near evaporitic rocks also have potential for high-salinity surface and 

ground water from dissolution of the evaporitic rocks and surficial deposits derived from them. 

The Eagle and Colorado Rivers are known to have high salinities downstream from the map area, 

and a large percentage of that salinity is probably derived from dissolution of evaporitic rocks and 

evaporitic surficial deposits (Kirkham and others, 1997; Kirkham, 1997; Scott and others, 1998).

EXPANSIVE AND CORROSIVE SOILS

Soils that tend to swell or shrink due to changes in moisture content are commonly known as 

expansive soils. Expansive soils are common in those areas underlain by sedimentary rock containing expansive clay minerals, or in areas underlain by colluvial and alluvial deposits derived from 

those rocks. Expansion and contraction of these soils can cause structural damage to rigid materials, such as foundations, pipes, and septic systems sitting on or buried in these expansive soils. In 

the map area the Benton Shale (Kb) and Pierre Shale (Kp) are known to contain bentonite, which 

is an expansive clay mineral (table 1). Soils developed directly on these units or on alluvium, colluvium, and landslide deposits derived from these units should be tested for expansive properties prior to new construction.

Potentially corrosive soils, surficial deposits, and bedrock in the map area are associated with 

those areas underlain by evaporite-bearing rocks—the Eagle Valley Evaporite (hee) and, to a lesser 

degree, the Eagle Valley Formation (he). In combination with moisture, the readily dissolved evaporite minerals (principally gypsum) can be highly corrosive to conventional concrete and uncoated 

metal pipes. Testing of soils, rock, and surficial deposits for the presence of corrosive evaporite 

minerals and proper selection of construction materials can mitigate evaporite-related corrosion.

During wet conditions, dirt roads become extremely slippery to impassable in rock units that 

form clay-rich or evaporite-rich soils. The most notable units in the map area that form these road 

conditions are Tertiary basalt (Tb), Pierre Shale (Kp), Niobrara Formation (Kn), Benton Shale (Kb), 

Eagle Valley Formation (he), Eagle Valley Evaporite (hee), and alluvial and colluvial deposits derived from these rock units (Qac, Qc, Qlsv, Qlsn, and Qlsb).

ENVIRONMENTAL HAZARDS

The Eagle County landfill is in the map area, about 2.5 km north of Wolcott, near the head of 

the west fork of Ute Creek, which is an intermittent tributary of the Eagle River. Of the two landfills (lf) shown on the map in that area, the southern one is an older reclaimed landfill, whereas the 

northern one is an active landfill. These sites are in shaly rocks of the Pierre Shale (Kp) and Niobrara Formation (Kn). These shaly rocks, unless fractured, should be relatively impervious to any 

seepage towards underlying ground water. Unusual precipitation events related to major storms 

might potentially cause some erosion of these landfills and, in a worst-case scenario, transport material down Ute Creek and into the Eagle River.

ECONOMIC GEOLOGY

The map area has a low potential for extractable mineral resources. There have been no exploratory oil and gas wells drilled in the map area and no producing wells are located nearby. The 

few exploratory wells that have been drilled nearby to the west have all been reported as dry (Tom 

Hemborg, Colorado Geological Survey; oral commun., 1997). No sign of mining prospects was 

found, no active mining claims are present, and no evidence could be found for intrusive rocks, 

veins, and associated mineralization and alteration.

The map area has some potential for small deposits of sand and gravel resources along the Eagle River and for gypsum in the southern part of the map area. The largest and potentially most 

exploitable sand and gravel deposits are in the younger terrace alluvium of the Eagle River (Qty). 

Mining of this gravel has occurred in the past. A golf course along the north side of the Eagle River, in section 23, now occupies one of the larger terrace remnants. The Eagle Valley Evaporite 

(hee) is exposed or shallowly buried in several localities in the southern part of the map area. This 

unit is actively mined for gypsum to the west near the town of Gypsum. Although the Eagle Valley 

Evaporite is a potential gypsum resource in this part of the map area, most of the localities underlain by the evaporite are undergoing rapid residential development. Larger undeveloped areas of 

evaporite are exposed in several areas west of the map area.

ACKNOWLEDGMENTS

This mapping project was funded by the National Cooperative Geologic Mapping Program. 

Kevin Tyson assisted with field studies during the summer of 1996. Jim Budahn and Dan Unruh 

provided geochemical analyses, Mick Kunk provided isotopic age analyses, and Frank Byers provided petrographic analyses of basaltic rock samples. Ralph Shroba and Paul Carrara provided advice 

and information concerning classification and delineation of surficial deposits. Scott Minor, Ric 

Page, and Bob Kirkham contributed insightful and helpful technical reviews, and Craig Brunstein 

did the technical edit. All graphics were digitized in GSMCAD (Williams and others, 1996), and 

the polygons were exported through ARC INFO, and then into Illustrator using MAPublisher. 

GSMCAD was also used to export DXF files, which were imported into Illustrator using MAPublisher. Denny Welp, Nancy Shock, Craig Brunstein, Jim Hoffman, Alex Donatich, Deke Young, Van 

Williams, and Karen Morgan all assisted, advised, or were principally responsible for different aspects of the manipulation of the digital files. Informal seminars, field trips, and discussions with 

many members of the Colorado and U.S. Geological Surveys (Bob Kirkham, Randy Streufert, 

Chris Carroll, Ralph Shroba, Paul Carrara, Bob Scott, Karl Kellogg, Bruce Bryant, Bill Perry, Jim 

Budahn, Dan Unruh, Mark Hudson, Tom Steven, and Frank Byers) influenced or contributed to aspects of the information presented. Will Henderson and Ted Archibeque of the Eagle County 

Community Development Department and Assessor's Office, respectively, helped with maps and 

other information concerning land status and ownership. Numerous landowners graciously granted permission to enter their land during the field studies.

REFERENCES CITED

American Geological Institute, 1982, Grain-size scales used by American geologists, modified 

Wentworth scale, in Data sheets (2nd ed.): Falls Church, Virginia, American Geological Institute, sheet 17.1.

Bass, N.W., and Northrup, S.A., 1963, Geology of the Glenwood Springs (30-minute) quadrangle 

and vicinity, northwestern Colorado: U.S. Geological Survey Bulletin 1142–7J, 74 p.

Beckett, R.L., 1955, Structural Geology of the Red Canyon area, Eagle County, Colorado: Boulder, University of Colorado, M.S. thesis, 36 p.

Braddock, W.A., 1978, Dakota Group rockslides, northern Front Range, Colorado, U.S.A., in

Voight, Barry, ed., Rockslides and avalanches, 1; natural phenomena: Developments in Geotechnical Engineering 14A, Elsevier Scientific Publishing Company, p. 440–479.

Burbank, W.S., and Lovering, T.S., 1933, relation of stratigraphy, structure, and igneous activity 

to ore deposition of Colorado and southern Wyoming, in Ore deposits of the Western States 

(Lindgren volume): New York, American Institute of Mining and Metallurgical Engineers, p. 

272–316.

Cobban, W.A., and Reeside, J.B., Jr., 1952, Correlation of Cretaceous Formations of the Western 

Interior of the United States: Geological Society of America Bulletin, v. 63, p. 1011–1044.

Colton, R.B., Holligan, J.A., Anderson, L.W., and Patterson, P.E., 1975, Preliminary map of landslide deposits, Leadville 1 ̊ × 2  ̊ quadrangle, Colorado: U.S. Geological Survey Miscellaneous 

Field Studies Map MF–697, scale 1:250,000.

Cruden, D.M., and Varnes, D.J., 1996, Landslide types and processes, in Turner, A.K., and 

Schuster, R.L., eds., Landslides-investigation and mitigation: National Research Council, 

Transportation Research Board Special Report 247, p. 36–75.

Davis, G.H., 1984, Structural geology of rocks and regions: New York, N.Y., John Wiley & Sons, 

492 p.

Dubiel, R.F., 1992, Sedimentology and depositional history of the Upper Triassic Chinle Formation in the Uinta, Piceance, and Eagle Basins, northwestern Colorado and northeastern Utah: 

U.S. Geological Survey Bulletin 1787–W, 25 p.

Freeman, V.L., 1971, Stratigraphy of the State Bridge Formation in the Woody Creek quadrangle, 

Pitkin and Eagle Counties, Colorado: U.S. Geological Survey Bulletin 1324–F, p. F1–F17.

Hongzhuan, Ye, Royden, L.H., Burchfiel, B.C., Schuepbach, M.A., 1996, Late Paleozoic deformation of interior North America; the greater ancestral Rocky Mountains: American Association of Petroleum Geologists Bulletin, v. 80, no. 9, p. 1397–1432.

Hubert, J.F., 1954, Structure and stratigraphy of an area east of Brush Creek, Eagle County, Colorado: Boulder, University of Colorado, M.S. thesis, 104 p.

Izett, G.A., Pierce, K.L., Naeser, N.D., Jaworowski, Cheryl, 1992, Isotopic dating of Lava Creek B 

Tephra in terrace deposits along the Wind River, Wyoming; implications for post 0.6 Ma uplift of theYellowstone hotspot [abs]: Geological Society of America Abstracts with Programs, 

v. 24, no. 7, p. 102.

Johnson, S.Y., Schenk, C.J., Anders, D.L., and Tuttle, M.L., 1990, Sedimentology and petroleum 

occurrence, Schoolhouse Member, Maroon Formation (Lower Permian), northwestern Colorado: American Association of Petroleum Geologists Bulletin, v. 74. no. 2, p. 135–150.

Kirkham, R.M., 1997, Late Tertiary and Quaternary collapse related to dissolution and flowage of 

Pennsylvanian evaporitic rocks in the Glenwood Springs area, Colorado, in McCalpin, J.P., 

compiler, The active geologic environment of central Colorado-Aspen-Glenwood Springs-Silt, 

Colorado: Field Guidebook, 1997 Friends of the Pleistocene field trip, Rocky Mountain cell, 

Day 2, p. 1–18.

Kirkham, R.M., Streufert, R.K., Scott, R.B., Lidke, D.J., Bryant, B., Perry, W.J., Jr., Kunk, M.J., 

Driver, N.E., and Bauch, N.J., 1997, Active salt dissolution and resulting geologic collapse in 

the Glenwood Springs region of west-central Colorado [abs.]: Geological Society of America 

Abstracts with Programs, v. 29, no. 6, p. A-416.

Kirkham R.M., and Widmann, B.L., 1997, Geologic map of the Carbondale quadrangle, Garfield 

County, Colorado: Colorado Geological Survey Open-File Report 97-3, scale 1:24,000.

Kunk, M.J., Kirkham, R.M., Streufert, R.K., Scott, R.B., Lidke, D.J., Bryant, B., and Perry, W.J., 

Jr., 1997, Preliminary constraints on the timing of salt tectonism and geologic collapse in the 

Carbondale and Eagle collapse centers, west-central Colorado [abs.]: Geological Society of 

America Abstracts with Programs, v. 29, no. 6, p. A-416.

Lovering, T.S., and Johnson, J.H., 1933, The meaning of unconformities in the stratigraphy of 

central Colorado: American Association of Petroleum Geologists Bulletin, v. 17, p. 

353–374.

Marvin, R.F., Mehnert, H.H., and Mountjoy, W.M., 1966, Age of basalt cap on Grand Mesa: U.S. 

Geological Survey Professional Paper 550–A, p. A81.

Murray, H.F., 1958, Pennsylvanian stratigraphy of the Maroon trough, in Curtis, B.F., ed., Symposium on Pennsylvanian rocks of Colorado and adjacent areas: Rocky Mountain Association 

of Geologists, p. 47–58.

Pierce, K.L., Obradovich, J.D., and Friedman, I., 1976, Obsidian hydration dating and correlation 

of Bull Lake and Pinedale glaciations near West Yellowstone, Montana: Geological Society of 

America Bulletin, v. 87, p. 703–710.

Richmond, G.M., 1986, Stratigraphy and correlation of glacial deposits of the Rocky Mountains, 

the Colorado Plateau, and the ranges of the Great Basin, in Sibrava, V., Bowen, D.Q., and 

Richmond, G.M., eds., Quaternary glaciations in the United States: Quaternary Science Reviews, v. 5, p. 99–127.

Richmond, G.M. and Fullerton, D.S., 1986, Introduction to Quaternary glaciations in the United 

States of America, in Sibrava, V., Bowen, D.Q., and Richmond, G.M., eds., Quaternary glaciations in the United States: Quaternary Science Reviews, v. 5, p. 3–10.

Schenk, C.J., 1992, Stratigraphic Correlations Between the Eagle Valley Evaporite and Minturn 

Formation, Eagle Basin, northwest Colorado: U.S. Geological Survey Bulletin 1787–GG, p. 

GG1–GG8.

Scott, R.B., Lidke, D.J., Shroba, R.R., Hudson, M.R., Kunk, M.J., Perry, W.J., Jr., and Bryant, 

Bruce, 1998, Large-scale active collapse in western Colorado: interaction of salt tectonism 

and dissolution, in Brahana, J.V. and others, eds. Gambling with groundwater—physical, 

chemical, and biological aspects of aquifer-stream relations: American Institute of Hydrology, 

Proceedings of the joint meeting of the XXVIII Congress of the International Association of 

Hydrogeologists and the annual meeting of the American Institute of Hydrologists, Las Vegas, 

Nevada, USA, p. 195–203.

Sheridan, D.S., 1950, Permian (?), Triassic, and Jurassic stratigraphy of the McCoy area, westcentral Colorado: Boulder, University of Colorado, M.S. thesis, 88 p.

Stauffer, J.E., 1953, Geology of and area west of Wolcott, Eagle County, Colorado: Boulder, University of Colorado, M.S. thesis, 56 p.

Thomas, C.R., McCann, F.T., and Raman, N.D., 1945, Mesozoic and Paleozoic stratigraphy in 

northwestern Colorado and northeastern Utah: U.S. Geological Survey, Oil and Gas Investigations, Preliminary Chart 16.

Tweto, Ogden, and Lovering, T.S., 1977, Geology of the Minturn 15-minute quadrangle, Eagle 

and Summit Counties, Colorado: U.S. Geological Survey Professional Paper 956, 96 p.

Tweto, Ogden, Moench, R.H., and Reed, J.C., Jr., 1978, Geologic map of the Leadville 1 ̊ × 2 ̊ , 

quadrangle, northwest Colorado: U.S. Geological Survey Miscellaneous Investigations Series 

Map I–999, scale 1:250,000.

Varnes, D.J., 1978, Slope movement types and processes, in Schuster, R.L., and Krizek, R.J., 

eds., Landslides, analysis, and control: National Academy of Sciences, Transportation Research Board Special Report 176, p. 11–33.

Wanek, L.J., 1953, Geology of an area east of Wolcott, Eagle County, Colorado: Boulder, University of Colorado, M.S. thesis, 62 p.

Warner, L.A., 1956, Tectonics of the Colorado Front Range, in American Association of Petroleum Geologists Rocky Mountain Section, Geologic Record, 1956, p. 129–144

Whitney, J.W., Piety, L.A., and Cressman, S.L., 1983, Alluvial history in the White River basin, northwest Colorado [abs.]: Geological Society of America Abstracts with Programs, v. 15., p. 328.

Williams, V.S., Selner, G.I., and Taylor, R.B., 1996, GSMCAD, a new program that combines the 

functions of GSMAP and GSMEDIT programs and is compatible with Microsoft Windows and 

ARC/INFO: U.S. Geological Survey Open-File Report 96-007, one 3.5-inch IBM-compatible 

diskette.

GEOLOGIC MAP OF THE WOLCOTT QUADRANGLE, EAGLE COUNTY, COLORADO

By

David J. Lidke

1998

GEOLOGIC INVESTIGATIONS SERIES

 I-2656

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Base from U.S. Geological Survey, 1962

Photorevised 1987

Lambert conformal projection. 10,000-foot grid ticks based on 

Colorado coordinate system, central zone. 1,000-meter 

Universal Transverse Mercator grid ticks, zone 13. 

1927 North American Datum

1 .5 0 1 KILOMETER

SCALE 1:24 000

1 1/ 2 0 1 MILE

COLORADO

MAP LOCATION APPROXIMATE MEAN

DECLINATION, 1998

MAGNETIC NORTH

12 / 

1 o

2

TRUE NORTH

Map, sections, correlation, and index map digitized by Dave Lidke

Layout design, and digital layout by Denny Welp and F. Craig Brunstein 

Manuscript approved for publication August 14, 1998

Geology mapped by Dave Lidke in 1996; assisted by Kevin M. Theissen

CONVERSION FACTORS

Multiply By To obtain

centimeters (cm) 0.3937 inches (in.)

meters (m) 3.281 feet (ft)

kilometers (km) 0.6214 miles (mi)

CONTOUR INTERVAL 40 FEET

DASHED LINES REPRESENT 20-FOOT CONTOURS

NATIONAL GEODETIC VERTICAL DATUM OF 1929

Any use of trade names in this publication is for 

descriptive purposes only and does not imply 

endorsement by the U.S. Geological Survey

This map was produced on request, directly

from digital files, on an electronic plotter.

For sale by U.S. Geological Survey Information Services 

Box 25286, Federal Center, Denver, CO 80225

This map is also available as a PDF file at http://greenwood.cr.usgs.gov

Erosion Ground

subsidence

Expansive and

corrosive

soils

Flooding

Mass wasting1 Gullying

Table 1.—Map units associated with potential geologic hazards in the Wolcott quadrangle

QTg

Tg

Qc

Qcd

Qcg

Qlsr

Qlsg

Qlsv

Qlsn

Qlsb

Qlsd

Qdu

Qac

Qfy

Qac

Qc

hee

he

Qac

Qc

Kb

Kp

hee

he

Qalc

Qal

Qac

Qfy

Qty?

1Includes landslides, debris flows, rockfalls and rock topples.

Qal

Qty

Qto

Tb

Kp

Kps

Kn

Kb

Kd

Jm

Je

dc

dPsb

Phms

Phm

he

hee

M

T

T

A

 40

 5

 15

 35

 20

 15

 5

QTdi

Tb

Kn

Kb

Kd

B'

A

B

106 ̊45'

39 ̊

45'

106 ̊37'30"

39 ̊

37'

30" 0

0 1 KILOMETER

1 MILE

0.5

0.5

Block-glide

folds

Collapsed

basaltic flows

Southern zone of tight folds

Figure 1. Sketch map of structural features in the Wolcott quadrangle, Colorado

T

High-angle fault

Tear fault

Reverse fault

Block-glide fault

General bedding or flow foliation orientation

Anticline or syncline

T

EXPLANATION

HORN RANCH ANTICLINE

WOLCOTT

SYNCLINE

UTE CREEK

SYNCLINE

Jm

dPsb

Phm

he

hee

dc

Qac Qalc Qfy

Qac

Phms

Je

dPsb

Jm

Qc

Qac

dc

Kd

Kd

he

hee

he hee he Qac

Qac

Qac

A

METERS Bellyache Ridge

A'

10,000

9,000

8,000

7,000

6,000

5,000

4,000

FEET

A A T T

3,000

2,500

2,000

1,500

1,000

Eagle River

HORN RANCH ANTICLINE

WOLCOTT SYNCLINE

Squaw Creek

SECTION

B-B'

I-70

Je

Phms

Kd

Qlsd

hee

he

Phm

Phms dPsb

dc

Je

Jm

Qcg Qlsd

Qlsb

Qcd

Qcd Qlsd

Qlsd

Qcd

Qcd

he

Phm

Phms

dPsb

dc

Je

Jm

he

Qc

Tb Tg Tb Qac

Qc

Qc

Qac Kd

af Qalc Qac Kp

Kn

Kd Kb

Jm Je

dc

dPsb

Phms

Phm

he

hee Jm

Qty

hee

Tb

METERS

B B'

10,000

9,000

8,000

7,000

6,000

5,000

4,000

FEET

? ?

?

3,000

2,500

2,000

1,500

1,000

REGION OF EVAPORITE REMOVAL AND COLLAPSE

3,000

Warren Gulch

Bellyache Ridge

Eagle River

UTE CREEK SYNCLINE

SECTION

A-A'

I-70

Holocene

Pleistocene

Pliocene

early Miocene

early Miocene

or Oligocene

Upper

Cretaceous

Lower

Cretaceous

Upper

Jurassic

Upper

Triassic

Lower Triassic

and Permian

Middle

Pennsylvanian

TERTIARY

QUATERNARY

CRETACEOUS

JURASSIC

TRIASSIC

PERMIAN

AND

PENNSYLVANIAN

PENNSYLVANIAN

? ? ? ?? ?

? ?

?

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity?

Minor unconformity?

Artificial fills deposits

Alluvial Alluvial and

Colluvial deposits Colluvial deposits

?

CORRELATION OF MAP UNITS

af lf

Qalc

Qal

Qtl

Qty

Qto

QTg

Qfy

Qfo

Qac

Qc

Qdu

QTdi

Tb

Tg

Kp Kps

Kn

Kb

Kd

Jm

Je

dc

dPsb

Phms

Phm

he

hee

Qcd Qcg

Qlsr

Qlsg Qlsv Qlsn Qlsb Qlsd