Geomorphology of Cement Creek 

and its Relation to Ferricrete Deposits

By Kirk R. Vincent, Stanley E. Church, and Laurie Wirt

Chapter E16 of

Integrated Investigations of Environmental Effects of Historical 

Mining in the Animas River Watershed, San Juan County, Colorado

Edited by Stanley E. Church, Paul von Guerard, and Susan E. Finger

Professional Paper 1651

U.S. Department of the Interior

U.S. Geological Survey
Contents

Abstract .......................................................................................................................................................747

Introduction.................................................................................................................................................748

Purpose and Scope ..........................................................................................................................748

Setting and Historical Background ................................................................................................748

Methods.......................................................................................................................................................750

Field Methods ....................................................................................................................................750

Dating Methods and Limitations ....................................................................................................750

Results: Nature and Distribution of Landforms .....................................................................................758

General Distribution of Landforms .................................................................................................758

Oldest Landforms ..............................................................................................................................758

Alluvial Fans .......................................................................................................................................759

Cement Creek Terrace and Flood Plain .........................................................................................759

Distribution of Ferricrete and Wetlands ........................................................................................760

Interpretations and Conclusions .............................................................................................................764

Earliest Geomorphic History ...........................................................................................................764

History of the Cement Creek Terrace and Flood Plain ................................................................766

Implications of Sedge Wetlands and Peat ...................................................................................767

Implications for the Timing and Processes of Ferricrete Precipitation ...................................768

References Cited........................................................................................................................................772

Plate

[Plate in this report is on accompanying CD-ROM]

5. Surficial geologic deposits along Cement Creek, upstream of Silverton, Colorado

Figures

 1. Location map of study area .....................................................................................................749

 2. Longitudinal profile of landforms along Cement Creek ......................................................751

 3. Longitudinal profile of landforms along Cement Creek near Ohio Gulch ........................752

 4. Drawing of stratigraphy of sediment composing the prominent terrace along 

Cement Creek ............................................................................................................................753

 5. Diagram showing time-stratigraphic relations of sediment composing prominent 

terrace along Cement Creek ....................................................................................................761

6. Plots comparing the spatial location of ferricrete, wetlands, and inflowing water 

along Cement Creek ..................................................................................................................762

 7. Box plots showing chemistry of silts found within peat and other sediment .................770

Table

 1. Data for radiocarbon samples collected along Cement Creek, organized by site,

landform, type of sample, and stratigraphic position ...........................................................754
Abstract

Latest Quaternary landforms, composed of gravel and 

peat, in the lower 11 kilometers of Cement Creek were mapped 

and dated in order to understand the geomorphic history of the 

area and thus provide a geomorphological context for understanding the formation of ferricrete, as part of the Animas 

River watershed study.

The San Juan Mountains were nearly covered by alpine 

glaciers during the latest Pleistocene and presumably numerous times before. During the most recent episode, generally 

referred to as the Pinedale glaciation, ice was about 520 meters 

thick at the present site of Silverton and 430 meters thick at 

the present site of Gladstone. That glaciation ended about 

12,000 14C years ago, and except for till, the sediments along 

Cement Creek were likely deposited after that. Other deposits 

apparently formed during deglaciation, including a canyonfilling stream-gravel deposit, a localized stream terrace about 

20 meters higher than the valley floor, and a large landslide. 

Prominent alluvial fans, located at the mouths of most tributaries, also probably began to accumulate as soon as ice disappeared from the floor of the Cement Creek valley. The fans 

probably accumulated rapidly until the surrounding hillslopes 

became stabilized by vegetation, which may have taken many 

centuries. The largest fans, or pairs of fans, aggraded the valley bottom perhaps more than 20 meters and caused Cement 

Creek to aggrade upstream of them. This resulted in a segmented longitudinal profile of the Cement Creek valley that 

is still present, because the bulk of the fan deposits have not 

been removed by Cement Creek. More than 6,000 years ago, 

however, Cement Creek partially breached the toes of the fans, 

incising through as much as 5 meters of fan sediment. This, 

in turn, caused the fans to become incised by their tributary 

streams. Between 3700 B.C. and A.D. 400 (5,650 to 1,550 cal. 

yr B.P.) Cement Creek migrated laterally, but its bed remained 

at the level of the present streambed. Starting about A.D. 400, 

which is slightly before the Medieval Warm Period, Cement 

Creek began to aggrade and rose as much as 3 meters above 

its previous level. Cement Creek incised back to its previous 

level, leaving behind a prominent stream terrace, between 

A.D. 1330 and A.D. 1700. This incision may have occurred 

before A.D. 1500 and perhaps about A.D. 1440. Incision of 

the Cement Creek terrace is roughly synchronous with the 

beginning of the Little Ice Age. Aggradation and incision of 

the Cement Creek terrace were not caused by local base level 

processes, and may have been the result of climate change, 

although the precise mechanisms involved are not known. 

Recent human activities, in contrast, have had little influence on the shape and physical processes of Cement Creek. 

The influence of human activities on geochemical process is 

beyond the scope of this study, with the exception of ferricrete 

formation.

Iron compounds locally cement clastic sediment of all 

origins, creating conglomerate-type ferricrete. Most ferricrete 

exposures are dry and presumably are inactive, and most are 

prehistorical in age. Defining the timing of conglomerate-type 

ferricrete cementation is problematic, even where the depositional age of the clastic sediment is precisely known, because 

cementation could have occurred at any time after clastic 

deposition. Cementation of distal fan sediment must have 

occurred more than 6,000 years ago, because that is when the 

ground water in the gravel was permanently drained by stream 

incision. A spring-deposited ferricrete encased charcoal that 

is 4,500 cal. years old, so presumably conglomerate ferricrete 

also formed during the middle Holocene. Stream gravels 

deposited in the past 500 years are also cemented by iron compounds. We conclude that ferricrete formed in Cement Creek 

clastic sediment at various times and locations throughout the 

Holocene, and thus most ferricrete is unrelated to mining.

Exposures of ferricrete are spatially discontinuous and 

do not correspond to the rate of emergence of ground water 

during the low-flow season. This suggests that the geochemical conditions necessary for ferricrete formation are not uniform in the watershed.

Exposures of wet, possibly active ferricrete are almost 

invariably located where there are sedge wetlands (underlain 

by peat) on an adjacent terrace. We conclude that both wetlands and wet ferricrete result from the perennial emergence of 

ground water that originated in tributary subbasins, rather than 

from Cement Creek itself. One must take care when using peat 

Chapter E16

Geomorphology of Cement Creek and its Relation 

to Ferricrete Deposits

By Kirk R. Vincent, Stanley E. Church, and Laurie Wirt
to date stream incision because peat can continue to accrete 

in wetlands after stream incision. This continuing accretion 

occurs because the water supply to wetlands is from emerging 

ground water that originated in tributary subbasins, not from 

the main stream. Thus, the presence of sedge peat in subalpine 

settings like Cement Creek is a paleo-environmental indicator 

of the emergence of ground water from the valley sides.

Introduction

Cement Creek, which flows into the Animas River at 

Silverton (fig. 1), was presumably named for the ferricrete 

so prevalent in Cement Creek basin. Iron-rich precipitates 

cement alluvium and colluvium, forming conglomerates, and 

also accumulate in iron springs and bogs as relatively pure 

deposits with few clasts incorporated. Both types of deposits 

are called ferricrete. Acidic geochemical conditions leading 

to the precipitation of ferricrete can adversely impact surface 

water chemistry and riparian ecology and thus are of environmental concern. Therefore, several studies investigated the 

physical, hydrologic, and geochemical conditions of ferricrete formation, in order to help understand the geochemical 

processes before and after mining began. Verplanck and others 

(this volume, Chapter E15) present a classification of ferricrete, and the spatial distribution of ferricrete in the region 

is documented on plate 2 of Yager and Bove (this volume, 

Chapter E1). Wirt and others (this volume, Chapter E17) 

explore the geochemical and hydrologic processes of formation of ferricrete, using Cement Creek as a major example. 

This chapter provides the geomorphological context for those 

chapters. Our study focuses on Cement Creek from the confluence with South Fork Cement Creek (near the Gladstone site) 

to the canyon mouth at Silverton (fig. 1).

Purpose and Scope

The purpose of this study is to provide the geomorphological context for the formation of ferricrete along Cement 

Creek by investigation of the following:

• The location and nature of clastic sediment that is 

locally cemented with iron-rich compounds. Alluvial 

deposits influence the movement of ground water, 

which is the medium of transport of the iron and the 

medium from which the iron compounds precipitate.

• The history of incision of alluvial deposits, because 

that influences the location of oxidation of iron-rich 

waters. Incision can cause dewatering of clastic sediment and a shift in the location of ferricrete formation.

• The dating of aggradation and incision of alluvium 

along Cement Creek, thus providing constraints on 

the timing of ferricrete formation within, or on, clastic 

sediment.

In so doing, we also contribute to an understanding of the 

Holocene geomorphic history and processes of Cement 

Creek, and of the region in general.

Setting and Historical Background

This chapter documents the nature and history of 

landforms along the lower 11 km of Cement Creek, from 

Gladstone to Silverton, and at the mouths of its major tributaries (fig. 1 and pl. 5). Cement Creek flows through a subalpine 

valley located in the San Juan Mountains of San Juan County, 

Colo., and joins the Animas River at Silverton. In the study 

area, Cement Creek ranges in altitude from 2,828 to 3,182 m, 

but the surrounding hillslopes extend up to mountain tops that 

reach 4,111 m. Those hillslopes are dominated by exposed 

bedrock and tundra vegetation above the treeline (at about 

3,600 m), and are dominated by Engelmann spruce and 

subalpine fir below the treeline. In the valley bottom, alluvial 

fans are dominated by spruce forest and open grasslands; 

stream terraces and flood plains support thickets of bog birch 

and willow, sedge wetlands, and spruce forests.

The Silverton weather station records mean annual 

temperature of 1.7°C (35°F) and about 61 cm average annual 

precipitation (National Weather Service data), but precipitation 

is about twice that in the surrounding mountains (von Guerard 

and others, this volume, Chapter B). Although precipitation 

in the area is predominantly snowfall, rain is delivered by 

summer thunderstorms and by Pacific tropical storms in the 

fall. Peak streamflows generally result from spring snowmelt, 

but substantial flows result from tropical storms in the fall 

(von Guerard and others, this volume, fig. 3; Pruess, 1996).

The bedrock geology, and the mineralization that made 

the area famous, are discussed by Yager and Bove (this volume, pl. 1) and Bove and others (this volume, Chapter E3). 

For this chapter the salient aspect of the bedrock geology is 

that it is composed of erosion-resistant volcanic rocks. Thus 

talus deposits are common, hillslope soils are thin and coarse 

textured, and alluvial deposits are coarse and contain little silt 

and clay. Blair and others (2002) presented the late Quaternary 

geologic history of the region, and we complement their work 

by subdividing certain map units along Cement Creek and 

by providing age constraints that were previously lacking. 

Atwood and Mather (1932) mapped the extent of glaciation 

in the San Juan region. Few if any surficial deposits predate 

the last glaciation; thus, the landforms we discuss are latest 

Pleistocene and Holocene in age.

Hard-rock mining was extensive in the Cement Creek 

watershed during the late 19th century and through most 

of the 20th century (Church, Mast, and others, this volume, 

Chapter E5, fig. 1), and the toxic effects of metals released 

by human efforts are the subject of several chapters in this 

volume. We conclude, however, that the actions of humans 

have had little lasting impact on the physical form (beyond 

localized disturbance) or physical function of Cement Creek 

and adjacent wetlands. We do not address chemical impacts. 

748 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 749

Dry Gulch

Lower bog

KILOMETERS

MILES

0 2

0

1

1 2

3 4

South Fork

Tiger Gulch

Hancock Gulch

Cascade Gulch

SUNNYSIDE MINE

(at Gladstone)

Soda Gulch

Henrietta

Gulch

Silverton

Cement

Creek

DENVER

USGS

Gauging Station

09358550

37o

50'

Start of profile

on figure 2

37o

52'

30"

107o41'30" 107o37'30"

former

Lake Emma

Cement Creek

basin

Cement

Creek

basin

Animas River

Colorado River

Arkansas River

South Platte R iver

Grassy Gulch

Illin ois Gulch

NiagaraGu lch

Topeka Gulch

Ohio

Gulch

Porcupine Gu lch

Minnesota Gulch

Fairview Gulch

Georgia Gulch

Prospect Gulch

Animas River

Middle Fork

Nort h Fo rk

COLORADO

Figure 4

Sample locality 

discussed in text

209

209

235

103

211

Unnamed 

tributaries 

discussed 

in text

EXPLANATION

Figure 1. Location of Cement Creek and its major tributaries, near Silverton, San Juan County, Colo.
Two historical floods are noteworthy and both involve 

the Gladstone site, at the upstream end of our study reach 

(fig. 1, pl. 5). One flood occurred on October 5, 1911, and is 

the flood of record on the Animas River (Durango Evening 

Herald, 1911). It is informally named after Gladstone because 

that is where the largest 24-hour rainfall (21 cm) for the storm 

was measured (Pruess, 1996). The second flood involved a 

bizarre event that occurred on June 4, 1978, when the bed of 

Lake Emma (a glacial tarn), having been literally undermined, 

accidentally caved into the Sunnyside mine below (Jones, 

this volume, Chapter C, figs. 26–29). A torrent of sediment 

and lake water drained through the mine workings, gushed 

out of the American tunnel portal at Gladstone (mine # 96), 

and rushed down Cement Creek (Bird, 1986). Neither flood, 

however, had a major physical impact on the geomorphology 

of Cement Creek that is identifiable today.

Methods

In order to understand the Holocene geomorphic history and processes of Cement Creek, and thus place ferricrete 

formation into a geomorphological and temporal context, we 

employed four methods. These involved mapping fluvial landforms (pl. 5) and surveying their longitudinal profile (figs. 2 

and 3). The stratigraphy and sedimentology of clastic and 

peat deposits were documented (for example, fig. 4), and the 

deposits were dated using radiocarbon (table 1) and tree rings.

Field Methods

Landforms were mapped in the field onto a photographic 

base (about 1:6,200) by standard field methods. The mapping 

from this study is presented on plate 5. This effort added detail 

and age constraints to the previous work of Blair and others 

(2002), which was regional in scope. Blair and others identified five surficial geologic map units in the Cement Creek 

watershed, in addition to colluvium and bedrock hillslopes 

and areas disturbed by human activities. We developed 

11 map units, which are used to distinguish alluvial fans, 

stream terraces, the flood plain and active channel of Cement 

Creek, glacial till, landslide deposits, and areas obscured by 

human activity. Exposed bedrock and colluvial hillslopes were 

not mapped. The sedimentology and stratigraphy of stream 

and wetland deposits were inspected at numerous cutbank 

exposures along Cement Creek, and the stratigraphy of one 

important site is illustrated in figure 4.

A longitudinal profile of Cement Creek (figs. 2 and 3) 

was surveyed in order to place landform surfaces and stratigraphy, ferricrete deposits, wetlands and other sites of inflowing water, and dated material into a common spatial context 

(fig. 5). A longitudinal profile is like a cross section, but is 

oriented parallel to the valley axis. The profile was surveyed 

using a 2-power hand level on a staff of fixed height, a surveyor’s rod, and a 100 m tape. The instrument “turned” (was 

moved) when the down-valley shot distance reached 25 m. 

The altitudinal datum was taken from the contour map, and 

thus altitude values are approximate. The “horizontal” datum 

was arbitrarily set to be zero at the confluence of Cement 

Creek with South Fork Cement Creek (fig. 1). The cumulative 

down-valley tape distance was not converted to true horizontal 

distance, but the resulting distortion of local gradients is insignificant for our purposes. The profile transect was designed as 

a compromise between the valley axis and the longer course 

of the meandering stream. In other words, the transect crossed 

point bars rather than following the thalweg or center of the 

channel. This means that the true stream gradients are locally 

slightly less steep than depicted, but again this distortion is 

acceptable given our purposes. This also means that our tape 

distances differ slightly from those measured by Kimball and 

others (2002) during their tracer dilution study of Cement 

Creek. We have used their inflow data and converted their 

tape distances to ours by knowing distances measured in both 

studies at the confluences of major tributaries. The profile 

extends from the South Fork Cement Creek confluence (near 

Gladstone) for just over 10 km to the mouth of Cement Creek, 

dropping 332 m in altitude over this distance. The survey 

progressed downstream and was not closed by surveying back 

up the valley; rather the total fall was independently estimated 

from USGS topographic maps. The difference in the surveyed 

and map-based altitude drop is 6 m, which is excellent agreement given the ≈13 m (40-foot) contour interval of the maps.

Dating Methods and Limitations

Three dating techniques were used to establish the age 

of clastic and peat deposits. (1) Crosscutting relationships 

were used to establish the relative age of the deposits. This 

involved crosscutting stratigraphy of exposed sediment, and 

also crosscutting landforms. For example, the toes of many 

alluvial fans (map unit fo) are truncated, and a prominent 

terrace (map unit t) is found at the foot of the escarpments. 

The fan surfaces and sediment must be older than the terrace 

surface and the terrace sediment. (2) We counted the growth 

rings of living and standing-dead Engelmann spruce trees that 

were rooted in surficial sediment of the Cement Creek terrace 

or flood plain, to determine when the trees were established. 

This provides a minimum age for deposition of the sediment in 

which the trees were rooted. (3) Lastly, we obtained radiocarbon ages for organic material found in sediment. The dating 

results for 43 samples are listed in table 1 along with sample 

types, localities, and laboratory methods and assumptions. All 

radiocarbon ages were calibrated (Stuiver and Reimer, 1993), 

and all dates are presented in calibrated (cal.) calendar years 

unless otherwise indicated. Dating results for the prominent 

terrace (map unit t) are presented in figure 5.

Three types of buried organic material (stumps, branches, 

and peat or twigs in peat) were dated for this study. The reliability of radiocarbon results for dating the deposition of clastic sediment depends in part on the type of organic material 

750 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 751 Figure 2. Surveyed longitudinal profile of bed and prominent terrace of Cement Creek, and other landforms. USGS stream gauge (Station 09358550) is located at 9.7 km. Dots represent measurements points. Straight line is drawn below profile to help illustrate convex shape of profile. DISTANCE DOWNSTREAM FROM CONFLUENCE OF SOUTH FORK CEMENT CREEK, IN KILOMETERS

ALTITUDE, IN METERS

Top of gravel, unit gHigh terrace, unit eCarved bedrock strathOld fan, unit foYoung fan, unit fyTerrace, unit tStreambed

EXPLANATION

Two fans

sites 103 and 211 are on South Forkupstream of confluence 305 m and 430 m,respectively

rockslotcanyon

3,2003,1503,1003,0503,0002,9502,9002,850

1234567 89 10 0

unit g

Ohio Gulch

Illinois Gulch

Landslide, unit ls

site 209

site 235, fig. 4

Tiger and Georgia Gulches

Prospect Gulch
3,140

3,1303,1203,1103,1003,0903,0803,0703,0603,0503,0403,0303,020

3.0 3.5 4.0 4.5 5.0 5.5 6.0

Ohio Gulch fan

site 235

fig. 4

site 209

DISTANCE DOWNSTREAM, IN KILOMETERS

2.5

Terrace, wetland unit tw

Terrace, unit tYoung fan, unit 

Streambed

fy

Old fan, unit fo

ALTITUDE, IN METERS

EXPLANATION

Figure 3. Surveyed longitudinal profile of bed and prominent terrace of Cement Creek (middle portion of Cement Creek profile, fig. 2, near Ohio Gulch), and tributary 

alluvial fans projected to the profile transect. Dots represent measurements points. Straight line is drawn below profile for reference.

752 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 753

{

0

0

5 10 15 20 25 30 35 40 45 50 55

654321

ground surface

covered

pebble gravel

white silts

pebble gravel

peat

peat covered

cobble gravel

sand

pebble gravel

silt sandy

gravel

covered

tree cored in 1998, 

142 rings, A.D. 1856 date

organic 

A horizon

silt

?

?

peat

peat

Cement Creek at low flow

peat

covered

covered

HORIZONTAL DISTANCE, IN METERS

HEIGHT, IN METERS

 1000 B.C.

GX-26397

stump rooted

in gravel

Beta-119771

A.D. 420

GX-24747

A.D. 1440

Beta-148495

A.D. 1000

Beta- 148496  A.D. 775

GX-24748

B.C. 759 or 544

GX-24749

A.D. 1280

GX-24750

modern

TERRACE MAP UNIT, SEDGE WETLAND SUBDIVISION (tw), PEAT FACIES TERRACE MAP UNIT (t), GRAVEL FACIES

Figure 4. Stratigraphy at site 235 of the prominent terrace (map unit t) along Cement Creek. Streambank exposure is located about 400 m upstream of Ohio Gulch (at 4.85 km 

in figs. 2 and 3) on east side of Cement Creek.
Table 1. Data for radiocarbon samples collected along Cement Creek, organized by site, landform, type of sample, and stratigraphic position.

[Note that “B.C.” and “A.D.” are both placed before dates in this table—for better readability and quicker recognition]

Field No. Lab No.1 δ13C per mil1 14C age years B.P.2 Calibrated ages3 Height above water, m6 Intercept(s)4 1-sigma range5 2-sigma range5

Samples from flood-plain sediments (map unit p) at various locations along Cement Creek

Wood not in growth position:

00-ABFC-207B Beta-147014 –25* Modern (117 %)7 0.85 5,690

00-ABFC-207A Beta-147013 –25* Modern (100 %)7 0.3 5,69000-ABFC-205 Beta-147012 –25* 30±40 A.D. 1953 A.D. 1951–1955 A.D. 1897–1915

A.D. 1949–1955

0.25 3,050

00-ABFC-204B Beta-147011 –25* 180±70 A.D. 1670–1950 A.D. 1650–1880

A.D. 1920–1950

A.D. 1530–1560A.D. 1630–1950

0.6 1,755; 20 OGR9

.

00-ABFC-210 Beta-147020 –25* 230±70 A.D. 1660 A.D. 1640–1680

A.D. 1740–1800A.D. 1930–1950

A.D. 1490–1890A.D. 1910–1950

1.0 7,560; wood in 

alluvial ferricrete.

Stumps in growth position:

00-ABFC-213 Beta-147030 –25* Modern (101 %)7 0.3 7,850

00-ABFC-208 Beta-147015 –25* 90±70 A.D. 1890–1950 A.D. 1680–1760

A.D. 1804–1954

A.D. 1660–1955 0.3 1,325; 

30 OGR9

.

00-ABFC-204A Beta-147010 –25* 180±50 A.D. 1670–1950 A.D. 1660–1690

A.D. 1730–1810A.D. 1920–1950

A.D. 1640–1950 0.1 1,755; 

20 OGR9

.

Samples from stratigraphic site 209, located on figs. 1, 2, and 3 (lat 37.8680° N., long 107.6745° W.)

Wood from flood-plain sediments (map unit p):

00-ABFC-209F Beta-147019 –25* 70±70 A.D. 1951 A.D. 1685–1732

A.D. 1808–1926A.D. 1948–1955

A.D. 1665–1784A.D. 1789–1955

1.2 3,180; wood in sand.

00-ABFC-209D Beta-147018 –25* 220±50 A.D. 1660 A.D. 1650–1680

A.D. 1770–1800A.D. 1940–1950

A.D. 1530–1560A.D. 1630–1690A.D. 1730–1810A.D. 1920–1950

1.0 3,180; wood in peat.

00-ABFC-209B Beta-147016 –25* 230±60 A.D. 1660 A.D. 1640–1680

A.D. 1770–1800A.D. 1940–1950

A.D. 1510–1600A.D. 1620–1700A.D. 1720–1820A.D. 1840–1880A.D. 1920–1950

0.2 3,180; wood in silt.

Wood in gravel (of map unit t) beneath unconformity at base of flood-plain sediments (map unit p):

00-ABFC-209C Beta-147017 –25* 1,910±70 A.D. 90 A.D. 30–150 B.C. 50–A.D. 250 0.0 3,180

754 Environmental Effects of Historical Mining, Animas River Watershed, Colorado Distance on profile8, m; comments
Cement Creek Geomorphology and Ferricrete Deposits 755

Table 1. Data for radiocarbon samples collected along Cement Creek, organized by site, landform, type of sample, and stratigraphic position.—Continued

Field No. Lab No.1 δ13C per mil1 14C age years B.P.2 Calibrated ages3 Height above water, m6 Intercept(s)4 1-sigma range5 2-sigma range5

Samples from stratigraphic site 211, located on fig. 1; site is 430 m8 up South Fork Cement Creek (lat 37.8879° N., long 107.6490° W.)

Wood in gravel of flood plain (map unit p):

00-ABFC-211B Beta-147022 –25* 110±50 A.D. 1700–1950 A.D. 1680–1740

A.D. 1800–1930

A.D. 1660–1960 1.1 30 OGR9

.

Stump in growth position at surface of terrace (map unit t):

00-ABFC-211F Beta-147025 –25* 300±60 A.D. 1640 A.D. 1500–1660 A.D. 1450–1670

A.D. 1770–1800

3.75 10 OGR9

.

Stumps in growth position within gravels of terrace (map unit t):

00-ABFC-211D Beta-147024 –25* 1,110±70 A.D. 960 A.D. 880–1000 A.D. 770–1030 2.73 10 OGR9

.

00-ABFC-211C Beta-147023 –25* 1,670±70 A.D. 400 A.D. 260–290

A.D. 320–430

A.D. 230–540 0.95 30 OGR9

.

00-ABFC-211A Beta-147021 –25* 1,690±80 A.D. 380 A.D. 250–430 A.D. 140–540 0.65 30 OGR9

.

Samples from stratigraphic site 103, located on fig. 1; site is 305 m8 up South Fork Cement Creek (lat 37.8895° N., long 107.6507° W.)

Wood from gravel of apparent flood plain (map unit p), in unconformable contact with terrace sediment (map unit t):

99-ABFC-103G GX-26409 –22.4 440±50 A.D. 1440 A.D. 1430–1480 A.D. 1410–1520

A.D. 1590–1620

0.10

Wood from gravel section of terrace (map unit t) on right bank:

99-ABFC-103K GX-26396 –22.4 570±100 A.D. 1330–1400 A.D. 1300–1440 A.D. 1270–1490 1.8599-ABFC-103I GX-26395 –23.8 1,030±40 A.D. 1000–1020 A.D. 980–1020 A.D. 900–920

A.D. 960–1150

1.35

99-ABFC-103H GX-26394 –23.4 1,540±40 A.D. 538 A.D. 435–564

A.D. 572–577A.D. 589–597

A.D. 423–620 0.9

Peat or twigs from peat section of terrace (map unit subdivision tw) on left bank:

00-ABFC-212A Beta-147026-AMS –23.9 2,300±40 B.C. 390 B.C. 400–370 B.C. 410–360

B.C. 290–230

2.85 Peat.

00-ABFC-212B Beta-147027-AMS –28.1 3,680±40 B.C. 2040 B.C. 2130–2010 B.C. 2190–2170

B.C. 2150–1940

1.8 Peat.

00-ABFC-212C Beta-147028 –25* 4,920±70 B.C. 3680 B.C. 3770–3650 B.C. 3920–3870

B.C. 3810–3630B.C. 3560–3540

0.17 Twig in peat.

Stump in growth position from gravel beneath peat section (map unit subdivision tw) on left bank:

00-ABFC-212D Beta-147029 –25* 4,960±80 B.C. 3710 B.C. 3900–3890

B.C. 3800–3660

B.C. 3960 - 3640 0 20 OGR9

.

Samples from stratigraphic site 235 (see fig. 4), on figs. 1, 2, and 3; site is 400 m upstream of Ohio Gulch (lat 37.8557° N., long 107.6754° W.)

Wood in peat of terrace (map unit subdivision tw):

98-ABB-235t3 GX-24750 –27.4 Modern (108 %)7 2.6 4,875

98-ABB-235p0 GX-24747-AMS –23.1 470±50 A.D. 1440 A.D. 1410–1450 A.D. 1330–1340

A.D. 1400–1490

2.4 4,853

98-ABB-235t4 GX-24749 –26.5 725±120 A.D. 1280 A.D. 1210–1330

A.D. 1340–1390

A.D. 1030–1430 1.45 4,866

Distance on 

profile8, m; 

comments
Table 1. Data for radiocarbon samples collected along Cement Creek, organized by site, landform, type of sample, and stratigraphic position.—Continued

Field No. Lab No.1 δ13C per mil1 14C age years B.P.2 Calibrated ages3 Height above water, m6 Intercept(s)4 1-sigma range5 2-sigma range5

Samples from stratigraphic site 235, continued

Peat of terrace (map unit subdivision tw):

00-ABF-C235-3a Beta-148495 –25* 1,040±60 A.D. 1000 A.D. 970–1030 A.D. 890–1150 1.85 4,854

00-ABF-C235-4a Beta-148496 –25* 1,240±50 A.D. 775 A.D. 700–870 A.D. 670–895 1.3 4,854

96-ABS-123 Beta-119771 –26.7 1,640±70 A.D. 420 A.D. 260–270

A.D. 340–470A.D. 480–530

A.D. 240–560A.D. 570–580A.D. 590–600

1.0±0.1 4,848

Wood in peat of terrace (map unit subdivision tw):

98-ABB-235p3 GX-24748-AMS –27.2 2,490±40 B.C. 759–544 B.C. 765–519 B.C. 792–409 0.87 4,854

Stump in growth position from gravel beneath peat section (map unit subdivision tw):

99-ABFC-109 GX-26397 –24.5 2,840±80 B.C. 1000 B.C. 1180–1190

B.C. 1120–900

B.C. 1260–1230B.C. 1220–820

0.0 4,830; 20 OGR9

.

Samples of wood or charcoal from terrace sediments (map unit t) at various locations along Cement Creek

99-ABFC-122A Beta-135103 –22.6 3,610±50 B.C. 1950 B.C. 2030–1880 B.C. 2140–1870

B.C. 1840–1810B.C. 1800–1780

0.2 1,865; charcoal 

in silt beneath wetland.

99-ABFC-111 GX-26398 –22.5 1,680±40 A.D. 388 A.D. 263–276

A.D. 337–418

A.D. 256–304A.D. 316–432

0.3 495; wood 

in alluvial ferricrete.

99-ABFC-130C GX-26404-AMS –22.2 2,470±40 B.C. 757–541 B.C. 762–482

B.C. 467–412

B.C. 787–405 0.9 4,950; wood in 

inter-peat silt.

Samples of wood from terrace sediments along Middle Fork Mineral Creek (lat and long locations indicated)

00-ABFC-201 Beta-147007 –25* 790±70 A.D. 1260 A.D. 1190–1280 A.D. 1050–1100

A.D. 1140–1300

0.7 37.8448° N.

107.7318° W.

00-ABFC-202A Beta-147008 –25* 870±50 A.D. 1180 A.D. 1060–1230 A.D. 1030–1270 0.4 37.8441° N.

107.7290° W.

00-ABFC-200 Beta-147006 –25* 4,520±70 B.C. 3340–3200 B.C. 3360–3090 B.C. 3500–3460

B.C. 3380–3000B.C. 2980–2940

1.2 37.8451° N.

107.7323° W.

756 Environmental Effects of Historical Mining, Animas River Watershed, Colorado Distance on profile8, m; comments
Cement Creek Geomorphology and Ferricrete Deposits 757

Table 1. Data for radiocarbon samples collected along Cement Creek, organized by site, landform, type of sample, and stratigraphic position.—Continued

Field No. Lab No.1 δ13C per mil1 14C age years B.P.2 Calibrated ages3 Height above water, m6 Intercept(s)4 1-sigma range5 2-sigma range5

Miscellaneous samples from Cement Creek basin

Sample of wood at base of colluvium and on contact with till (lat 37.8926° N., long 107.6477° W.):

98-ABFC-177C GX-26406 –22.8 9,150±50 B.C. 8290 B.C. 8450–8280 B.C. 8530–8260

Sample of wood in alluvial ferricrete in Cement Creek above Gladstone (lat 37.8959o N., long 107.6463o W.):

99ABFC-176C GX-26405 –24.3 2,170±70 B.C. 200 B.C. 360–110 B.C. 390–40

B.C. 30–20B.C. 10–0

0.0

Wood in iron spring deposit overlying colluvial ferricrete in Dry Gulch (lat 37.8641o N., long 107.6791o W.):

99ABFC-185D GX-26407 –21.4 380±40 A.D. 1481 A.D. 1447–1622 A.D. 1437–1638 1.0

Charcoal from bog iron deposit overlying canyon-filling gravel (map unit g):

99ABFC-141A GX-26392 –24.4 4,010±110 B.C. 2560–2490 B.C. 2830–2350 B.C. 2880–2200 25 9,275 1Two commercial laboratories, Beta Analytic Inc (Lab. No. Beta-xxxxxx) and Geochron Laboratories (Lab. No. GX-xxxxx), analyzed the radiocarbon samples. Sample analysis done using accelerator mass 

spectrometry is denoted with the letters AMS. For 13C/12C results marked with an asterisk (*) the values were estimated to be –25 per mil based on values typical of the material type. 

2Conventional radiocarbon age (δ13C corrected), based on the Libby half life (5,570 years) for 14C, as reported by the laboratory in years before A.D. 1950 (B.P.). The error is ±1 sigma as judged by the 

analytical errors alone. The sample was crushed if necessary and dispersed in water. The eluted clay/organic fraction was treated in hot dilute 1N HCl to remove any carbonates. It was then filtered, washed, 

dried, and combusted in oxygen to recover carbon dioxide for analysis.

3Ages calibrated using the CALIB4.3 program based on Stuiver and Reimer (1993) with data from Stuiver and Braziunas (1993) and Stuiver and others (1998). The 1998 atmospheric decadal data set and 

laboratory error multiplier K=1 were used in the calculations.

4Maximum and minimum values given if the radiocarbon age intercepted the calibration curve at multiple locations. Age(s) rounded to the nearest decade if the standard deviation in the radiocarbon age 

was ≥50 years.

5Calibrated age range(s) using the intercept method. Ages rounded to the nearest decade if the standard deviation in the radiocarbon age was ≥50 years. 1 sigma = square root of (sample standard deviation2 + 

curve standard deviation2); 2 sigma = 2 × square root of (sample standard deviation2+ curve standard deviation2).

6Height above late summer (base flow) water level of stream at the site of the sample.7Sample with “modern” age had the given percentage of the A.D. 1950 14C activity, and is considered younger than A.D. 1850. 8Distances are positions on surveyed longitudinal profile (figs. 2 and 3). Sites 103 and 211 are on South Fork Cement Creek at specified position measured upstream from confluence with Cement Creek.9Specified number of outer growth rings (OGR) of the plant were sampled and analyzed.

Distance on 

profile8, m; 

comments
sampled. For example, large plant stems are always older than 

the deposit containing them, by a day or by centuries, because 

they grew before being incorporated into the deposit, and they 

may have been exhumed (reworked) from much older deposits. Logs and branches found in clastic deposits were dated, 

but care was taken in interpreting their ages. Lamina of wellstratified peat are unlikely to have been reworked, but care 

must be taken when the history of streams is inferred from 

dated peat, as discussed in the section titled, “Implications 

of Sedge Wetlands and Peat.” Twigs found in clastic deposits 

were not dated because of the possibility of misidentifying a 

root as a plant stem. Numerous tree stumps were discovered 

in growth position, having been buried and subsequently 

reexposed by streambank erosion. A stump rooted in gravel 

provides a highly reliable age constraint for the gravel deposits 

above and below its root crown, because a rooted stump in 

growth position could not have been reworked. We rely most 

heavily on stumps rooted in gravel and peat beds that were 

stratigraphically linked to channel gravel-deposits.

An inherent limitation clings to “young” radiocarbon 

samples, because the true calendar age of such samples is 

difficult to determine. The shape of the radiocarbon calibration curve between calendar years A.D. 1650 and 1950 is such 

that a radiocarbon age between 50 and 200 years intercepts 

the curve at more than one point. The correct intercept can be 

determined only by ruling out the others with independent evidence. For that reason we based our youngest age constraints 

on the growth rings of trees rooted in the Cement Creek flood 

plain.

Results: Nature and Distribution 

of Landforms

The results of our field observations and dating results 

are presented herein following the general chronological order 

of the landforms. Other observations are contained in the Map 

Unit Explanation of plate 5. This section is largely descriptive, 

and most interpretations are discussed in other sections.

General Distribution of Landforms

The longitudinal profile of Cement Creek (fig. 2) is 

convex in overall shape, with the lower half of the profile 

(below 5.2 km) being steeper than the upper half. The location 

of the tributary fan shed from Ohio Gulch coincides with this 

inflection at 5.2 km. The profiles of many alluvial streams, in 

contrast, are typically straight or concave.

In detail, the longitudinal profile consists of numerous 

straight reaches, with a conspicuous pairing of less steep 

reaches leading to steep reaches (fig. 2). One such pairing 

is shown in figure 3. There are also reaches of intermediate 

gradient that locally separate these pairs. The approximate 

gradients are 0.03 m/m for the “less steep” reaches, 0.04 m/m 

for the “intermediate” reaches, and 0.06 m/m for the “steep 

reaches.” The less steep reaches are 1–3 km long, and their 

streambeds are composed of pebble or cobble gravel, whereas 

the steep reaches are only 0.1–0.4 km long, and their streambeds are composed of boulders. The three main inflection 

points, where the profile becomes steep, are coincident with 

the locations of paired alluvial fans (fig. 2) shed from tributaries. At two locations tributaries enter Cement Creek from 

both sides such that they are almost directly opposed. These 

are Tiger and Georgia Gulches, and Ohio and Illinois Gulches 

(figs. 1 and 2; pl. 5). At the third inflection a pair of unnamed 

tributaries enter Cement Creek in close proximity from the 

northeast (fig. 1 and pl. 5).

Several landforms are found along the entire length of 

the study reach, but others are restricted to or absent from the 

lower 3 km of Cement Creek. Alluvial fans at the mouths of 

tributaries are generally spaced at regular intervals, but they 

are few in number in the lower reach where the contributing 

watersheds are small (fig. 1 and pl. 5). Landforms restricted 

to this lower reach are among the oldest in the Cement Creek 

basin.

Oldest Landforms

Glacial till (map unit m) is probably the oldest surficial 

deposit in the study area and is found on hillslopes adjacent 

to Cement Creek. Till occurs locally near Ohio Gulch, and 

more extensively near the mouth of Cement Creek including hillslopes adjacent to the town of Silverton. The deposits 

consist of clasts ranging in size from pebbles to boulders in a 

clayey sand to sand matrix. Deposit thickness is unknown. The 

till was deposited principally as lateral moraines. A minimum 

age for the till is 9,150±50 14C yr B.P. or 10,240 cal. yr B.P., 

the age of a log in colluvium overlying till (sample GX-26406, 

table 1).

A large landslide complex (map unit ls) was mapped on 

the northeast side of Cement Creek and surrounding Hancock 

Gulch (between 6 and 8.5 km in fig. 2). This large deposit 

consists of angular, poorly sorted clasts of variable size and 

variable matrix texture. The landslide has unknown thickness. 

It is surrounded by bedrock composed of Silverton Volcanics 

(Yager and Bove, this volume, pl. 1) and has high electrical 

conductivity indicating that it is wet and probably composed 

of altered rock (Smith and others, this volume, Chapter E4). 

The landslide surface is hummocky, is not obviously glaciated, and is locally cut by an alluvial fan (map unit fo), but is 

otherwise undated. We have no documented evidence that the 

landslide is active at present, and it does not appear to have 

had a large influence on the longitudinal profile of Cement 

Creek (fig. 2).

Canyon-filling stream gravels (map unit g) crop out 

500–800 m upstream of the mouth of Cement Creek (between 

9 and 9.3 km in fig. 2). These form nearly vertical cliffs of 

758 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 759

gravel on both sides of Cement Creek, and the top of the 

section is about 25 m higher than the streambed. The bottom 

of the section is about 9 m higher than the streambed and 

rests on a carved-bedrock platform that slopes downstream 

(fig. 2). The deposits are poorly sorted, weakly stratified, clastsupported, subrounded cobble gravel that has a sand matrix. 

The gravels are weakly imbricated and stratification dips about 

1° down Cement Creek. The gravel is strongly cemented with 

iron oxides. A minimum age for the gravels is 4,500 yr B.P., 

from charcoal (sample GX-26392 in table 1) found in bog-iron 

spring deposits capping the gravel. Other age constraints are 

inferred in the section titled, “Earliest Geomorphic History.”

A single remnant of a high stream terrace (map unit e) 

is about 20 m higher than the valley floor and is inset into a 

glacial moraine on the northwest side of the town of Silverton. 

The terrace deposits are well-rounded, clast-supported cobble 

gravel with a sand matrix, and have unknown stratification 

and thickness. The terrace surface is close to and is inclined 

away from the mouth of Cement Creek (fig. 2) and thus was 

likely formed by Cement Creek. The terrace surface projects 

to the base of the section of canyon-filling stream gravels (map 

unit g) in the Cement Creek canyon (fig. 2). The terrace thus 

formed after unit g.

Alluvial Fans

Alluvial fans in the Cement Creek valley emanate from 

tributaries and were classified as either old inactive fans (map 

unit fo) or young and active, or possibly active, fans (map 

unit fy) that are inset into the older fans.

Old alluvial fans emanate from the mouths of nearly all 

of the tributary gulches. The deposits consist of poorly sorted, 

subangular to subrounded, pebble to boulder gravel, and either 

are weakly stratified and clast supported with a sand matrix or 

are nonstratified and partially supported by a matrix of sand, 

silt, and some clay. The maximum exposed thickness is 6 m. 

Most of the fans are incised. On all of them, the bar and swale 

topography of the streams that formed them has been largely 

obscured by surface-intensive processes. The old fans are thus 

inactive, although some are sites of snow avalanche run-out. 

At and above Dry Gulch (at 1 km in fig. 2), the distal surfaces 

of the fans smoothly merge with the Cement Creek terrace 

(map unit t). Downstream of Dry Gulch, however, the toe 

of every fan is truncated by an erosional escarpment and the 

fan surfaces project as much as 5 m higher than the Cement 

Creek terrace (fig. 3). One streambank exposure reveals a 

buried disconformity where fan deposits are truncated and 

overlain by terrace deposits. These fans thus became inactive 

before the terrace formed, prior to 6 ka as discussed in the 

following section. Certain fans appear to have influenced the 

shape of Cement Creek’s longitudinal profile (figs. 2 and 3), 

as mentioned in the section titled, “General Distribution of 

Landforms.”

The young alluvial fans are typically inset into and are 

smaller than the older fans. The largest tributaries have young 

fans, whereas the smaller tributaries typically do not have 

young fans. The deposits consist of poorly sorted, subangular 

to subrounded, pebble to cobble gravel (with few boulders); 

they are either weakly stratified and clast supported with a 

sand matrix, or nonstratified and partially supported by a 

matrix of sand, silt, and some clay. The maximum exposed 

thickness is 2 m. The distal surfaces of these fans either merge 

smoothly with the Cement Creek terrace or are truncated by 

erosional escarpments. Many of these fans are active, such that 

roads that cross them occasionally require maintenance after 

water-flow or debris-flow events. The young fans may have 

been active for more than the last 6,000 years.

Cement Creek Terrace and Flood Plain

A prominent stream terrace, a remnant of a former flood 

plain, is found nearly continuously along Cement Creek and is 

designated map unit t and subdivision tw. The terrace surface 

is 1.5–3.5 m higher than the streambed. The terrace is typically 5–25 m wide, but locally may be as wide as 70 m. The 

maximum exposed thickness of the terrace deposits is 3.5 m. 

These deposits consist of two facies, one gravel and the other 

peat, which interfinger as shown in figure 4. Where the gravel 

facies is exposed, its surface is dominated by grasses or forest. 

Where the peat facies is exposed, its surface is dominated by 

sedge wetlands, including relict and active beaver ponds, or 

thickets of willow and bog birch. Where they occur adjacent 

to each other, the surfaces of gravel sections are generally 

0.5–1 m lower than the surfaces of peat sections, as illustrated 

in figure 4. Where the wetlands were large enough to be 

mapped separately they are designated with map unit tw.

The gravel facies is poorly sorted, weakly stratified, 

clast-supported, subrounded pebble or cobble gravel with sand 

matrix. Locally the gravel contains thin or medium (3–30 cm) 

beds of sand and sand lenses. The peat facies is subhorizontally 

stratified thin to thick (3–100 cm) beds of peat. Locally the 

peat contains very thin or thin (1–10 cm) beds of white or very 

light gray silt, which locally grade laterally into channel sands 

and gravels (fig. 4). The white silt layers contain no macrofossils of aquatic or terrestrial life forms (Elisabeth Brouwers, 

written commun., 2001). They have elevated aluminum 

contents, but no identifiable aluminum-bearing minerals were 

identified by X-ray diffraction (Rhonda Driscoll, analyst, 2001; 

mineralogical data in database, Sole and others, this volume, 

Chapter G). These observations are interpreted in the conclusions section titled, “Implications of Sedge Wetlands and Peat.”

The terrace deposits represent roughly synchronous channel and flood-plain aggradation, followed by stream incision, 

as interpreted in the conclusions section titled, “History of 

the Cement Creek Terrace and Flood Plain.” Dated materials 

found within terrace gravels, or dated peat observed to interfinger with gravels, are herein distinguished from dated peat 
located above gravels or not stratigraphically linked to gravel. 

The reason for this distinction is that peat continued to accrete 

after Cement Creek incised, as discussed in the section titled, 

“Implications of Sedge Wetlands and Peat.”

Dated materials from the terrace sediment span the past 

6,000 years (table 1 and fig. 5). Dated materials that are both 

directly associated with terrace gravels and located close 

to (within 1 m of) the present streambed level range in age 

from 3700 B.C. to about A.D. 400 (5,650–1,550 cal. yr B.P.). 

After A.D. 400, successively younger materials are found at 

successively higher positions within the gravel facies. Some 

dated peat samples, in contrast, are substantially older than 

gravels at the same stratigraphic level (fig. 5). The youngest datable material clearly associated with terrace gravels 

(sample GX-26396, table 1) has an age of A.D. 1330 or 1400 

(620 or 550 cal. yr B.P.), and provides a maximum age constraint on the cessation of gravel aggradation and thus stream 

incision. Two dated samples that are stratigraphically younger 

than terrace gravel provide an uncertain age constraint because 

the organic materials could have grown before or after Cement 

Creek incised. The oldest age of peat overlying terrace gravel 

is A.D. 1440 (510 cal. yr B.P.) for sample GX-24747. A wood 

fragment (sample GX-26409) found in sediment near the present streambed and against terrace sediment also had an age of 

A.D. 1440.

The Cement Creek flood plain, or low terrace (map 

unit p), also consists of gravel and peat facies. The nature of 

the flood-plain surface and sediment is nearly identical to the 

terrace (unit t) just described, so only unique aspects of the 

flood plain are presented. The flood plain is discontinuous 

and is found along only one-third of Cement Creek (pl. 5). 

The flood-plain surface is 0.5–1 m higher than the streambed 

and is typically 5–60 m wide. The maximum exposed thickness of the flood-plain deposits is 1 m. About half the area 

of the flood-plain surface is sedge wetlands (underlain by 

peat), including a few active beaver ponds and localized iron 

bogs. The landform is generally absent just downstream of 

Gladstone, and may have been remobilized during the 1978 

Lake Emma flood. Elsewhere, Lake Emma flood deposits have 

persisted on gravel bars that are slightly below the surface of 

this landform, suggesting that the landform is no longer being 

“actively constructed and maintained” by Cement Creek. As 

“active construction and maintenance” is the geomorphological definition of a flood plain, this landform may be a 

low terrace. In any case, the landform started to form before 

A.D. 1700, and the wetland and bog sediment continue to 

accumulate.

Eleven samples of organic material from flood-plain 

sediment were dated (table 1), and of these 4 are modern and 

the remainder could be modern. Five samples could have ages 

of A.D. 1660 (290 cal. yr B.P.), as that is their calibration 

intercept-age. However, given the 2-sigma radiocarbon laboratory uncertainties, four of these could be as old as A.D. 1510 

(440 cal. yr B.P.), or as mentioned, they all could be modern. 

The oldest materials associated with the flood plain that 

were precisely dated are tree stumps rooted in the flood-plain 

surface. For two Engelmann spruce stumps, slabs were cut 

and sanded, and the growth rings counted. Both stumps were 

observed in 1995, are situated about 1 m above low-flow water 

level, and retain about one-third of their bark. Both stumps had 

previously been cut with chain saws, but they could have been 

either standing dead snags or live trees when cut. One stump is 

located on the north side of the channel about 200 m downstream of the confluence with South Fork Cement Creek. A 

slab was cut from that stump: 271 growth rings were counted 

along one radius and 274 rings along a second radius. The 

second stump is located on the north side of the channel at 

1.95 km in figure 2 and has 222 growth rings (Church, Fey, 

and Unruh, this volume, Chapter E12, fig. 21, stump shown 

in photograph at far right).

The active stream channel and unvegetated bars of 

Cement Creek (map unit c), consists of sandy pebble-gravel, 

cobble-gravel, and, locally, boulders. Bedrock is exposed in 

the bed only in the slot canyon at the mouth of Cement Creek 

(between 9.2 and 9.7 km in fig. 2). Stream channels are typically 5–10 m wide, but locally are as wide as 40 m.

Large areas obscured by human activities, principally 

related to mining since A.D. 1871, are designated with map 

unit h.

Distribution of Ferricrete and Wetlands

Although ferricrete is exposed at many locations along 

the length of Cement Creek, ferricrete is not continuous. 

Ferricrete exposures are most abundant in two reaches, and 

exposures of wet ferricrete show a correspondence with the 

locations of sedge wetlands. Many of the observations presented here set the stage for Wirt and others (this volume).

Ferricrete exposures visible from the channel were noted 

while we were surveying the longitudinal profile, and their 

heights above the streambed and distances downstream were 

measured (fig. 6A). We also noted whether the ferricrete was 

wet with water seeping from the exposure or was dry at the 

time, which was late summer 2000 when the flow was low 

(von Guerard and others, this volume, fig. 3). In general, wet 

ferricrete exposures are much less frequent than dry ferricrete 

exposures. Most surficial sediment along Cement Creek is 

part of the terrace and flood plain (pl. 5). For that reason most 

ferricrete visible from the channel consists of cemented terrace and flood-plain gravel, and these deposits do not extend 

higher than 3 m above the channel bed. Higher exposures are 

of cemented fan gravel (unit fo), colluvium, and canyon-filling 

stream gravel (unit g, between 9 and 9.3 km in fig. 6A). For 

convenience in discussing the spatial pattern of ferricrete and 

wetlands, we subdivided our study area into the upper, middle, 

and lower ferricrete reaches shown in figure 6A. The upper 

reach extends from the South Fork confluence to just below 

Fairview Gulch (from 0 to 3.3 km in fig. 6A; see fig. 1). The 

760 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 761

43210

-4,000 -3,000 -2,000 -1,000 0 1,000 2,000

Site 211

Site 235 (fig. 4)

Site 103 on south bank

Site 103 on north bank

Site 103, peatSite 103, wood in gravelSite 211, peatSite 211, wood in gravelSite 235, peatSite 235, wood in gravelOther sites, peatOther sites, wood in gravel± 2 sigma

EXPLANATION

HEIGHT ABOVE LOW WATER, IN METERS

CALIBRATED AGE, YEARS BC (-) AND AD (+)

Figure 5. Time-stratigraphic relations of sediment composing the prominent terrace (map unit t) along Cement Creek, for four stratigraphic sequences 

(numbered localities of fig. 1), and several individual beds. See table 1 for methods and other details on radiocarbon ages, and site localities. For the stratigraphic exposure at site 235 (fig. 4) ages are plotted relative to water level at dated stump at base of the section, and the age of sample GX-24749 was placed in correct stratigraphic position at horizontal distance of 25 m in figure 4.
-2

86420

1210

0 1 2 3 4 5 6 7 8 9 10

HEIGHT ABOVE STREAMBED, IN METERS

DISTANCE DOWNSTREAM FROM SOUTH FORK CEMENT CREEK, IN KILOMETERS

Sedge

wetlands

EXPLANATION

lower middle ferricrete reach upper

Ferricrete

not

surveyed

Wetlands, left bankWetlands, right bankDry ferricreteWet ferricreteExposed rock

A

762 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 763 Figure 6. Ferricrete and wetland locations correlated with inflow discharge along Cement Creek. 

A, Spatial location of ferricrete exposures visible from the 

channel, expressed as height above streambed and location along surveyed longitudinal profile (fig. 2), and location along the profile of sedge wetlands. Ferricrete polygons show continuity and varying thickness. Bars connecting surveyed wetland points show wetland continuity. 

B, Magnitudes, and spatial locations along 

the longitudinal profile, of water flowing into Cement Creek during late summer 1996 (Kimball and others, 2002). Height of each bar represents change in Cement Creek discharge over a short section of stream depicted by width of bar. As such, the areas of the bars are meaningless.

50

40353025201510

0 1 2 3 4 5 6 7 8 9 10

INFLOW DISCHARGE, 

IN LITERS PER SECOND

DISTANCE DOWNSTREAM, IN KILOMETERS

B

From tributary or mineFrom springs or seepsFrom bogs and wetlandsSource not observed

EXPLANATION
middle reach extends downstream to Hancock Gulch (from 3.3 

to 7.2 km in fig. 6A). The lower reach extends downstream to 

Soda Gulch (from 7.2 to 8.8 km in fig. 6A). Our surveying of 

ferricrete stopped at 8.2 km, for logistical reasons, so we rely 

on the ferricrete mapping of Yager and Bove (this volume, 

pl. 2) to define the downstream end of the lower reach to be 

at 8.8 km. Elsewhere our documentation of the occurrence of 

ferricrete is consistent with that of Yager and Bove.

Along Cement Creek exposures of both wet and dry ferricrete, and also wetlands, are most abundant in the upper reach. 

Considering all the wet ferricrete exposures along Cement 

Creek, we noted that more than half occur in the upper reach. 

Most of the wet ferricrete exposures in the upper reach, and all 

the large ones, are found on the west side of Cement Creek in 

a short subreach. That subreach (from 1.3 to 2.4 km in fig. 6A) 

corresponds to the confluence of Prospect Gulch, which enters 

from the west (fig. 1). Most of the sedge wetlands, 80 or 

90 percent of the area of wetlands in the upper reach, are also 

found on the west side of Cement Creek. In the middle reach, 

in contrast, both ferricrete exposures and wetlands are sparse. 

Ferricrete exposures are also abundant in the lower reach, but 

both wet ferricrete and wetlands are relatively sparse in that 

reach. In conclusion, where exposures of wet ferricrete occur, 

wetlands are usually located nearby. Where there are wetlands, 

however, exposures of wet or dry ferricrete are not necessarily 

located nearby.

Ferricrete exposures do not correspond to the magnitude 

of water flowing into Cement Creek or to the hydrological 

source of that water. During low-flow conditions the discharge 

in Cement Creek increases as one moves downstream, but 

not at a steady rate. Figure 6B depicts the changes in Cement 

Creek discharge quantified in September 1996 by Kimball 

and others (2002) as part of their tracer dilution study. They 

noted the locations and hydrological source of any visible 

inflowing water. By “source” we mean surface flow from a 

tributary stream or mine, surface flow from a bog or wetland, 

or subsurface flow emerging as springs or seeps. Ferricrete 

exposures are found in subreaches where Cement Creek 

gained water from each type of source, and even in subreaches where the discharge increased but no discrete source 

of inflowing water was observed in the field. We documented 

no obvious association between the occurrences of ferricrete 

exposures and the hydrological source of inflowing water. 

Ferricrete exposures are found in subreaches where the rate 

of water flowing into Cement Creek was relatively small and 

relatively large. Three exposures of wet ferricrete (at 0.65, 

4.75, and 7.5 km in fig. 6A) and numerous exposures of dry 

ferricrete are located in subreaches where no increase in the 

base flow of Cement Creek was detected (Kimball and others, 

2002). In addition, the middle reach where ferricrete exposures are least frequent (from 3.3 to 7.2 km in fig. 6A) is also 

the reach where Kimball and others (2002) found the greatest 

increase in the base flow of Cement Creek (fig. 6B). There 

is no obvious association between the presence of ferricrete 

deposits and the rate at which water flows into Cement Creek 

nearby.

Interpretations and Conclusions

Earliest Geomorphic History

The high San Juan Mountains were nearly covered by 

alpine glaciers during the latest Pleistocene (Atwood and 

Mather, 1932) and presumably numerous times before. During 

the most recent episode, generally referred to as the Pinedale 

glaciation, ice was about 520 m thick at the present site of 

Silverton and 430 m thick at the present site of Gladstone. 

The general timing of that glaciation is presented because it 

constrains the ages of deposits found along Cement Creek.

The onset of the latest glaciation in the Rocky 

Mountains is poorly understood, but it may have begun 

about 30,000 14C yr B.P. (Nelson and others, 1979). The timing of the glaciers’ maximum extent has been estimated based 

on radiocarbon ages for sediment of lakes impounded at ice 

margins near terminal moraines. Two such results are that 

valley glaciers reached their maximum extent slightly after 

23,000 (Rosenbaum and Larson, 1983) and 22,000 14C yr B.P. 

(Madole, 1986). Thus it apparently took about 7,000 years 

(30–23 ka 14C years) for the ice to advance to its maximum 

extent, which is a reasonable general duration for expansion 

of the San Juan glaciers because ice volume was large and 

some individual glaciers were long. Ice flowed more than 

60 km from the Silverton area to the terminus at Durango. 

Leonard (1989) concluded that during the full glacial, summer 

temperatures were 8°–13° C cooler than present. The timing 

of the glaciers’ initiation of retreat has been estimated from 

radiocarbon ages of material found in sediment resting on till 

upstream of terminal moraines. Two such results indicate that 

valley glaciers began to retreat before 13,800 (Madole, 1976) 

and 13,700 14C yr B.P. (Nelson and others, 1979). The altitudes 

of the sites where those dates were obtained are relatively 

low, 2,640 and 2,882 m respectively. Note that the altitudes 

of Silverton and Durango are 2,830 and 1,990 m, respectively. 

A similar age of 13,700 14C yr B.P. was obtained by Madole 

(1980) for a high-altitude site at 3,156 m. In addition, Gosse 

and others (1995) concluded that glaciers began to retreat 

from their terminal moraines in the Wind River Range of 

Wyoming about that same time (assuming the years from their 

10Be method are equivalent to calendar years). Deglaciation 

probably began earlier in the San Juan Mountains, however, 

compared to more northerly Wind River Range. At face value, 

one could conclude that full glacial conditions lasted for 

about 9,000 years (23–14 ka 14C years), and that most of the 

ice disappeared quite rapidly; but this may be misleading for 

three reasons. First, it may have taken considerable time for 

vegetation both to colonize the previously glaciated terrain and 

then to become incorporated into sediment. Second, the oldest 

organic material in a given deposit may not be exposed and 

available for sampling. Third, the duration of the unconformity 

between glacial and postglacial deposits is uncertain. For these 

reasons, estimates of when areas became ice free, based on 

radiocarbon dating, are likely too young by centuries if not 

millennia.

764 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 765

Cirques were the last sites to become ice free, and the 

cause of the disappearance of the last remaining ice may have 

been the anomalously warm Bölling-Alleröd interval, which 

occurred between 12,500 and 10,700 14C yr B.P. (Stuiver and 

others, 1995). Estimates for deglaciation of individual cirques 

in Colorado typically range from 10,000 to 12,000 14C yr B.P. 

Several reasons are advanced for the relatively large range 

in these ages. Variable relief and aspect of individual cirques 

made the conditions for ice retention variable. Some individual dates may be too young, for the reasons just mentioned. 

Lastly, there is some evidence for minor glacial advance 

(Menounos and Reasoner, 1997) in the Colorado Front 

Range, during the Younger-Dryas event between 10,700 and 

10,100 14C yr B.P. (Reasoner and Jodry, 2000). Whether this 

minor glacial advance occurred in the San Juan Mountains is 

not known. One site, (former) Lake Emma at an altitude of 

3,740 m, deserves discussion because it is close to Cement 

Creek (fig. 1). Elias and others (1991), following Carrara and 

others (1984), studied the lake sediment and found disparate 

radiocarbon ages for moss, wood, and insects. We mention 

only the ages for wood fragments, because wood is probably the most reliable material for dating. The oldest age for 

a wood fragment was 9,400±70 14C yr B.P., but that was not 

from the base of the lake sediment. A slightly older fragment 

of wood (9,580±130 14C yr B.P.) was found in a nearby bog by 

Carrara and others (1984). Elias and others (1991) concluded 

that Lake Emma was ice free “by 10,000 yr B.P. and perhaps 

as early as 11,000 yr B.P.” Note that the oldest radiocarbon 

sample (GX-26406, table 1) from this study has an age of 

9,150±50 14C yr B.P. for a spruce log resting on till at an altitude of 3,230 m. Clearly hillslopes adjacent to Cement Creek 

were forested by that time.

The oldest stream deposits along Cement Creek likely 

formed, or began to form, during deglaciation. During deglaciation a net loss of ice occurs as the equilibrium line shifts 

upstream. The terminus recedes upstream by melting and 

sublimation, but below the equilibrium line the ice also thins, 

which can lead to fragmentation of the ice. The glacier in 

Cement Creek was relatively narrow and more than 20 m thinner than the main glacier in Bakers Park into which it flowed. 

(Bakers Park is the early name for the uniquely wide portion 

of the Animas River valley within which Silverton is now 

located (Bird, 1986).) Two features suggest that the Cement 

Creek tributary glacier had receded from the mouth of Cement 

Creek, while a massive body of stagnant ice still partially 

filled Bakers Park. The top of the canyon-filling gravels (map 

unit g) currently projects to a position about 40 m higher than 

the current floor of Bakers Park, based on the dip of bedding 

(fig. 2). We interpret this deposit to be glacial outwash (from 

Cement Creek) that was impounded by stagnant ice in Bakers 

Park. After map unit g aggraded, Cement Creek incised 

through it as ice in Bakers Park ablated. Deposits similar in 

appearance occur at, and downstream of, the confluence of 

Mineral Creek and Middle Fork Mineral Creek (Yager and 

Bove, this volume, pl. 2). Those deposits are tens of meters 

thick and are limited in spatial extent, suggesting that they 

too formed behind a temporary ice impoundment.

A high stream terrace (map unit e) is inset into till about 

20 m higher than the floor of Bakers Park, and its surface 

projects to the base of unit g gravels in the mouth of Cement 

Creek (fig. 2 and pl. 5). We interpret this terrace to have been 

formed by Cement Creek as it flowed along the margin of 

stagnant ice in Bakers Park. Thus, the high terrace formed 

during deglaciation, and there may be no other correlative 

landforms in the region. The large landslide in the vicinity of 

Hancock Gulch (unit ls, pl. 5) is not obviously active, has no 

obvious effect on the longitudinal profile of Cement Creek 

(fig. 2), and is crosscut by and thus predates an old alluvial 

fan (unit fo). It is plausible that when ice disappeared from 

that part of Cement Creek and Hancock Gulch, the release of 

confining pressure on the presumably saturated unconsolidated 

material allowed the landslide to move. Glacial till, unit m, 

obviously stopped forming by the end of glaciation. We suspect that the canyon-filling gravels (map unit g) and the high 

stream terrace (map unit e), and possibly the landslide (map 

unit ls), are on the order of 12,000±2,000 14C years old, 

because of the apparent association of those landforms with 

deglaciation.

Prominent alluvial fans (unit fo), located at the mouths 

of most tributaries, also probably began to accumulate as soon 

as ice disappeared from the floor of the Cement Creek valley. 

This timing is inferred because as small glaciers retreated up 

tributaries, the newly exposed rubble on hillslopes, lacking 

the stabilizing influence of vegetation, would have quickly 

become subject to sub-aerial detachment and transport 

process. The fans may have accumulated rapidly until the 

surrounding hillslopes did become stabilized by vegetation. In 

any case, the absence of paleosols in the fan gravels indicates 

that no significant pauses in deposition occurred. The fans 

stopped forming because their streams incised when Cement 

Creek breached their toes. This occurred prior to 6,000 years 

ago, as discussed in the next section. The bulk of the fan 

deposits have not been removed by Cement Creek (pl. 5), and 

certain fans remain the primary influence on the segmented 

shape of the longitudinal profile of Cement Creek (figs. 2 and 

3). Certain large fans, or pairs of fans, apparently caused the 

Cement Creek valley to aggrade (upstream of the fans), or 

kept the upper reaches from incising. The Ohio Gulch fan is 

a particularly good example (fig. 3). This is the largest fan in 

the study area and thus has had the largest sediment supply, 

although the watershed is not particularly large (fig. 1). Compared to other subbasins, large areas of Ohio Gulch hillslopes 

are composed of extensively altered bedrock (Bove and others, 

this volume) and are devoid of vegetation, which apparently 

allowed rapid erosion. Cement Creek was not capable of 

removing all of the sediment supplied by that tributary, and 

Cement Creek aggraded upstream of the obstruction. The 

resulting segmented profile (fig. 3) is common along montane 

streams (Miller and others, 2001; K.R. Vincent, unpub. data, 

2003), particularly where the valleys are narrow. The Cement 

Creek valley bottom is less than 150 m wide, in contrast to 

the 400 m wide bottom of the Animas River valley between 

Howardsville and Eureka (Vincent and Elliott, this volume, 
Chapter E22, fig. 1). In that Animas River reach, tributary 

fans enter the valley in direct opposition and at those locations 

cover nearly the full width of the valley floor, but without 

effect in that the longitudinal profile of that portion of the 

Animas River valley is not segmented (Vincent and Elliott, 

this volume, fig. 3).

History of the Cement Creek Terrace and 

Flood Plain

The aggradation and incision of the Cement Creek 

terrace may have been driven by something related to climate, 

so the results of two studies of postglacial climate are presented. The former Lake Emma site (fig. 1) is currently above 

treeline, but its lake sediments contain numerous fragments 

of early Holocene wood. Carrara and others (1991), using 

data from former Lake Emma and several bogs in the area, 

concluded that between 9,600 and 5,400 14C yr B.P., treeline 

in the northern San Juan Mountains was at least 80 m higher 

than at present, suggesting that average July temperatures 

were 0.5°–0.9° C higher than at present. Between 5,400 and 

3,500 14C yr B.P., treeline was near its present-day limits, and 

after 3,500 14C yr B.P., it was generally lower than present 

(Carrara and others, 1991). Fall (1997) studied pollen and 

plant macrofossils from various sites in the Colorado Rocky 

Mountains and arrived at similar conclusions. In the early 

Holocene (from 9,000 to 6,000 14C yr B.P.), the climate was 

warmer and wetter than today, cooler and dryer conditions 

dominated the middle Holocene (6,000–4,000 14C yr B.P.), and 

the modern climatic regime was established about 2,000 14C yr 

B.P. with slightly warmer than present conditions persisting 

until about 1,000 14C yr B.P. (Fall, 1997). Shorter term climatic 

fluctuations also occurred during the Holocene, such as the 

so-called Medieval Warm Period and Little Ice Age discussed 

at the end of this section.

The early Holocene history of Cement Creek is not 

known, but the streambed was probably at a higher level 

than today, having been impounded by alluvial fans. By 

about 6,000 years ago the stream had partly breached the 

fans (fig. 3), and the bed of Cement Creek was at the same 

level as it is today. This is evidenced by the 3700 B.C. age 

(5,650 cal. yr B.P.) of an upright stump found in terrace sediment at the present low-flow water level (fig. 5). During the 

middle Holocene, Cement Creek migrated laterally but 

apparently remained at the same level as it is today for about 

4,400 years. This is evidenced by six radiocarbon ages from 

samples found within 30 cm of present low-flow water level 

(fig. 5), at various locations along the stream. These ages 

range from 3700 B.C. to A.D. 380 (5,650 to 1,570 cal. yr 

B.P.). In apparent contrast, one sample of peat obtained from 

1.8 m higher than the streambed (fig. 5) had a radiocarbon age 

within this period. In the next section, we use that observation 

as supporting evidence that peat accumulated in sedge wetlands, and continues to accumulate, independently of the level 

of the gravel bed of Cement Creek.

The bulk of the deposits of the prominent terrace along 

Cement Creek represent channel and flood-plain aggradation, followed by stream incision. The gravel facies is the 

key because it represents deposition on the bed and bars of 

the stream. After A.D. 400 (1,550 cal. yr B.P.), successively 

younger materials are found at successively higher positions 

within the gravel facies, within all three well-dated stratigraphic sections (fig. 5). The aggradation persisted at least 

until A.D. 1330 or 1400 (620 or 550 cal. yr B.P.), because that 

is the age of the youngest datable material clearly associated 

with terrace gravel (sample GX-26396, table 1). The episode 

of channel and flood-plain aggradation was not unique to 

Cement Creek. A terrace is found at many locations along 

Mineral Creek that is similar to the Cement Creek terrace in 

appearance and height above streambed. Mineral Creek drains 

the area immediately to the north and west of the Cement 

Creek watershed (Yager and Bove, this volume, pl. 2). Two 

wood fragments (Beta-147008 and 07, table 1) from the terrace along Middle Fork Mineral Creek have radiocarbon ages 

of A.D. 1180 and 1260 (770 and 690 cal. yr B.P.), and these 

ages fall within the period of aggradation of the Cement Creek 

terrace.

The timing of stream incision has constraints in addition to the ones already mentioned. The oldest age of peat 

overlying terrace gravel is A.D. 1440 (510 cal. yr B.P.) for 

sample GX-24747, and a wood fragment (sample GX-26409) 

found in sediment near the present streambed and against terrace sediment also has an age of A.D. 1440. Interpretation of 

these data is tenuous, however, because the sediments encasing 

these two samples are stratigraphically younger than terrace 

gravel; thus the organic materials could have grown before or 

after Cement Creek incised. The Cement Creek flood plain 

developed after stream incision, but radiocarbon ages for that 

landform are inconclusive. Of the 11 dated samples of organic 

material from flood-plain sediment, 4 are modern. A “modern” radiocarbon age means it is younger than A.D. 1850. 

Five samples could have ages of A.D. 1660 (290 cal. yr B.P.), 

as that is their calibration intercept-age. Taken at face value, 

the flood plain could have been present as early as A.D. 1660. 

Given the radiocarbon analytical uncertainty and the problem 

of calibrating young samples, however, four of these samples 

could be as old as A.D. 1500 (450 cal. yr B.P.), or all radiocarbon samples from the flood plain could be modern. The oldest 

accurately dated materials associated with the flood plain are 

tree stumps rooted in the flood-plain surface. One Engelmann 

spruce stump has 274 growth rings, and since we observed this 

stump in 1995 the tree must have germinated by A.D. 1721 

if not long before, depending on when the tree died. About 

one-third of the bark remained on the stump at the time of 

sampling, and thus it had been dead for several decades, but 

less than a century (J.M. Friedman, written commun., 2002). 

We conclude that the tree germinated around A.D. 1700, and 

the Cement Creek terrace necessarily incised before that. A 

second spruce stump has 222 growth rings, confirming the 

general antiquity of spruce rooted in the flood plain.

766 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 767

In conclusion, the Cement Creek terrace incised between 

A.D. 1330 and A.D. 1700, strictly based on our most conservative interpretation of the dating results. Yet the preponderance of less reliable evidence suggests that incision may have 

occurred before A.D. 1500 and perhaps about A.D. 1440.

The aggradation and incision of the Cement Creek 

terrace were not caused by local base-level processes, based 

on the extensive nature of the terrace and other factors. 

Aggradation driven by a base-level rise, such as impoundment behind an aggrading fan, does not extend long distances 

upstream (Leopold and Bull, 1979). Since the Cement Creek 

aggraded only 3 m, that distance would be on the order of 

several hundred meters. Yet the terrace is nearly continuous 

along the 10 km long study reach. The sites where synchronous aggradation was documented (fig. 6) are separated by 

as much as 5.3 km. (As mentioned, a similar terrace is found 

along Mineral Creek.) In addition an agent of base-level rise is 

lacking. Aggradation of a young alluvial fan might have been 

such an agent for localized aggradation of Cement Creek, but 

young fans are nearly absent from the reach above Prospect 

Gulch, for example, where the terrace is continuous and not 

segmented. Similar arguments can be made against terrace 

incision being caused by base-level fall, in that the stream is 

everywhere incised and the terrace profile is parallel to the 

streambed profile (figs. 2 and 3).

A common explanation for the aggradation and incision of a landform like the Cement Creek terrace is to call 

upon climate change (Bull, 1991), but the exact processes or 

process interactions are rarely understood. Indeed the onset 

of Cement Creek aggradation occurred after the “modern 

climatic regime” became established about A.D. 0 (Fall, 

1997) and shortly before the so-called Medieval Warm Period, 

which began around A.D. 850 (Briffa and Osborn, 2002). In 

addition the stream incised about the time the Little Ice Age 

began about A.D. 1550 (Beniston and others, 1997), although 

the exact nature and timing of the so-called Little Ice Age 

are debated. In simple terms, streams aggrade because more 

sediment is supplied than can be carried downstream. That 

might result if sediment yields from hillslopes increased 

(due to decreased vegetation cover or some other cause) 

while the basic hydrology was unchanged, or if the frequency 

or magnitude of large floods decreased while sediment yield 

was unchanged. Other explanations include increased flow 

drag due to increased cover by riparian plants, decreasing the 

ability of the stream to transport sediment. Beaver foster riparian plants, and changes in beaver populations might change 

the density of woody riparian plants and the flow drag they 

create. Beaver were present in the San Juan Mountains during 

prehistorical times, as evidenced by the 173 growth rings we 

extracted from a living spruce rooted in a relict beaver dam 

near Gladstone, in 2000. That beaver dam was constructed 

before A.D. 1818. Stream incision is also explained by climate 

change altering the balance of sediment supply and transport, 

but streams can incise during single large floods, which is not 

exactly “climate change.” When multiple streams in an area 

are documented to have changed in similar ways at the same 

time, something regional in nature like climate is assumed 

to be the cause. For example, Hereford (2002) summarized 

information for seven well-studied streams in the southern 

Colorado Plateau region and concluded that they had similar dynamics occurring roughly synchronously. Following 

a period of incision or “major channel adjustment,” those 

streams began to aggrade about A.D. 1400, and all incised 

within several decades of A.D. 1880. Hereford (2002) called 

upon changes in the magnitude and frequency of floods as 

the cause of the changes. The spatial scale over which similar 

and synchronous, climate-driven channel changes can occur 

is limited, however, because the timing and nature of climate 

change vary spatially, and because streams vary in size and 

physical and biological properties and processes. The dynamics of Cement Creek were out of phase with those of the 

streams studied by Hereford (2002), for example, even though 

Cement Creek is only 400 km northeast of the area of most of 

the streams that he discussed.

Finally, the aggradation and incision of the Cement Creek 

terrace were not caused by local base-level processes, but 

rather something basin wide in scope. These changes may be 

related to climate change, but the exact mechanisms are probably not knowable.

Implications of Sedge Wetlands and Peat

Our study of the prominent terrace along Cement 

Creek leads to three important conclusions regarding the 

paleo-environment of flood-plain peat in subalpine settings. 

Currently, ground water from the valley margin supplies the 

sedge wetlands, not the mainstem stream, and this may have 

always been the case. Great care must be taken when one uses 

peat to date stream incision, because the peat can continue 

to accrete after stream incision. The low pH of water in the 

sedge wetlands has leached the inter-peat silt strata of their 

trace-element and some major-element constituents, and the 

original mineralogy of the detrital phases was converted to 

that of amorphous phases.

The stratigraphy of the prominent terrace along Cement 

Creek (fig. 4) represents a period of channel and flood-plain 

aggradation followed by stream incision. The aggradation 

occurred over a thousand years, and the surface became a 

terrace when Cement Creek incised more than three centuries 

ago, as discussed in the preceding section. The gravel facies 

represents aggradation of the channel bed and bars, and the 

peat facies represents aggradation of flood-plain sedge wetlands. These facies interfinger at their margins (fig. 4), demonstrating contemporaneous aggradation at equivalent rates, but 

the processes differed. Stream aggradation resulted from the 

deposition on the bed of coarse clastic sediment supplied from 

upstream, whereas wetland aggradation was dominated by the 

growth and local deposition of plant material. If peat accumulation had been slower than channel aggradation, low areas 

of the flood plain would have been filled with gravel during 

lateral migration of the channel. Stratigraphic evidence of this, 

thin peat beds between thicker gravels, was observed (fig. 4), 

but only locally. The silt beds found within the peat section in 
figure 4 can be traced laterally to where they grade into sand 

and gravel streambed facies. We interpret the silt beds to be 

the result of deposition of suspended sediment advected onto 

the flood plain during major floods, but the silt beds are thin 

and few in number. For example, in figure 4 only three silt 

beds are visible in a stratigraphic section representing about 

2,500 years of aggradation of peat. Thus overbank flows did 

not supply the bulk of the flood-plain sediment, nor do they 

now, and apparently overbank flows did not supply the water 

required to support the wetland plants.

The terrace surface is 1.5–3.5 m higher than the streambed. The flood-plain deposits are inset into terrace deposits, 

and the flood-plain surface is 0.5–1 m higher than the streambed. Debris-flow deposits from the accidental draining of Lake 

Emma (Jones, this volume) are found on the channel margin 

just below the flood-plain level, and these appear to have 

experienced very little reworking since they were deposited in 

1978. Streamflows probably never flood the terrace and only 

rarely cover the flood plain. At present, wetlands are found at 

various locations on both the Cement Creek terrace and flood 

plain. Sedges dominate these wetlands, and willow and bog 

birch shrubs are found locally. Relict and active beaver ponds 

are found locally. Even during the dry season of late summer, 

the wetland soils remain either moist or are covered by shallow standing water (see Yager and Bove, this volume, pl. 2, 

figs. 14 and 15). These wet places must be supplied by ground 

water seeping through hillslope colluvium or tributary fan 

alluvium along the valley margin. The surface of the terrace 

is higher where it is underlain by peat compared to where it 

is underlain by gravel (fig. 4). Even though the terrace may 

have incised as early as A.D. 1400, peat overlying gravel has 

ages ranging from A.D. 1440 to modern, and the wetlands 

are actively accumulating peat. The terrace peat sections continued to accrete in wetlands after the gravel section stopped 

aggrading when Cement Creek incised.

From this we draw two conclusions. As the wetlands on 

the terrace are still accreting, great care must be taken when 

one uses radiocarbon ages of peat to constrain the timing of 

stream incision. In our discussion of the timing of incision of 

Cement Creek, we used dated peat only where the peat clearly 

interfingers with clastic streambed sediment. We also conclude 

that peat beds composed of sedges in settings like Cement 

Creek accumulated independently from stream processes, and 

that they are an indicator that ground water seeping from the 

valley sides was the source of water in prehistorical times.

Acidic ground water has also modified thin layers of 

fine-grained (silty) overbank sediment within the peat. The 

chemistry and mineralogy of the inter-peat silt are significantly different from those of modern streambed sediment 

in Cement Creek, and worthy of discussion. In figure 7, we 

compare the chemistry of the major (7A) and selected trace 

elements (7B) in the inter-peat silt, modern bed sediment from 

Cement Creek, and premining sediment from sites B13 and 

B16 on Cement Creek (Church, Fey, and Unruh, this volume). 

Compared to both the sediment from the active bed of Cement 

Creek and the premining sediment samples, the inter-peat 

silt strata are depleted in iron, and enriched in potassium and 

aluminum. In addition, inter-peat samples generally have the 

lowest overall concentrations of calcium and magnesium, and 

the highest concentrations of titanium. With regard to the trace 

elements that reflect contributions from historical mining, the 

concentrations of most trace elements except arsenic (fig. 7B) 

are enriched in modern streambed sediment relative to the 

premining sediment samples from both sites. The median 

concentration of all trace elements except cadmium in the 

inter-peat silt is significantly less than in the lower of the two 

premining sediment sites.

The inter-peat silt samples represent the end result of 

acidic weathering of overbank sediment from the Cement 

Creek drainage that has been leached by acidic ground water 

in the sedge bogs that form the peat deposits. Results from 

X-ray analysis of the mineralogy of all the inter-peat silt 

samples failed to identify primary minerals bearing aluminum. 

Trace amounts of muscovite and albite were identified (Sole 

and others, this volume), but not in adequate abundance to 

account for the large amount of aluminum (>7.5 wt. percent) present in the inter-peat silt strata. Quartz was always 

identified. Given the low pH of water in the sedge wetlands 

(3.0–4.5; Wirt and others, this volume), the depleted traceelement concentrations, and the lack of primary minerals, we 

interpret the data to indicate that the inter-peat silt strata have 

been leached of many trace- and some major-element constituents. The original detrital minerals have been converted to 

amorphous compounds. Since their deposition, the inter-peat 

deposits have been a minor source of dissolved trace elements 

and are currently a source of amorphous aluminum to Cement 

Creek, as erosion removes the peat deposits.

Implications for the Timing and Processes 

of Ferricrete Precipitation

Water is required for the formation of ferricrete, because 

it is the medium of transport of iron and sulfur, but the locations of exposed ferricrete along Cement Creek show no correspondence with the rate of water flowing into Cement Creek 

during low-flow conditions and only one association with the 

hydrological source of water. Ferricrete exposures are found 

in subreaches where water flowing into Cement Creek is from 

tributary streams, wetlands, bogs, springs, and seeps; ferricrete 

is even found in subreaches where the discharge increased but 

no discrete source of inflowing water was observed (fig. 6B). 

Ferricrete exposures are found in subreaches where the rates 

of water flowing into Cement Creek are relatively large, 

relatively small, and not detected using the tracer method 

(Kimball and others, 2002). In addition, the reach where ferricrete exposures are least frequent is also the reach of greatest 

increase in the base flow of Cement Creek (Kimball and others, 2002). In conclusion, the hydrological source (tributary 

streams, ground-water seeps, and so forth) and the rates at 

which water flows into Cement Creek during base-flow conditions are not useful predictors of the occurrence of ferricrete 

deposits and might not be linked to the processes of ferricrete 

formation in a general way.

768 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 769

One spatial association of hydrology and alluvial ferricrete was observed by this study, and that of Wirt and others 

(this volume). Wet ferricrete exposures are almost invariably 

located where there are wetlands nearby, although the opposite is not true. Ferricrete formation is thus not caused by 

the wetlands; rather, both wetlands and wet ferricrete result 

from perennial emergence of ground water. The source of 

the ground water is from tributary subbasins. Where ground 

water passes through alluvial fan sediment or colluvium along 

the valley margin, it emerges in a diffuse manner, and large 

sedge wetlands result if broad and low gradient substrates (like 

stream terraces) are present. That same water eventually flows 

to the stream where the banks, and any ferricrete, remain moist. 

Localized emergence of ground water forming iron springs or 

bogs may indicate that the water was conducted to the surface 

by fracture zones in bedrock (Wirt and others, this volume).

Ferricrete accretion is thought to be variable in space and 

time. It requires a sustained supply of acidic iron-rich water 

over a sustained time frame, although it may also occur intermittently in time and spatial extent. Ferricrete that is wet during late summer low-flow conditions is presumed wet throughout the year, and could be precipitating either continuously 

or seasonally, depending on whether or not the geochemical 

conditions are optimal (Wirt and others, this volume). Ferricrete that is dry during the summer, in contrast, may be 

active at other times of the year or during certain wet years, 

but it cannot be forming continuously. As we demonstrate 

next, certain exposures of this dry ferricrete are no longer 

actively forming, and not surprisingly dry ferricrete exposures 

are much more common than wet ferricrete exposures along 

Cement Creek. Cementation of clastic sediment decreases the 

sediment’s permeability; thus, ferricrete formation itself may 

decrease the flow of ground water and affect the rate and location of subsequent ferricrete formation.

The geomorphic history of Cement Creek has influenced 

the location and timing of ferricrete formation. Obviously, 

clastic sediment must be present, or alluvial and colluvial 

ferricrete could not form, and various clastic deposits formed 

at various locations along Cement Creek throughout the 

Holocene. The age of deposition of clastic sediment is a 

maximum-age constraint for the timing of ferricrete cementation, but cementation could have occurred shortly after, 

long after, or continually since clastic deposition. The rate of 

ferricrete formation may be rapid or slow. Historical sediment and cultural artifacts have become cemented or encased 

in ferricrete in a matter of years or decades (Verplanck and 

others, this volume), indicating that ferricrete formation can 

be quite rapid under ideal geochemical circumstances. Under 

less ideal circumstances ferricrete probably formed more 

slowly. Flood-plain gravel is locally cemented, and this must 

have occurred in the past 600 years. Some of the flood-plain 

ferricrete likely formed centuries ago, yet others, such as bog 

iron deposits on the flood plain, are currently forming. Most of 

the ferricrete in the Cement Creek watershed is prehistorical, 

as ferricrete exposures were abundant at the onset of mining (Comstock, 1883). Much of the ferricrete likely formed 

thousands of years ago, but the minimum-age constraints 

necessary to prove it are available at only a few exposures. The 

minimum-age constraints rely on the observation that stream 

incision permanently drained ground water from certain gravel 

deposits, and in one case charcoal was discovered encased in 

a bog iron deposit. Canyon-filling stream gravels (map unit g) 

form nearly vertical cliffs about 15 m high on both sides of 

Cement Creek (see Yager and Bove, this volume, pl. 2, fig. 3), 

at the mouth of Cement Creek (between 9 and 9.3 km in figs. 2 

and 6A). These gravel deposits are strongly cemented with 

iron oxides. At least part of the ferricrete is 4,500 years old, 

given the age of charcoal (sample GX-26392, table 1) found in 

bog iron spring deposits capping the cemented gravel. Cement 

Creek incised through these gravel deposits before 6,000 years 

ago and probably about 12,000 years ago, and this drained the 

ground water from the alluvium. The horizon of active cementation probably moved downward in the section as Cement 

Creek incised; thus, the bulk of the ferricrete is likely early 

Holocene in age. Similar thick ferricrete deposits were mapped 

at the mouth of Mineral Creek and at Blair Gulch on the 

Animas River (Yager and Bove, this volume, pl. 2). The gravel 

deposits in Cement Creek only thinly cover bedrock, however, 

so some of the cementation may have occurred at seeps in the 

exposure face that were fed by ground water emerging from 

bedrock fractures nearby. The age of the bog iron deposit just 

mentioned, because it is younger than the timing of incision, 

indicates that this process occurred at least locally thousands 

of years after incision, and it could have occurred locally at any 

time during the Holocene. The timing of ferricrete formation is 

unequivocal for another type of deposit. Old alluvial fans (map 

unit fo) were emplaced perhaps 12,000 years ago and became 

incised before 6,000 years ago. Incision of Cement Creek and 

the tributary streams that formed the fans permanently drained 

the ground water from the near-surface, distal portions of these 

fans. In numerous locations we observed zones of ferricrete 

cropping out of erosional escarpments cut into the distal ends 

of the fans. The ferricrete zones are 1 to several decimeters 

thick, are subparallel to and are on the order of 1 m beneath 

the original fan surfaces, and are several meters higher than 

the terrace or streambed at the foot of the escarpments. These 

ferricrete zones must have formed before stream incision, when 

the ground-water levels were closer to the fan surfaces, because 

after incision ground water emerged from the permeable fan 

alluvium several meters beneath the ferricrete zones. These 

ferricrete zones must have formed entirely during the early 

to middle Holocene.

We conclude that ferricrete has been forming in the 

Cement Creek valley throughout the Holocene, although the 

locations of ferricrete formation have shifted as clastic sediment 

was deposited or eroded away. The rates of ferricrete formation 

have also been variable, because the geochemical requirements 

for ferricrete formation are spatially variable (Wirt and others, 

this volume). Low-pH iron-rich ground water is required for the 

formation of ferricrete. Ferricrete exposures are most frequent 

in reaches of Cement Creek where the ground-water supply is 

from subbasins composed of intensely hydrothermally altered 

bedrock containing abundant pyrite and other sulfide minerals 

(Wirt and others, this volume).
6.0

6.5

7.0

7.5

8.0

8.5

9.0

Modern sediment, Cement Creek

White inter-peat silt

Premining baseline sediment, site B16

Premining baseline sediment, site B13

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0

2

4

6

8

10

12

14

16

0.35

0.40

0.45

0.50

0.55

0.60

0.65

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

A

EXPLANATION

90th percentile

10th percentile

75th percentile

25th percentile

Median

The information displayed in box plots 

 is summarized in the graphic below.

ALUMINUM

CALCIUM

POTASSIUM

MAGNESIUM

IRON

CONCENTRATION, IN WEIGHT PERCENT

CONCENTRATION, IN WEIGHT PERCENT

TITANIUM

770 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 771

0

20

40

60

80

100

120

0

1

2

3

4

5

6

0

20

40

60

80

100

120

140

160

180

200

200

400

600

800

1,000

1,200

1,400

1,600

100

150

200

250

300

350

400

450

500

550

600

650

0

200

400

600

800

1,000

1,200

1,400

1,600

CONCENTRATION, IN PARTS PER MILLION

ARSENIC

CADMIUM

COPPER

MANGANESE

LEAD

ZINC

B

CONCENTRATION, IN PARTS PER MILLION

Figure 7 (above and facing page). Box plots of A, major-element and B, selected trace-element concentrations for premining 

sediment samples from site B13 (n=23), premining sediment samples from site B16 (n=16) as discussed in Church, Fey, and Unruh 

(this volume), inter-peat silt samples (n=17) discussed in this chapter, and modern sediment samples from Cement Creek (n=10). 

All analytical and mineralogical data are in the database (Sole and others, this volume).
References Cited

Atwood, W.W., and Mather, K.F., 1932, Physiography and 

Quaternary geology of the San Juan Mountains, Colorado: 

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

Beniston, M., Diaz, H.F., and Bradley, R.S., 1997, Climate 

change at high elevation sites—An overview: Climate 

Change, v. 36, nos. 3–4, p. 1–19.

Bird, A.G., 1986, Silverton gold–The story of Colorado’s 

largest gold mine: Lakewood, Colo., privately published 

(ISBN 0-9619382-2-6), 152 p.

Blair, R.W., Jr., Yager, D.B., and Church, S.E., 2002, Surficial geologic maps along the riparian zone of the Animas 

River and its headwater tributaries, Silverton to Durango, 

Colorado, with upper Animas River gradient profiles: 

U.S. Geological Survey Digital Data Series DDS–71.

Briffa, K.R., and Osborn, T.J., 2002, Blowing hot and cold: 

Science, v. 295, p. 2227–2228.

Bull, W.B., 1991, Geomorphic responses to climatic change: 

New York, Oxford University Press, 326 p.

Carrara, P.E., Mode, W.N., Rubin, Meyer, and Robinson, 

S.W., 1984, Deglaciation and postglacial timberline in the 

San Juan Mountains, Colorado: Quaternary Research, v. 21, 

p. 42–55.

Carrara, P.E., Trimble, D.A., and Rubin, Meyer, 1991, 

Holocene treeline fluctuations in the northern San Juan 

Mountains, Colorado, U.S.A., as indicated by radiocarbondated conifer wood: Arctic and Alpine Research, v. 23, 

p. 233–246.

Comstock, T.B., 1883, Notes on the geology and mineralogy 

of San Juan County, Colorado: Transactions of the American Institute of Mining Engineers, v. 11, p. 165–191.

Durango Evening Herald, 1911, Oct. 5–11 editions, Durango, 

Colo.

Elias, S.A., Carrara, P.E., Toolin, L.J., and Jull, A.J.T., 1991, 

Revised age of deglaciation of Lake Emma based on new 

radiocarbon and macrofossil analyses: Quaternary Research, 

v. 36, p. 307–321.

Fall, P.L., 1997, Timberline fluctuations and late Quaternary 

paleoclimates in the southern Rocky Mountains, Colorado: 

Geological Society of America Bulletin, v. 109, no. 10, 

p. 1306–1320.

Gosse, J.C., Klein, J., Evenson, E.B., Lawn, B., and 

Middleton, R., 1995, Beryllium-10 dating of the duration 

and retreat of the last Pinedale glacial sequence: Science, 

v. 268, p. 1329–1333.

Hereford, Richard, 2002, Valley-fill alluviation during the 

Little Ice Age (ca. A.D. 1400–1880), Paria River basin and 

southern Colorado Plateau, United States: Geological Society of America Bulletin, v. 114, no. 12, p. 1551–1563.

Kimball, B.A., Runkel, R.L., Walton-Day, Katherine, and 

Bencala, R.L., 2002, Assessment of metal loads in watersheds affected by acid mine drainage by using tracer 

injection and synoptic sampling; Cement Creek, Colorado, 

U.S.A: Applied Geochemistry, v. 17, p. 1183–1207.

Leonard, E.M., 1989, Climatic change in the Colorado Rocky 

Mountains—Estimates based on modern climate at late 

Pleistocene equilibrium lines: Arctic and Alpine Research, 

v. 21, p. 245–255.

Leopold, L.B., and Bull, W.B., 1979, Base level, aggradation, 

and grade: Proceedings of the American Philosophical Society, v. 123, no. 3, p. 168–202.

Madole, R.F., 1976, Bog stratigraphy, radiocarbon dates, and 

Pinedale to Holocene glacial history in the Front Range, 

Colorado: U.S. Geological Survey Journal of Research, v. 4, 

p. 163–169.

Madole, R.F., 1980, Time of Pinedale glaciation in northcentral Colorado—Further considerations: Geology, v. 8, 

p. 118–122.

Madole, R.F., 1986, Lake Devlin and Pinedale glacial history, Front Range, Colorado: Quaternary Research, v. 25, 

p. 43–54.

Menounos, B., and Reasoner, M.A., 1997, Evidence of cirque 

glaciation in the Colorado Front Range during the Younger 

Dryas chronozone: Quaternary Research, v. 48, p. 38–47.

Miller, J., Germanoski, D., Waltman, K., Tausch, R., and 

Chambers, J., 2001, Influence of late Holocene hillslope 

processes and landforms on modern channel dynamics in 

upland watersheds in central Nevada: Geomorphology, 

v. 38, no. 2–4, p. 373–391.

Nelson, A.R., Millington, A.C., Andrews, J.T., and Nichols, 

H., 1979, Radiocarbon-dated upper Pleistocene glacial 

sequence, Fraser Valley, Colorado Front Range: Geology, 

v. 7, p. 410–414.

772 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Cement Creek Geomorphology and Ferricrete Deposits 773

Pruess, J.W., 1996, Paleoflood reconstructions within the 

Animas River basin upstream from Durango, Colorado: Fort 

Collins, Colo., Colorado State University M.S. thesis, 192 p.

Reasoner, M.A., and Jodry, M.A., 2000, Rapid response of 

alpine vegetation to the Younger Dryas climate oscillation 

in the Colorado Rocky Mountains, U.S.A: Geology, v. 28, 

p. 51–54.

Rosenbaum, J.G., and Larson, E.E., 1983, Paleomagnetism of 

two late-Pleistocene lake basins in Colorado—An evaluation of detrital remanent magnetization as a recorder of the 

geomagnetic field: Journal of Geophysical Research, v. 88, 

p. 10611–10624.

Stuiver, Minze, and Braziunas, T.F., 1993, Sun, ocean, climate 

and atmospheric 14CO2

—An evaluation of causal and spectral relationships: The Holocene, v. 3, no. 4, p. 289–305.

Stuiver, Minze, and Reimer, P.J., 1993, Extended 14C data base 

and revised CALIB 3.0 14C age calibration program: Radiocarbon, v. 35, p. 215–230.

Stuiver, Minze, Reimer, P.J., Bard, Edouard, Beck, J.W., 

Burr, G.S., Hughen, K.A., Kromer, Bernd, McCormac, 

Gerry, van der Plicht, Johannes, and Spurk, Marco, 1998, 

INTCAL98 radiocarbon age calibration, 24,000-0 cal. BP: 

Radiocarbon, v. 40, p. 1041–1083.