U. S. DEPARTMENT OF THE INTERIOR

U. S. GEOLOGICAL SURVEY

Preliminary Report on Deposit Models for

Sand and Gravel in the Cache la Poudre River Valley

(on-line edition)

Open File Report 99-587

By William H. Langer1 and David A. Lindsey1

Photo of gravel processing along the Front Range, Colorado

This report is preliminary and has not been reviewed for conformity with U. S. Geological Survey editorial

standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for

descriptive purposes only and does not imply endorsement by the U. S. Government.

1

U. S. Geological Survey, Denver, Colo., 80225
Contents

Abstract 1

Introduction 1

Purpose and Scope 3

Overview of Methods 3

Acknowledgements 3

Geologic Setting 3

General Setting 3

Bedrock Geology 3

Surficial Geology 3

Geomorphic Deposit Models of the Cache la Poudre River Valley 6

Dissected Terraces 7

Geologic Environment 7

Deposit Size 7

Deposit Characteristics 7

Environmental Impacts of Mining 7

Alluvial Fans 7

Geologic Environment 7

Deposit Size 7

Deposit Characteristics 7

Environmental Impacts of Mining 7

High Terraces 7

Geologic Environment 7

Deposit Size 7

Deposit Characteristics 7

Environmental Impacts of Mining 8

Floodplains and Low Terraces 8

Geologic Environment 8

Deposit Size 8

Deposit Characteristics 9

Environmental Impacts of Mining 9

Composition and Quality of Gravel in Floodplains and Low Terraces 9

General Stratigraphy 9

Sampling Plan 12

Sample Processing 12

Particle Size 12

Lithology and Gravel Quality 14

Pebble Shape 17

Pebble Roundness 18

Conclusions 18

References Cited 20

i
List of Figures

Figure 1.—Map showing location of Cache la Poudre River and the study area.

Figure 2.—Map showing location of Cache la Poudre River, sample sites, and sampling plan.

Figure 3.—Valley cross sections showing alluvial deposits of the Cache la Poudre River valley between Fort Collins and

Greeley, Colorado.

Figure 4.—Block diagram showing landforms of alluvial deposits of the Cache la Poudre River valley between Fort

Collins and Greeley, Colorado.

Figure 5.—Photograph showing large size of clasts at Western Mobile (LaFarge) Taft Hill pit near Fort Collins,

Colorado.

Figure 6.—Photograph showing size of clasts at Western Mobile (LaFarge) Greeley West pit near Greeley, Colorado.

Figure 7.—Photograph showing soil and overbank sediment in erosional contact with gravel, Hall Irwin Greeley pit, near

Greeley, Colorado.

Figure 8.—Photograph showing changes in texture and color between the two stratigraphic units at the Western Mobile

(LaFarge) POE pit near Fort Collins, Colorado.

Figure 9.—Photograph showing details of clay lenses in the upper stratigraphic unit at the Western Mobile (LaFarge)

POE pit near Fort Collins, Colorado.

Figure 10.—Photograph showing changes in texture and color between the two stratigraphic units at the Hall Irwin

Windsor pit, near Windsor, Colorado.

Figure 11.—Photograph showing changes in texture and color between the upper and lower stratigraphic units at the

Western Mobile (LaFarge) 35th Street pit near Greeley, Colorado.

Figure 12.—Photograph showing dirty gravel at Western Mobile (LaFarge) Timnath pit near Fort Collins, Colorado.

Figure 13.—Downstream variation in particle size, Cache la Poudre River valley gravel.

Figure 14.—Plot showing ratio of gravel sizes in upper versus lower gravel unit, Cache la Poudre River valley.

Figure 15.—Downstream variation in pebble lithology, Cache la Poudre River valley gravel.

Figure 16.—Downstream variation in gravel quality, Cache la Poudre River valley.

Figure 17.—Plot showing ratio of pebble lithology in upper versus lower gravel unit, Cache la Poudre River valley.

Figure 18.—Plot showing ratio of gravel quality in upper versus lower gravel unit, Cache la Poudre River valley.

Figure 19.—Zingg diagram showing pebble shape, Cache la Poudre River valley gravel.

Figure 20.—Histogram showing calculated pebble sphericity, Cache la Poudre River valley gravel.

Figure 21.—Plot showing ratio of pebble shape in upper versus lower gravel unit, Cache la Poudre River valley.

Figure 22.—Downstream variation in pebble roundness, Cache la Poudre River valley gravel.

Figure 23.—Plot showing ratio of pebble roundness in upper versus lower gravel unit, Cache la Poudre River valley.

List of Tables

Table 1.—Variance components for gravel particle size parameters, Cache la Poudre River.

Table 2.—Variance components for gravel lithology and quality, Cache la Poudre River.

Table 3.—Variance components for gravel pebble shape, Cache la Poudre River valley.

Table 4.—Variance components for gravel pebble roundness, Cache la Poudre River valley.

List of Appendix Tables

Table A1.—Particle-size data, sieved gravel samples, Cache la Poudre River valley.

Table A2.—Gravel lithology, Cache la Poudre River valley.

Table A3.—Gravel quality, Cache la Poudre River valley.

Table A4.—Gravel shape, Cache la Poudre River valley.

Table A5.—Gravel roundness, Cache la Poudre River valley.

Table A6.—Sample sites, site symbols, and miles downstream, Cache la Poudre River valley.

ii
Open File Report 99-587

Preliminary Report on Deposit Models for

Sand and Gravel in the Cache la Poudre River Valley

By William H. Langer and David A. Lindsey

ABSTRACT

The stratigraphy, sedimentary features, and physical characteristics of gravel deposits in the Cache la Poudre River

valley were studied to establish geologic models for these deposits. Because most of the gravel mined in the valley is

beneath the low terraces and floodplain, the quality of these deposits for aggregate was studied in detail at eight sites in a

25.5-mile reach between Fort Collins and Greeley, Colorado. Aggregate quality was determined by field and laboratory

measurements on samples collected under a consistent sampling plan.

The Broadway terrace is underlain by Pleistocene alluvium and, at some places, by fine-grained wind-blown

deposits. The Piney Creek terrace, low terraces, and floodplain are primarily underlain by Holocene alluvium.

Pleistocene alluvium may underlie these terraces at isolated locations along the river. Gravels beneath the Piney Creek

terrace, low terraces, and floodplain are divisible into two units that are poorly distinguishable at the upstream end of the

study area, but are readily distinguishable about 7 miles downstream. Where distinguished, the two gravel units are

separated by a sharp, locally erosional, contact. The upper gravel is probably of Holocene age, but the lower gravel is

considered to be Holocene and Pleistocene.

The primary variation in particle size of the gravels beneath the floodplain and low terraces of the Cache la Poudre

River valley is the downstream decrease in the proportion of particles measuring 3/4 inch and larger. Above Fort Collins,

about 60 pct of the gravel collects on the 3/4 inch sieve, whereas about 50 pct of gravel collects on the same sieve size at

Greeley. For 1.5-inch sieves, the corresponding values are about 50 pct for Fort Collins and only about 30 pct for

Greeley. Local differences in particle size and sorting between the upper and lower gravel units were observed in the

field, but only the coarsest particle sizes appear to have been concentrated in the lower unit.

Field measurements of aggregate quality, pebble lithology, and shape show little significant downstream variation.

Pebble lithology is about 25 percent granite; 48 percent pegmatite; 5-7 percent each of gneiss, quartz, and quartzite; and

minor amounts of diabase, schist, volcanic porphyry, and sandstone. Among the rock types, only the volcanic porphyries

might be reactive with Portland cement.

Pebble shape is dominantly equidimensional with a tendency to form thick, disc-shaped particles. Disc-shaped and

spherical particles comprise about 39 percent and 31 percent of the pebble-size fraction, respectively. Rod and blade

shapes comprise about 18 and 12 percent of the pebble-size fraction, respectively. The relatively large proportion of

equidimensional particles in the Cache la Poudre may be due to the small proportion of layered gneiss in gravel. Pebbles

having axial ratios less than 0.5, which might be structurally weak, are rare.

The two gravel units show subtle local differences and evidence for derivation of the younger gravel from the older

gravel. At many sites, the upper gravel unit tends to contain more quartz plus quartzite, has poorer physical quality, and

contains more angular pebbles than the lower gravel. Weathering, followed by transport in the river, might be expected to

concentrate quartz and quartzite, degrade physical quality, and break pebbles into angular fragments. This conclusion is

consistent with local evidence of an erosional contact between the two gravel units.

INTRODUCTION

Infrastructure, such as roads, buildings, and airports,

are built and maintained by use of large quantities of

natural aggregate. About 80 percent of the aggregate

produced in Colorado is sand and gravel, which comes

from floodplains and terraces along stream and river

valleys, or occasionally from high dissected terraces and

alluvial fans along the mountain front. About 20 percent

of Colorado aggregate production is crushed stone, which

comes from rock quarries located mostly in the mountains

or, in a few cases, from buttes or mesas on the plains.

As the Front Range urban areas expand, the very

growth that demands aggregate resources also precludes

its development. Other competitive land uses have a

higher value than aggregate resources. For example,

gravel cannot be mined from under a subdivision. Growth

in Fort Collins, Greeley, and the corridor between those

cities, has precluded some aggregate development. In

addition, zoning and environmental restrictions can

further restrict development of natural resources. If

aggregate resources are to remain economically available,

current resource information must be available for use in

well-reasoned decisions about future land use. The failure

to plan for the protection and extraction of aggregate

resources may result in increased consumer cost,

environmental damage, and an adversarial relationship

between industry and the public.

The gravel deposits of the Cache la Poudre valley

between Fort Collins and Greeley (Figs. 1 and 2) are the

principal source of aggregate for urban development in

the valley. This report describes the sand and gravel

deposits of the Cache La Poudre valley from Fort Collins
LINE OF SECTION

105°00' 104°52'30"

104°45' 104°37'30"

105°7'30"

40°

30'

40° 22'

30"

40°

37'

30"

2 2 2

2 2 2

EXPLANATION

Sample site in gravel pit

 W, abbreviation for pit name

Sampled sections in pit

 2, number of replicate

 samples for particle size

 SAMPLE LOCATIONS

AND DESIGN (SCHEMATIC)

Ft. Collins

Greeley

TH

POE

W

HIW

HIG

W GW

35TH ST

Cache

River

la Poudre

5 MILES

5 KILOMETERS

0

0

2

TM

A

B

C

D

D'

C'

B'

A'

E' F'

G'

E

F

G

Figure 1.—Map showing location of Cache la Poudre River and the

study area.

Figure 2.—Map showing location of Cache la

Poudre River, sample sites, and sampling

plan. Abbreviations for sites at gravel pits:

TH, LaFarge Taft Hill; POE, LaFarge Portof-Entry; TM, LaFarge Timnath; W, LaFarge

Weitzel; HIW, Hall Irwin Windsor; HIG,

Hall Irwin Greeley; 35TH ST, LaFarge 35th

Street Greeley; and GW, LaFarge Greeley

West (25th Street).

Ft. Collins AREA OF

FIGURE 2

Greeley

La Porte

Rocky

Mountain

National

Park

42°30'

Kinikinik

Windsor

Cache

South

Platte River

la Poudre

River

105°30' 105° 104°30'

42°45'

42°15'

25 MILES I-25

30 KILOMETERS

0

0

2
eastward to Greeley to provide information for

appropriate utilization of this important local resource.

Purpose and Scope

The U.S. Geological Survey (USGS), in

collaboration with the Colorado Geological Survey

(CGS), the National Stone Association, and other

cooperators, is developing information, defining tools,

and demonstrating ways to evaluate aggregate resources

(crushed stone, sand, and gravel) in the Front Range

Urban Corridor. This report addresses one project task, to

prepare deposit models for aggregate resources.

Floodplains and low terraces are the principal

sources of aggregate that are developed in the Cache la

Poudre River valley. Sand and gravel also underlie small

high terraces and alluvial fans along the Cache la Poudre

River, but commonly are not used as specification

aggregate because of inferior quality. This report

describes deposit models for all the types of gravel

deposits along the Cache la Poudre River, but focuses on

a model for the floodplain low terraces and the

description of those deposits.

The stratigraphy and sedimentary structures of the

sand and gravel deposits in the Cache la Poudre River

valley were studied to establish a framework for the

assessment of the quantity and quality of the gravel

resources. Although the regional distribution of the gravel

resources is well-established (Colton and Fitch, 1974;

Schwochow and others, 1974b; Shelton and Rogers,

1987), information on the stratigraphy and quality of

gravel is generally unavailable. Some subsurface

information includes postings of reconnaissance data on

gravel thickness, particle size, and pebble composition

(Colton and Fitch, 1974; Schwochow and others, 1974b)

and well and test hole logs of surficial deposits (Schneider

and Hershey, 1961).

Overview of Methods

New and existing data on the subsurface of the

Cache la Poudre River valley were collected and

synthesized to establish a new stratigraphic framework.

Well and test hole logs (Schneider and Hershey, 1961)

were compiled into seven cross sections showing the

stratigraphy of valley deposits. Stratigraphic relationships

and sedimentary features of gravel beneath the floodplain

and terraces were mapped and described from exposures

at eight gravel pits (Fig. 2) in the Cache la Poudre valley.

Samples for measurement of particle size and

determination of pebble lithology, shape, roundness,

physical quality, and chemical quality were collected from

vertical trenches taken at one to three locations and from

one or two horizons at each of eight gravel pits. Sample

spacing was guided by results of earlier studies (Lindsey

and Shary, 1997; Lindsey and others, 1998a and 1998b).

Each sample was processed in the field by dry sieving and

weighing size fractions. The 3/4-1.5 inch pebble fraction

was reserved for lithologic identification and laboratory

measurements as described in Lindsey and Shary (1997).

Acknowledgements

We thank the employees of LaFarge (previously

known as Western Mobile) and Hall Irwin for granting

access to company gravel pits and for sharing their

knowledge of individual gravel deposits and mining

operations. B. F. Arbogast, U. S. Geological Survey, took

the title page photograph.

GEOLOGIC SETTING

General Setting

The Cache la Poudre River is in north-central

Colorado and heads on the Continental Divide in Rocky

Mountain National Park (Fig. 1). The river flows

northward to Kinikinik, then eastward through the

mountains, foothills, and hogback, where it emerges on

the Great Plains at La Porte. From La Porte, the river

flows southeastward through Fort Collins, past Windsor;

it joins the South Platte River east of Greeley. This study

is restricted to the lower part of the river from the

mountain front to its confluence with the South Platte

River.

Bedrock Geology

The surficial deposits in the study area were

deposited on two formations of Lower Cretaceous age,

the Pierre Shale and the overlying Fox Hills Sandstone.

The best location to view the contact between these

formations is approximately five miles east of Windsor at

the escarpment along the south side of the valley of the

Cache la Poudre River. From that location westward, the

Cache la Poudre River alluvium lies directly on various

members of the Pierre Shale. For the most part, the Pierre

Shale is shale, claystone, and sandy siltstone. The Fox

Hills Sandstone consists of well-sorted, medium-grained,

thick-bedded sand.

The surficial deposits are sufficiently thin that,

upstream from Windsor, the excavation of gravel

generally extends to bedrock (Fig. 3 - Sections A-A’

through E-E’). Downstream from Windsor, the bedrock

valley deepens and the deposits thicken to where

excavation of aggregate commonly stops before reaching

bedrock (Fig. 3 - Sections F-F’, G-G’), except where pits

are located along the margins of the valley.

Surficial Geology

The surficial geologic units of the study area are

mostly Quaternary in age. The mountainous part of the

study area was exposed to repeated glaciation. Glacial

activity provided large amounts of sediment and

meltwater to transport the sediment to the lower part of

the Cache la Poudre River between Fort Collins and

Greeley. Glacial advances in the mountainous headwaters

of the Cache la Poudre River correspond to geomorphic

cycles in the lower part of the river. Each geomorphic

cycle consists of downward stream cutting, sideward

3
B7-68-15ddc

Interstate 25

Broadway Terrace

Floodplain

Broadway Terrace

B7-68-21aab

C C’

4850

4900

4950

B7-68-18ccc

B7-68-18cdc

B7-68-19ada

B7-68-18ddd

B7-68-17ccc

B7-68-17cdd

Cache La Poudre River

B7-68-16ccc

B7-68-17dcd

B7-68-16cdc

B7-68-16cdd

B7-68-15ccc

B7-68-22bab

B7-68-22aba1

B7-68-14ccd

FEET ABOVE SEA LEVEL

Vertical Exaggeration 40:1

4900

4950

5000

B B’

Dirty gravel B7-68-5acc B7-68-5cad

B7-68-8baa

Broadway Terrace B7-68-8abc B7-68-8caa

B7-68-8cda

B7-68-8cdd

B7-68-17bac

B7-68-17cba

B7-68-20cbb

B7-68-17ccc

Mulberry St

Cache La Poudre River

Floodplain

FEET ABOVE SEA LEVEL

Vertical Exaggeration 40:1

4800

4850

Interstate 25

D D’

4900

B6-68-3cdd

B6-68-3dad

B6-68-2abd

B6-68-1bbb

Bend in section

B7-68-36cad

Poudre River

Cache La

Floodplain

Broadway Terrace

Upland deposits

FEET ABOVE SEA LEVEL

Vertical Exaggeration 40:1

Vertical Exaggeration 40:1

5000

FEET 

4
Vertical Exaggeration 40:1

E E’

? ? 4750

FEET ABOVE SEA LEVEL

Cache La Poudre River

B6-67-35add

B6-67-35aad

B6-67-25ccb

B6-67-25cbb

B6-67-26adc

B6-67-23ddd

B6-67-23dda

Broadway Terrace

Floodplain

4800

4700

Vertical Exaggeration 40:1

F F’

4650

4700

4600

4550

FEET ABOVE SEA LEVEL

Broadway

Terrace

Floodplain

B5-66-1ccc

B5-66-2add

B5-66-1bbc

B6-66-36ccc

B6-66-35dad

B6-66-36bcb

B6-66-35aaa

B6-66-25ccc

B6-66-26daa

B6-66-26ada

B6-66-24ccd

B6-66-23dad

B6-66-23daa

Cache

La Poudre

River

Seeley Lake

G G’

4700

4650

4600

4550

?

? ?

Cache

La Poudre

River

FEET ABOVE SEA LEVEL

B5-65-9ccc

B5-65-9cba

B5-65-9baa

B6-65-33ddc

B6-65-34bcc

B6-65-34bba

Vertical Exaggeration 40:1

Broadway Terrace

Floodplain

Piney Creek Terrace

Figure 3 (continued). —Valley cross sections showing alluvial deposits of the Cache la Poudre River valley between

Fort Collins and Greeley, Colorado.

5

Base of Holocene (estimated)

Silt and clay

Sand

Sand > Gravel

Gravel > Sand

Soil and overbank silt

EXPLANATION

Dirty gravel
stream cutting, alluviation, wind erosion and deposition,

and soil formation (Scott, 1963). Each cycle formed a

separate stream terrace. Four terraces, or cycles, are

recognized in the study area. They are, from oldest to

youngest, the Verdos Alluvium, Louviers Alluvium,

Broadway Alluvium, and the Piney Creek Alluvium. A

fifth, incomplete cycle, is represented by the modern

floodplain, referred to as the post-Piney Creek Alluvium

(Colton, 1978). Other terraces mapped by Scott (1963)

along the Platte River south of the study area have not

been mapped along the Cache la Poudre River.

GEOMORPHIC DEPOSIT MODELS OF THE

CACHE LA POUDRE RIVER VALLEY

Deposit models contain information about 1)

geologic environment, 2) deposit size, 3) deposit

characteristics, and 4) environmental impacts from

mining (Cox and others, 1986). Deposits are defined as

entire geomorphic units such as alluvial terraces along

streams (Bliss, 1993). Geologic environment refers to the

present geologic (and geomorphic) setting, which are the

product of both the sedimentary environment in which the

deposit formed and the subsequent geologic history of the

site. The area and thickness of a deposit describe size

(volume). Deposit characteristics include sedimentary

features and physical properties that are relevant to

aggregate quality or suitability, such as particle size and

composition. In this report, only impacts of mining on the

geologic environment, such as the water table, are

considered. Engineering impacts such as noise and dust

are beyond the scope of this report.

Working models of sand and gravel deposits of the

Front Range Urban Corridor have been prepared by

preliminary analysis of three kinds of available data: 1)

deposit maps showing distribution of sand- and gravelbearing units, 2) subsurface sections, based on borehole

logs showing depth and thickness of sand, gravel, and

other sediment, and 3) compilations of particle size and

composition, which are useful for assessing aggregate

quality or suitability. Using these data, the geologic

environment of a deposit can be identified and,

consequently, the deposit can be assigned to the

appropriate model. Deposit characteristics can be

identified by integrating data on particle size and

composition with lithologic maps and subsurface sections.

Statistical models of deposit size (volume) can be

prepared from dimensions of individual deposits shown

on maps and cross-sections.

Lindsey (1997) described five general geomorphic

deposit models in the Colorado Front Range that reflect

the map units utilized by Schwochow and others (1974a).

These five models are dissected fans, high dissected

terraces, high terraces, low terraces and modern

floodplains, and eolian sand. Lindsey and others (1998a)

prepared a detailed stratigraphic model of gravel beneath

the low terraces and modern floodplain of the South Platte

River valley between Denver and Brighton, Colorado.

They then used the new stratigraphic model to guide

sampling and analysis of gravel deposits for aggregate

quality (Lindsey and others, 1998b). Deposit models, field

sampling methods, laboratory methods, and statistical

techniques for estimating aggregate quality in gravel

TERRACE

LANDSCAPE FORMS

ROLLING UPLAND

SAND DUNES MOUNTAIN VALLEY

FLOODPLAIN AND

 LOW TERRACES

TERRACE

HIGH DISSECTED

 TERRACE

EXPLANATION

ALLUVIAL FAN

F

T

H

A

DEPOSIT MODELS

HOGBACK

F

F

F

A

A

T

T

T

T

T

T

T

H

H

H

Unconsolidated

gravel, sand, and silt

Gneiss and

igneous rock of

Precambrian age

from E. J. Crosby, 1978

Figure 4.—Block diagram showing

landforms of alluvial deposits of

the Cache la Poudre River valley

between Fort Collins and

Greeley, Colorado.

6
deposits of the Front Range Urban Corridor were

summarized by Lindsey and Langer (1988; 1999). Their

methods were used for the present study. Langer and

others (1999) illustrated the use of deposit model data for

decision-making.

Four gravel deposit models are defined for the

Cache la Poudre River valley. These models correspond to

four classes of landforms underlain by gravel deposits

(Fig. 4) that were mapped by Schwochow and others

(1974a) in the Cache la Poudre River valley between Fort

Collins and Greeley, Colorado. From the highest (oldest)

to lowest (youngest) the major landforms are dissected

terraces, alluvial fans, high terraces, and floodplains and

low terraces (Lindsey, 1997).

Dissected Terraces

Geologic Environment—Dissected terraces (referred

to as Upland Deposits by Schwochow and others, 1974a),

occur as isolated erosional remnants on hills overlooking

the Cache la Poudre River valley. The deposits were once

nearly continuous surfaces of floodplain alluvium (Verdos

and Louviers Alluviums) of the Pleistocene Cache la

Poudre River. The surfaces commonly are 150 to 175 feet

above the modern Cache la Poudre River floodplain.

Deposit Size—Deposits occupy small isolated areas

south of Greeley and north of Windsor. The dissected

terrace deposits are up to 20 feet thick, but commonly

between 10 and 15 feet thick.

Deposit Characteristics—Deposits commonly

consist of pebble, cobble, and boulder gravel and sand.

Thick, well-developed soil profiles contain much clayey

material. Thick caliche (calcium carbonate) locally

cements the gravel. The gravel-size clasts are mostly

weathered, partly decomposed, and coated with caliche.

Environmental Impacts of Mining—Sand and gravel

mining in dissected uplands commonly does not penetrate

the water table. The aggregate is dry and can be extracted

by using conventional earth-moving equipment. The

impacts from excavating aggregate from a dry pit are

commonly engineering related.

Alluvial Fans

Geologic Environment—The alluvial fans occur

beneath high surfaces isolated from, and overlooking, the

Cache la Poudre River. The deposits once underlay a

nearly continuous surface of alluvial fans (Slocum

Alluvium of Pleistocene age) spreading from the

mountain eastward onto the Great Plains. The surfaces

commonly are 50 to 100 feet above the Cache la Poudre

River floodplain.

Deposit Size—Deposits occupy large areas in, south,

and west of Fort Collins. The alluvial fan deposits are as

much as 20 feet thick.

Deposit Characteristics—Deposits commonly

consist of pebble, cobble, and boulder gravel and sand.

Thick, well-developed soil profiles contain much clayey

material. Thick caliche (calcium carbonate) locally may

cement the gravel. Gravel-size clasts are mostly

weathered, partly decomposed, and coated with caliche.

Environmental Impacts of Mining—Sand and gravel

mining in alluvial fans commonly does not penetrate the

water table. The aggregate is dry and can be extracted by

using conventional earth-moving equipment. The impacts

from excavating aggregate from a dry pit are commonly

engineering-related.

High Terraces

The alluvial stratigraphy of the Cache la Poudre

River valley is delineated on the set of cross sections (Fig.

3). For the most part, the cross sections show two ages of

deposits: the older, Pleistocene alluvium underlying the

high (Broadway) terrace, and the younger, Holocene

alluvium underlying the floodplain and the low (Piney

Creek) terrace (described below). The alluvial deposits of

Holocene age are separated on the cross sections (Figs.

3B, 3C, and 3D) from the underlying deposits of

Pleistocene age by a dotted contact. The separating

contact is inferred from textural changes noted in drill

logs, or from relief shown on topographic maps. Age

classification of deposits is from Colton (1978).

Geologic Environment—High terraces are

intermediate in elevation between the dissected terraces

and alluvial fans and the low terraces and floodplains

(Fig. 3, section D-D’). The terraces were once nearly

continuous surfaces of floodplain alluvium (Broadway

Alluvium) of a late Pleistocene age Cache la Poudre

River. Terrace surfaces are about 15 to 40 feet above the

present Cache la Poudre floodplain.

Deposit Size—High terraces form large contiguous

surfaces along most of the north side of the river, and on

both the north and south sides of the river at Greeley.

Gravel deposits beneath the Broadway terrace commonly

are 50 feet thick (Fig. 3, sections B-B’ and C-C’); near

Greeley, they are over 100 feet thick (section G-G’). The

thickest deposits occur on the north side of the Cache la

Poudre River.

Deposit Characteristics—The Broadway alluvium,

which is a highly variable deposit, underlies the high

terraces. The deposits contain coarse, high quality gravel

that constitutes a major aggregate resource, but in places

they contain weathered gravel that is less desirable as a

source of aggregate. The deposits are well to poorly

sorted and well to poorly layered. Deposits can contain

significant amounts of very fine sand, silt and clay

occurring as layers within the deposits (Fig 3, sections CC’ and G-G’), or mixed with sand and gravel resulting in

dirty gravel (section B-B’). The clasts range in size from

boulders to pebbles and generally decrease in size

downstream.

Wind-blown sand, silt, and clay cover much of the

Broadway terrace between Fort Collins and Greeley (Fig.

3, section F-F’). At some places the thickness of

windblown deposits is as much as 25 feet. Near Fort

Collins the wind-blown deposits extend onto the low

7
terraces and floodplain deposits as well (Fig. 3, sections

A-A’, B-B’, and C-C’).

Environmental Impacts of Mining—Sand and gravel

mining in high terraces may or may not penetrate the

water table, depending on local conditions. If the

excavation does not penetrate the water table, the

aggregate is dry and can be extracted by using

conventional earth-moving equipment. The impacts from

excavating aggregate from a dry pit are commonly

engineering related.

If the excavation penetrates the water table and the

pit is mined dry, water must be pumped or otherwise

removed from the pit because the pit will become a

discharge point. Ground water discharge may lower the

water table near the operation and, in turn, impact nearby

wells. In highly permeable deposits, slurry walls might be

necessary to isolate the pit from the water table. Water

removed during dewatering can be returned to nearby

streams; these streams may recharge the aquifer

downstream from the pit. The impacts to the water table

from dewatering can be monitored by use of observation

wells.

In some situations where the sand and gravel pits

penetrate the water table, the pit may not be able to be

drained, or the operator may prefer to extract the sand and

gravel by using wet mining techniques. Material may be

excavated by using draglines, clamshells, bucket and

ladder, or hydraulic dredges. In semiarid climates such as

the Front Range Urban Corridor, evaporation from water

in pits can lower the water table. Monitoring wells

installed around the pit and stream gauges can be used to

observe effects on the water table.

Floodplains and Low Terraces

Geologic Environment—The low terraces (postPiney Creek Alluvium of Holocene age) and the modern

floodplain form the lowest surfaces in the Cache la

Poudre River valley. Commonly the lowermost terrace

(post-Piney Creek) lies at elevations only a few feet

higher than the modern floodplain, so that they cannot be

distinguished on most topographic maps.

Deposit Size—For most of the reach from Fort

Collins to Greeley the floodplains and low terraces

occupy about half the width of the valley as defined by all

Figure 5.—Photograph showing large size

of clasts at Western Mobile (LaFarge)

Taft Hill pit near Fort Collins,

Colorado.

Figure 6.—Photograph showing size of

clasts at Western Mobile (LaFarge)

Greeley West pit near Greeley,

Colorado. Clast size is much smaller

than that upstream at Haft Hill pit.

8
alluvial landforms. The deposits range in thickness from

about 20 feet near Fort Collins to about 50 feet near

Greeley.

Deposit Characteristics—Deposits are well sorted

and well layered. Near Fort Collins the clasts range in size

from pebbles to boulders (Fig. 5). Near Greeley (Section

G-G’) the deposits do not contain boulder-sized clasts

(Fig. 6). Overburden, consisting mostly of overbank silt,

clay, and very fine sand, is generally thin.

Environmental Impacts of Mining—If excavation

penetrates the water table and the pit is mined dry, water

will be pumped or otherwise removed from the pit. The

pit will become a discharge point, and this ground water

discharge may lower the water table near the operation

and, in turn, impact nearby wells. In highly permeable

deposits, slurry walls might be necessary to isolate the pit

from the water table. Water removed during dewatering

can be returned to nearby streams, which may serve to

recharge the aquifer downstream from the pit. The

impacts to the water table from dewatering can be

monitored by use of observation wells.

In some situations where the sand and gravel pits

penetrate the water table, the pit may not be able to be

drained, or the operator may prefer to extract the material

by using wet mining techniques. Material may be

excavated by using draglines, clamshells, bucket and

ladder, or hydraulic dredges. The greatest potential impact

of mining a wet pit occurs when the pit is located on an

active floodplain. Cut-offs and avulsions of a stream

during floods may cause flooding and (or) capture of the

pit.

Changes in stream channel geometry have been

documented where sand and gravel have been mined

directly from stream channels in rivers in northern

California (Kondolf, 1998). However, in-stream channel

mining commonly is not conducted in the Front Range

Urban Corridor. Even so, some streams, in particular

those underlain by large gravel layers deposited under

conditions other than those prevailing at the present, may

support gravel extraction with no serious environmental

impacts. Jiongxin (1996) described such a situation on the

Hanjiang River in China where downcutting stopped

when coarse bed material was reached. The Cache la

Poudre is a similar situation, where coarse gravel of

glacial origin underlies modern stream deposits.

Sand and gravel pits located on active floodplains

may be captured by cut-offs and avulsions of streams

during floods. Pit capture can result in trapping of

bedload. Levees or dikes may protect floodplain pits from

flooding, and can keep bedload in the channel. Dikes

prepared with engineered armored spillways at low points

allow controlled flooding and can help prevent pit capture

(Norman and others, 1998).

COMPOSITION AND QUALITY OF GRAVEL IN

FLOODPLAIN AND LOW TERRACES

The floodplain and low terraces are underlain by

coarse gravel, which currently is the most exploited

aggregate resource in the Cache la Poudre River valley.

Therefore, detailed studies were conducted for this

deposit model.

General Stratigraphy

From top to bottom, the floodplain and low terraces

are underlain by 1) soil and overbank deposits (commonly

3-5 feet thick) and 2) two poorly defined gravel units

beneath the overbank deposits. The soils and overbank

deposits commonly consist of gray to brownish-gray silt,

clay, and sand, and in places contain small amounts of

gravel (Fig. 7). The contact between the soil and overbank

unit and the underlying sand and gravel deposit

commonly is a sharp erosional contact that is marked by

an abrupt change in grain size from clay, silt and sand to

sand and gravel.

Figure 7.—Photograph showing soil and

overbank sediment in erosional contact

with gravel, Hall Irwin Greeley pit,

near Greeley, Colorado.

9
Figure 8.— Photograph showing changes in

texture and color between the two

stratigraphic units at the Western

Mobile (LaFarge) POE pit near Fort

Collins, Colorado.

Figure 9.— Photograph showing details of

clay lenses in the upper stratigraphic

unit at the Western Mobile (LaFarge)

POE pit near Fort Collins, Colorado.

Figure 10.— Photograph showing changes

in texture and color between the two

stratigraphic units at the Hall Irwin

Windsor pit, near Windsor, Colorado.

Two gravel units can be distinguished beneath the

floodplain and low terraces of the Cache la Poudre River

valley, but they are not everywhere visible and their

appearance is not consistent from one locality to the next.

Two units can not be distinguished upstream, at the

Western Mobile (LaFarge) Taft Hill pit (Fig. 5), where the

entire section of alluvial gravel is very coarse.

Two poorly defined gravel units can be observed at

the Western Mobile (LaFarge) POE pit (Fig. 8), located

approximately 7.1 miles downstream from the Taft Hill

pit. The contact commonly is marked by changes in

texture and color. The upper unit contains irregular

masses and lenses of organic-rich clay ranging from a few

inches to a few feet across. These lenses tend to be

concentrated in channel scours, either at the base of the

upper unit or within the upper unit (Fig. 9). The irregular

clay masses probably represent overbank deposits that

were undercut and caved by floodwaters.

The Hall Irwin Windsor pit is approximately 14.3

miles downstream from the Taft Hill pit. Two stratigraphic

units are easily observed (Fig. 10). The upper unit is about

3 feet thick, and is overlain by clay overbank deposits and

10
soil. The unit consists of sand and gravel, is dark grayish

brown and brown, and is classified as 10YR 4/2 and

10YR 5/3 on the Munsell soil color chart (Anonymous,

1994). The contact with the lower unit is an irregular

erosional surface overlain by clay lenses. The lower unit

is coarser than the upper unit and consists of imbricate

cobble gravel that dips to the south. The lower unit

contains sand lenses up to two feet thick. The color is

grayish brown to dark grayish brown (10YR 5/2 to 10YR

4/2 on the Munsell soil color chart).

Two stratigraphic units are also readily

distinguishable at the Western Mobile (LaFarge) 35th

Street pit (Fig. 11), approximately 24.4 miles downstream

from the Taft Hill pit. In the 35th Street pit, distinct

differences in structure, grain size distribution, and color

can be observed between the two units. The upper unit is

about 15 feet thick, and consists of interlayered pebble

gravel with clay lenses and pebbly sand lenses. The color

is light gray to light brownish gray (10YR 7/2 to 10YR6/2

on the Munsell soil color chart). The lower unit is about 8

feet thick. The top seven feet of the unit consists of cobble

to pebble gravel, is poorly stratified, and is pale brown to

brown (10YR 6/3 to 10YR6/4 on the Munsell soil color

chart). The bottom foot of the unit is clean sand. The sand

rests on clayey bedrock. The entire deposit thickens to the

south.

A third, very localized stratigraphic unit, described

as “dirty gravel” (Schneider and Hershey, 1961), was

encountered in a number of test holes (Fig. 3, sections AA’ and B-B’). At the Western Mobile (LaFarge) Timnath

pit (Fig. 12) approximately four feet of very indurated

Figure 11.— Photograph showing changes in texture and color between the upper and lower stratigraphic units at the

Western Mobile (LaFarge) 35th Street pit near Greeley, Colorado.

Figure 12.—Photograph showing dirty gravel at Western

Mobile (LaFarge) Timnath pit near Fort Collins,

Colorado.

11
cobble/pebble gravel with a silt matrix (dirty gravel)

overlies clean sand and pebble gravel. The color is very

dark grayish brown (10YR 3/2 on the Munsell soil color

chart). The dirty gravel was observed on the floodplain of

the Cache la Poudre River at the Timnath pit, but from

drill hole data, appears to occur within the Broadway

Alluvium. The unit may be a debris flow that originated

on the valley side and slid onto the floodplain.

Sampling Plan

Gravel deposits beneath the floodplain and low

terraces of the Cache la Poudre River valley were

sampled to measure downstream and stratigraphic

variation in particle size and other characteristics that

might be indicative of aggregate quality. Although the

primary target of sampling was downstream variation,

field observations revealed local stratigraphic units in the

gravel that presented the possibility of subtle vertical

variation.

A multilevel sample design was used to measure

variation in the gravel deposits (Fig. 2). Strategies for

sampling gravel have been discussed by Lindsey and

Langer (1998, 1999). Eight sites were selected at gravel

pits from Fort Collins to Greeley, representing about 25

miles of the Cache la Poudre valley (Fig. 2). Although

sites are more or less evenly spaced about 4-5 miles apart,

distance between sites ranges from 1-3 miles for a cluster

of 3 sites to 7 miles for the sites located at the upstream

end. At all sites except the Western Mobile (LaFarge) 35th

Street pit near Greeley, two or three vertical sections

through the entire thickness of gravel were selected

approximately 500 feet apart and then sampled. At three

sites, replicate samples of sections were collected and

sieved to estimate sample error for particle size.

The use of a sample design allows partition of the

natural variation in the character of the gravel. The model

described above targets estimation of downstream

variation and compares this variation to other sources of

variation, such as local variation or sampling error, that

may obscure downstream trends. The equation for the

variance model is:

S

 T

2

 = sα

2

 + s

β

2

 + se

2

Where ST

2

 = total variance, sα

2 = variance among sites

(downstream variation), s

β

2 = variance among sections

(local variation), and is se

2

 = sample (error variance).

In addition to the above-described sampling plan,

individual gravel units at all sections and sites except the

Taft Hill pit were sampled to test for differences between

the upper and lower units. As described under General

Stratigraphy, the upper and lower gravel units at each

locality exhibit subtle differences in color and sorting.

Although the two units are not consistent in appearance

throughout the Cache la Poudre River valley, they were

sampled separately to verify that observation.

Sample Processing

Vertical trench samples (Krumbein and Pettijohn,

1938) were collected by using a shovel or hoe to scrape

gravel from a vertical indentation cut in a clean pit face.

All of the gravel that falls is caught in a bucket. Samples

ranged from 36 to 203 pounds, with an average weight of

69 pounds. In general, the sample size was largest for the

coarsest gravel, as recommended by Wentworth (1926).

Samples were sieved into size fractions greater than

3/16, 3/8, 3/4, 1.5, and 3 inches; the proportion of

particles greater than 6 inches, present at one site, was

estimated in the field. All material that passes through the

3/16-inch sieve is classified as fines. Particle size was

measured in inches and fractions of inches; however, it is

convenient to convert measurements into millimeters for

calculating some size parameters, such as median and

quartile sizes. Accordingly, both English and metric units

are used to express particle size.

The 3/4 to 1.5-inch pebble fraction of each sample

was reserved for determination of rock type, shape, and

roundness. Fifty pebbles from each sample were selected

by cone-and-quarter splitting. A total of 1,450 pebbles

from all eight sample sites were examined. In addition to

rock identification, each pebble was classified by physical

and chemical aggregate quality (Langer and Knepper,

1998). Shape was determined by measuring the long (A),

intermediate (B), and short (C) dimensions with a ruler.

The tendency for equidimensional shape, termed

sphericity, was calculated from three axes (Krumbein,

1941). Roundness was estimated by visual comparison

with standards. Study methods, statistical analysis, and

interpretation of particle size, shape, and roundness are

described by Pettijohn (1975) and additional details on

methods of measuring and statistical analysis are provided

by Krumbein and Pettijohn (1938). Sample data are

summarized in Appendix Tables A1-A5; downstream

distances used for plotting data are given in Table A6.

Particle Size

The quantity of coarse gravel, expressed as average

cumulative weight percent (pct) at each site, decreases

downstream from Fort Collins to Greeley (Fig. 13).

Above Fort Collins, about 50 pct of gravel collects on the

1.5-inch sieve, whereas barely 30 pct collects on the same

sieve size at Greeley. Likewise, three-quarter inch gravel

ranges from 60 pct above Fort Collins to about 50 pct at

Greeley. For gravel finer than 3/4 inch, the downstream

decline is barely noticeable. Most variation in the quantity

of fine gravel is local. Linear regression lines were fitted

to site averages to enable prediction of downstream

trends, but considerable local variation is evident.

The variation (expressed as variance) of particle size

at each level of the sample design was estimated to assess

downstream versus local effects (Table 1; computational

methods of Krumbein and Graybill, 1965). The percent of

total variance between sites (representing downstream

variation) increases with increasing particle size, from

12
SIZE CLASS

0

10

20

30

40

WEIGHT PERCENT

SIZE CLASS 0

10

20

30

40

WEIGHT PERCENT

SIZE CLASS 0

10

20

30

40

WEIGHT PERCENT

0

10

20

30

40

WEIGHT PERCENT

SIZE CLASS

SIZE CLASS 0

10

20

30

40

WEIGHT PERCENT

0

10

20

30

40

WEIGHT PERCENT

SIZE CLASS

SIZE CLASS 0

10

20

30

40

WEIGHT PERCENT

EXPLANATION

SIZE CLASS

0

10

20

30

40

WEIGHT PERCENT

0 5 MILES

0 5 KILOMETERS

N

SIZE CLASS (INCHES)

+ 6

+ 3 - 6

+ 1.5 - 3

+ 3/4 - 1.5

+ 3/8 - 3/4

+ 3/16 - 3/8

- 3/16

PERCENT > 3 INCHES

PERCENT > 1.5 INCHES

PERCENT > 3/4 INCH

PERCENT > 3/8 INCH

PERCENT > 3/16 INCH

EXPLANATION

0

10

20

30

40

50

60

70

80

100

CUMULATIVE WEIGHT PERCENT

90

0 10 15 20 25 30

DOWNSTREAM DISTANCE IN MILES

35

PERCENT > 3 INCHES

PERCENT > 1.5 INCHES

PERCENT > 3/4 INCH

PERCENT > 3/8 INCH

PERCENT > 3/16 INCH

35TH ST HIW HIG TM TH POE W GW

Ft. Collins

Greeley

TH

POE

W

HIW

HIG

35TH ST GW

TM

Figure 13.—Downstream variation in particle size, Cache la Poudre River valley gravel. Histograms and graph show

average values at each site. Abbreviations for sites at gravel pits: TH, LaFarge Taft Hill; POE, LaFarge Port-ofEntry; TM, LaFarge Timnath; W, LaFarge Weitzel; HIW, Hall Irwin Windsor; HIG, Hall Irwin Greeley; 35TH ST,

LaFarge 35th Street Greeley; and GW, LaFarge Greeley West (25th Street).

Table 1.--Variance components for gravel particle size parameters, Cache la Poudre River valley

from Fort Collins to Greeley, Colorado. PERCENT>, cumulative weight percent for stated

particle size; Q1, first quartile (25th percentile); Q3, third quartile (75th percentile); MM,

millimeters. Average sample weight, 70 lbs; range, 36-203 lbs. *, F-test for differences among

means significant at 0.05 level.

SIZE PARAMETER

TOTAL

VARIANCE

BETWEEN

SITES (8)1

BETWEEN

SECTIONS2

(1-3/SITE)

BETWEEN

SAMPLES 3

(1-2/SECTION)

Percent of Total Variance

PERCENT > 3/16 INCH 24 0 38 62

PERCENT > 3/8 INCH 35 17 4 79

PERCENT> 3/4 INCH 49 43* 0 57

PERCENT > 1.5 INCHES 91 67* 0 33

PERCENT > 3 INCHES 121 77* 0 23

Q1 (MM) 4 0 0 100

MEDIAN (MM) 60 58* 2 40

Q3 (MM) 307 68* 0 32

ESTIMATED

MAXIMUM SIZE (MM) 1215 67* 0 33

1

 Downstream variation

2

 Local variation

3

 Sample error

13
zero for the cumulative weight percent of particles > 3/16

inch, to 77 pct for the cumulative weight percent of

particles > 3 inches. Analysis of variance (F-tests) shows

that the sample design was sufficient to distinguish

cumulative weight percent of particle sizes > 3/4 inch

among sites. Thus, downstream trends for coarse sizes

(Fig. 13) are considered stable; trends for cumulative

weight percent of particle sizes of > 3/8 and > 3/16 inch

are not predictable, mainly because the cumulative values

for these sizes do not vary appreciably downstream. For

small particle size, downstream variation is too small

relative to sample error to measure with confidence.

Potential stratigraphic (vertical) variation in gravel

coarseness was tested by comparing samples taken from

the upper unit of the gravel with those taken from the

lower unit (Fig. 14). At some places, the upper and lower

units of the gravel could be distinguished by subtle

differences in color, size, or sorting, but the available data

suggests only minor stratigraphic effect on particle size.

For particle size, the large magnitude of sampling error

(Table 1) severely limits comparisons based on

unreplicated samples from the upper and lower units of

the gravel. If the upper and lower units of the gravel at

each site have the same particle size, their ratios should be

near one. For cumulative weight percent of particles > 3

inches, the ratio is less than one, suggesting concentration

of the coarsest gravel in the lower unit. For ratios of other

particle sizes, considerable scatter is evident (Fig. 14).

Ratios are not consistent among adjacent sites, possibly

indicating lateral variation in particle size within each

stratigraphic unit. However, much of the variation in

ratios may also be interpreted as random because sample

error is large.

Lithology and Gravel Quality

Aggregate quality is related in part to gravel

lithology; hence the two are discussed together. Potential

sources of aggregate are defined according to general

physical and chemical quality. Physical quality is defined

as satisfactory, fair, or poor, and relates to lithology and

weathering. Satisfactory aggregate has physical properties

that make it suitable for most purposes. Satisfactory

aggregate contains clasts that generally are strong, hard,

relatively free from fractures, and not chiplike; capillary

absorption is very small or absent; and the surface texture

is relatively rough. Fair aggregate has physical properties

that make it useful for many purposes, but commonly can

not be used where engineering specifications are strict

such as in concrete or asphalt. Clasts are generally friable,

moderately fractured, and flat or chiplike; capillary

absorption is small to moderate; and fracture surfaces are

relatively smooth and impermeable. Poor aggregate has

physical properties that greatly limit its use. Clasts are

generally weak, highly fractured, and friable; capillary

absorption is moderate to high; and fracture surfaces are

relatively smooth and impermeable. In many

circumstances, potential sources of crushed stone can be

processed to improve quality to meet special

requirements.

Chemical quality is defined as either innocuous or

deleterious, and is related to mineralogy and presence of

other deleterious material. Innocuous aggregate contains

no constituents that dissolve or react chemically to a

significant extent in the atmosphere, water, or hydrating

portland cement in concrete or mortar under ordinary

conditions. Deleterious aggregate contains a significant

proportion of constituents that are known to react

chemically under conditions ordinarily prevailing in

portland cement concrete or mortar. The reaction may

produce significant volume change, interfere with the

normal course of hydration of portland cement, and

produce other harmful effects on concrete. Deleterious

material is commonly found in silicic volcanic rocks and

ash beds, which may contain volcanic glass and unstable

silica polymorphs.

Overall, the lithology of gravel pebbles in the Cache

0

.2

.4

.6

.8

1

1.2

1.4

1.6

1.8

2

0 5 10 15 20 25 30 35

DOWNSTREAM DISTANCE IN MILES

> 3/16 INCH

> 3/8 INCH

> 3/4 INCH

EXPLANATION

UPPER GRAVEL/LOWER GRAVEL

DOWNSTREAM DISTANCE IN MILES

-.2

0

.2

.4

.6

.8

1

1.2

1.4

1.6

1.8

0 5 10 15 20 25 30 35

UPPER GRAVEL/LOWER GRAVEL

> 1.5 INCHES

> 3 INCHES

EXPLANATION

TM HIW HIG 35TH ST

W

POE

GW

SITES TM HIW HIG 35TH ST

W

POE

GW

SITES

Figure 14.—Plot showing ratio of gravel sizes in upper versus lower gravel unit, Cache la Poudre River valley. Values

for individual samples shown at each site.

14
la Poudre valley is dominated by granite (24.9 pct) and

pegmatite (48.3 pct) (Fig. 15). Gneiss, quartz, and

quartzite make up another 5-7 pct each. These rocks were

derived from the upper part of the Cache la Poudre

drainage basin in the Front Range. Among the minor rock

types in the gravel, diabase and schist were also derived

from the mountains. Volcanic porphyry and sandstone

were derived from lithologic units exposed in the

mountain-front hogbacks or on the plains. The sparseness

of volcanic porphyries accounts for the relatively high

average of 98.9 pct chemically innocuous pebbles in the

gravel.

Variance among sites is low for most rock types, but

quartz, quartzite, and the volcanic porphyries show

significant variation among sites, as does chemical quality

(Table 2). For most other rock types, except for gneiss and

perhaps diabase, variance is concentrated among samples.

Among those lithologic characteristics showing

significant variance among sites, only quartz and quartzite

appear to vary downstream in a systematic fashion (Fig.

15). Quartz plus quartzite is a measure of the physical and

chemical maturity of the gravel. A straight line fitted to

average site values can approximate the downstream

increase in quartz plus quartzite. As expected, the

proportion of physically satisfactory gravel, which has a

high degree of between-sample variation (Table 2), shows

no systematic downstream trend (Fig. 16). The proportion

of chemically innocuous gravel, which varies

significantly among sites, does not follow a discernible

downstream trend either, but is lowest in the vicinity of

Greeley (Fig. 16), where volcanic porphyry is most

abundant (Fig. 15). The abundance of porphyry may, like

quartz plus quartzite, reflect the greater hardness and

durability of porphyry relative to other rock types during

transport. However, the abundance of porphyry does not

appear to increase gradually downstream, like that of

quartz plus quartzite, but is concentrated near Greeley. No

point source of porphyry that could account for an abrupt

appearance is known.

Potential stratigraphic variation in gravel lithology

and quality is weak (Figs. 17 and 18). At a few stations,

the ratio of quartz plus quartzite in the upper versus lower

gravel units exceeds 2, suggesting local enrichment of

these resistant rock types by weathering or reworking. In

contrast, ratios for innocuous chemical quality are

consistently near one, indicating that whatever reworking

occurred was not sufficient to affect chemical quality.

Gravel of satisfactory physical quality appears to be less

PERCENT

ROCK NAME

0

10

20

30

40

50

GRANITEGNEISS

PEGMATITE

SANDSTONEQUARTZ

DIABASE

SCHISTQUARTZITE

FELSIC PORPHYRY

60

PERCENT

ROCK NAME

0

10

20

30

40

50

GRANITEGNEISS

PEGMATITE

SANDSTONEQUARTZDIABASESCHISTQUARTZITEFELSIC PORPHYRY

PERCENT

0

10

20

30

40

50

GRANITEGNEISS

PEGMATITE

SANDSTONEQUARTZDIABASESCHIST

QUARTZITE

MAFIC PORPHYRY

ROCK NAME

PERCENT

ROCK NAME

GRANITEGNEISS

SANDSTONEQUARTZ

DIABASESCHIST

QUARTZITE

PEGMATITE

0

10

20

30

40

50

PERCENT

ROCK NAME

0

10

20

30

40

50

INTERMEDIATE PORPHYRY

GNEISS

SANDSTONE

QUARTZ

DIABASE

SCHIST

QUARTZITE

FELSIC PORPHYRY

MAFIC PORPHYRY

PEGMATITE

GRANITE

PERCENT

ROCK NAME

GRANITE

GNEISS

SANDSTONEQUARTZSCHIST

QUARTZITE

MAFIC PORPHYRY

0

10

20

30

40

50

PEGMATITE

PERCENT

ROCK NAME

0

10

20

30

40

50

GRANITEGNEISS

SANDSTONEQUARTZDIABASESCHIST

QUARTZITE

FELSIC PORPHYRY

INTERMEDIATE PORPHYRY

PEGMATITE

PERCENT

ROCK NAME

0

10

20

30

40

50

GRANITE

GNEISS

SANDSTONE

QUARTZ

DIABASESCHISTQUARTZITE

FELSIC PORPHYRY

PEGMATITE

0 5 MILES

0 5 KILOMETERS

N

0

5

10

15

20

0 5 10 15 20 25 30 35

DOWNSTREAM DISTANCE IN MILES

R = .0.84

QUARTZ + QUARTZITE IN PERCENT

35TH ST

HIG HIW

TM

POE

GW

TH

TM

SUMMARY

ROCK NAME

0

10

20

30

40

50

PERCENT

GRANITE 24.9 PCT

GNEISS 7.1 PCT

SANDSTONE 1.9 PCT

QUARTZ 4.9 PCTDIABASE 2.5 PCT

SCHIST 1.2 PCT

QUARTZITE 7.4 PCT

FELSIC PORPHYRY 1 PCT

INTERMEDIATE PORPHYRY 0.1 PCT

PEGMATITE 48.3 PCT

MAFIC PORPHYRY 0.7 PCT

Ft. Collins

Greeley

TH

POE

W

HIW

HIG

35TH ST GW

TM

Figure 15.—Downstream variation in pebble lithology, Cache la Poudre River valley gravel. Histograms and graph

show average values at each site. Abbreviations for sites are same as fig. 2.

15
Table 2.--Variance components for gravel lithology and quality, Cache la Poudre River valley

from Fort Collins to Greeley, Colorado. Rock type and quality determined for 50 pebbles per

sample. *, F-test for differences among means significant at 0.05 level. PCT, percent.

ROCK TYPE OR

QUALITY (IN PCT)

TOTAL

VARIANCE

BETWEEN

SITES (8)1

BETWEEN

SECTIONS2

(1-3/SITE)

BETWEEN

SAMPLES 3

(1-2/SECTION)

Percent of Total Variance

GRANITE 23 18 27 55

GNEISS 18 0 71* 29

PEGMATITE 81 9 0 91

SANDSTONE 5.4 9 0 91

QUARTZ 21 49* 0 51

DIABASE 10 0 46 54

SCHIST 3.0 22 13 65

QUARTZITE 19 39* 0 61

FELSIC PORPHYRY 3.0 54* 0 46

INTERMEDIATE

PORPHYRY 0.3 17 0 83

MAFIC PORPHYRY 3.2 18* 0 82

QUARTZ + QUARTZITE 33 28* 0 72

TOTAL PORPHYRY 6.3 23* 0 77

FAIR PHYSICAL 81 36 5 59

POOR PHYSICAL 32 17 7 76

SATISFACTORY

PHYSICAL 50 1 10 89

DELETERIOUS

CHEMICAL 3.0 62* 0 38

INNOCUOUS CHEMICAL 3.0 62* 0 38

1

 Downstream variation

2

 Local variation

3

 Stratigraphic variation (includes sampling error)

INNOCUOUS CHEMICAL IN PERCENT

95

96

97

98

99

100

0 5 10 15 20 25 30 35

DOWNSTREAM DISTANCE IN MILES

TM HIW HIG 35ST

W

POE

GW

SITES TH

65

70

75

80

85

0 5 10 15 20 25 30 35

DOWNSTREAM DISTANCE IN MILES

SATISFACTORY PHYSICAL IN PERCENT

TM HIW HIG 35ST

W

POE

GW

SITES TH

Figure 16.—Downstream variation in gravel quality, Cache la Poudre River valley. Average values for each site shown.

Abbreviations for sites are same as fig. 2.

16
Figure 17.—Plot showing ratio of pebble lithology in

upper versus lower gravel unit, Cache la Poudre

River valley. Values for individual samples shown.

Figure 18.—Plot showing ratio of gravel quality in upper

versus lower gravel unit, Cache la Poudre River

valley. Values for individual samples shown.

0

.5

1

1.5

2

2.5

3

3.5

4

UPPER GRAVEL/LOWER GRAVEL

0 5 10 15 20 25 30 35

DOWNSTREAM DISTANCE IN MILES

PCT QUARTZ + QUARTZITE

PCT PEGMATITE

PCT GNEISS

PCT GRANITE

EXPLANATION

TM HIW HIG 35TH ST

W

POE

GW

SITES

.6

.7

.8

.9

1

1.1

1.2

UPPER GRAVEL/LOWER GRAVEL

0 5 10 15 20 25 30 35

DOWNSTREAM DISTANCE IN MILES

PCT INNOCUOUS CHEMICAL QUALITY

PCT SATISFACTORY PHYSICAL QUALITY

EXPLANATION

TM HIW HIG 35TH ST

W

POE

GW

SITES

Figure 19.—Zingg diagram showing pebble shape, Cache

la Poudre River valley gravel. Shape axes defined in

Table 3.

The shapes of pebbles in gravel of the Cache la

Poudre River valley follow a pattern comparable but not

identical to gravel of the South Platte River valley (Figs.

19 and 20) (Lindsey and others, 1998b). As classified on a

Zingg diagram, pebbles are disc-shaped (38.8 pct versus

44.0 pct for the South Platte), spherical (30.8 pct versus

25.4 pct for the South Platte), rod-shaped (18.3 pct versus

16.0 pct for the South Platte) and blade-shaped (12.1 pct

versus 14.6 pct for the South Platte). The shift toward

spherical particles in the Cache la Poudre versus the

South Platte may be due to the smaller proportion of

layered gneiss in Poudre gravel than in South Platte

0

.1

.2

.3

.4

.5

.6

.7

.8

.9

1

B/A

0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1

C/B

BLADE-SHAPED 12.1 PCT ROD-SHAPED

 18.3 PCT

SPHERICAL

 30.8 PCT

 DISCSHAPED

38.8 PCT

abundant in the upper unit of the gravel than in the lower

unit (ratios less than one, Fig. 18). Perhaps, the source of

the upper unit has been weathered more than the lower

unit.

Pebble Shape

Particle shape affects strength; flat and blade-shaped

particles tend to be weak, whereas equidimensional

particles tend to be strong (Langer and Knepper, 1998).

Measurements of pebble shape in gravel of the Cache la

Poudre River valley show a preponderance of strong

particle shapes.

0

10

20

30

40

.3 .4 .5 .6 .7 .8 .9 1

SPHERICITY

PERCENT OF MEASUREMENTS

Figure 20.—Histogram showing calculated pebble

sphericity, Cache la Poudre River valley gravel.

Sphericity defined in caption, Table 3.

17
gravel. Pebbles having axial ratios of less than 0.5, which

might be structurally weak, are rare. Average calculated

sphericity (0.707) is close to that of South Platte gravel

(0.692).

Variance among sites is low for all shape parameters

(Table 3). Most variation is local or among samples. No

statistically significant downstream trend in shape can be

detected in the available data.

Potential stratigraphic variation in pebble shape was

examined, but ratios comparing the upper and lower units

of the gravel were found to vary widely. The overall

average of ratios is near one, suggesting no general

stratigraphic control of shape (Fig. 21).

Pebble Roundness

Roundness refers to the degree to which particles

lack angular corners; roundness may affect the behavior

of aggregate particles in some applications, such as the

tendency to shift under load in loose aggregate or asphalt.

Gravel in the Cache la Poudre valley tends to be

rounded to well-rounded (Fig. 22). Angular pebbles are

few. For the proportion of rounded and well-rounded

pebbles, as well as average roundness, variance among

sites is significant (Table 4). Average roundness values

decline to a point 5-10 miles above Greeley but rise

abruptly at Greeley (Fig. 22). The reason for this high,

and unpredictable, variation among sites downstream is

not known. In the streambed of the South Platte, pebble

roundness was observed to decrease downstream (Lindsey

and Shary, 1997).

Stratigraphic variation in pebble roundness is

suggested by high ratios for angular, subangular, and

subrounded particles in the upper unit versus the lower

unit of gravel at some sites (Fig. 23). Sparse angular

Figure 21.—Plot showing ratio of pebble shape in upper

versus lower gravel unit, Cache la Poudre River

valley. Values for individual samples shown.

pebbles tend to be more abundant in the upper unit of the

gravel. Freezing and thawing during weathering of the

source gravel, or breakage during stream transport, might

account for the greater abundance of angular pebbles in

the upper unit.

CONCLUSIONS

Four different gravel deposit models are defined for

the Cache la Poudre River valley. From the highest

Table 3.--Variance components for gravel pebble shape, Cache la Poudre River valley from

Fort Collins to Greeley, Colorado. A = long axis; B = intermediate axis; C = short axis.

MEAN SPHERICITY (Ψ) = 3

√(BC/A2), measures degree of equidimensional shape; when

A=B=C, Ψ = 1. PCT, percent of pebbles in 3/4-1.5 inch size fraction. Shape classified as

spherical, disc-, rod-, and blade-shaped by criteria of Zingg (Pettijohn, 1975, fig. 3-18), 50

pebbles per sample. *, F-test for differences among means significant at 0.05 level. PCT,

percent.

SHAPE PARAMETER

TOTAL

VARIANCE

BETWEEN

SITES (8)1

BETWEEN

SECTIONS2

(1-3/SITE)

BETWEEN

SAMPLES 3

(1-2/SECTION)

Percent of Total Variance

MEAN B/A 0.00058 0 51* 49

MEAN C/B 0.00063 6 33 61

MEAN SPHERICITY (Ψ) 0.00038 0 53* 47

PCT SPHERICAL 49 0 53* 47

PCT DISC-SHAPED 57 9 6 85

PCT ROD-SHAPED 30 0 40 60

PCT BLADE-SHAPED 29 0 38 62

1

 Downstream variation

2

 Local variation

3

 Stratigraphic variation (includes sampling error)

TM HIW HIG 35TH ST

W

POE

GW

SITES

-.5

0

.5

1

1.5

2

2.5

UPPER GRAVEL/LOWER GRAVEL

0 5 10 15 20 25 30 35

DOWNSTREAM DISTANCE IN MILES

PCT BLADE-SHAPED

PCT ROD-SHAPED

PCT DISC-SHAPED

PCT SPHERICAL

EXPLANATION

18
Table 4.--Variance components for gravel pebble roundness, Cache la Poudre River valley from

Fort Collins to Greeley, Colorado. PCT, percent of pebbles in 3/4-1.5 inch size fraction;

roundness estimated by visual comparison with roundness classes (Pettijohn, 1975, fig. 3-24),

50 pebbles per sample. AVERAGE ROUNDNESS = ((PCT A x .125) + (PCT B x .2) +

(PCT C x .315) + (PCT D x .5) + (PCT E x .8)) /100. *, F-test for differences among means

significant at 0.05 level. PCT, percent.

ROUNDNESS

PARAMETER

TOTAL

VARIANCE

BETWEEN

SITES (8)1

BETWEEN

SECTIONS2

(1-3/SITE)

BETWEEN

SAMPLES 3

(1-2/SECTION)

Percent of Total Variance

PCT A (ANGULAR) 7 5 0 95

PCT B (SUBANGULAR) 21 0 11 89

PCT C (SUBROUNDED) 50 23 0 77

PCT D (ROUNDED) 75 37* 0 63

PCT E (WELL-ROUNDED) 160 51* 0 49

AVERAGE ROUNDNESS 0.0036 40* 0 60

1

 Downstream variation

2

 Local variation

3

 Stratigraphic variation (includes sampling error)

60

0

10

20

30

40

50

PERCENT

A B C D E

ROUNDNESS

N

0

10

20

30

40

50

PERCENT

A B C D E

ROUNDNESS

0

10

20

30

40

50

PERCENT

A B C D E

ROUNDNESS

0

10

20

30

40

50

PERCENT

A B C D E

ROUNDNESS

0

10

20

30

40

PERCENT

A B C D E

ROUNDNESS

0

10

20

30

40

50

A B C D E

ROUNDNESS

PERCENT

0

10

20

30

40

50

60

PERCENT

A B C D E

ROUNDNESS 0

10

20

30

40

50

60

PERCENT

A B C D E

ROUNDNESS

0 5 MILES

0 5 KILOMETERS

AVERAGE ROUNDNESS

.

.5

.

.6

.

0 51 1 2 2 3 3

DOWNSTREAM DISTANCE IN MILES

35TH ST

HIG HIW

TM POE

GW

TH

W

0

10

20

30

40

50

PERCENT

A B C D E

ROUNDNESS

SUMMARY

Ft. Collins

Greeley

TH

POE

W

HIW

HIG

35TH ST GW

TM

Figure 22.—Downstream variation in pebble roundness, Cache la Poudre River valley gravel.

Histograms and graph show average values at each site. Roundness classes are: A, angular; B,

subangular; C, subrounded; D, rounded; and E, well-rounded. Average roundness (defined in

Table 4) at each site shown in upper right.

19
(oldest) to lowest (youngest) the models are 1) dissected

terraces, 2) alluvial fans, 3) high terraces, and 4)

floodplains and low terraces. The floodplain and low

terraces are underlain by coarse gravel, which currently is

the most exploited aggregate resource in the Cache la

Poudre River valley.

From top to bottom, the floodplain and low terraces

are underlain by 1) 3-5 feet of soil and overbank deposits

and 2) two well-sorted, stratified, gravel deposits. A third

stratigraphic unit, described as “dirty gravel,” locally

occurs in the study area; it consists of very indurated

cobble/pebble gravel with a silt matrix.

Gravels of the Cache la Poudre River valley are

coarsest upstream. Downstream decrease in the

proportion of coarse gravel can be estimated by statistical

sampling and analysis, but the proportion of fine gravel

and sand is too variable to estimate with confidence. With

only a few exceptions, downstream changes in other

characteristics such as lithology and shape cannot be

predicted with confidence. One exception is the content of

quartz plus quartzite, a measure of physical and chemical

maturity, which increases downstream. Pebble roundness

decreases downstream but rises abruptly at Greeley. The

proportion of physically satisfactory gravel, which is high

at all sites, is too variable to predict downstream. The

proportion of chemically innocuous gravel, everywhere

measured at 96 pct or greater, appears to be highest in the

middle reaches of the Cache la Poudre River valley.

The lower gravel unit contains the highest

proportion of coarse (> 3 inch) particles. The upper gravel

unit tends to contain more quartz plus quartzite, has

poorer physical quality, and contains a higher proportion

of angular pebbles than the lower gravel. Although

weakly defined, these stratigraphic differences are

consistent at most sites. Weathering, followed by transport

in the river, might be expected to concentrate quartz and

quartzite, degrade physical quality, and break pebbles into

angular fragments. This conclusion is consistent with the

appearance at most sites of a sharp, erosional contact

between the upper gravel unit and the lower gravel. The

erosional contact between gravel units, together with

evidence of degradation by weathering and transport prior

to deposition of the upper gravel, indicates that the upper

gravel was derived mainly by reworking of the older,

lower gravel.

REFERENCES CITED

Anonymous, 1994, Munsell soil color charts: New Windsor,

New York, Macbeth Division of Kollmorgen

Instruments Corporation.

Bliss, J.D., 1993, Modeling sand and gravel deposits—initial

strategy and preliminary examples: U. S. Geological

Survey Open-File Report 93-200, 31 p.

Colton, R.B., 1978, Geological map of the Boulder-Fort

Collins-Greeley area, Front Range Urban Corridor,

Colorado: U. S. Geological Survey Miscellaneous

Investigations Map I-855-G, scale 1:100,000.

Colton, R.B., and Fitch, H.R., 1974, Map showing potential

sources of gravel and crushed-rock aggregate in the

Boulder-Fort Collins-Greeley Area, Front Range Urban

Corridor, Colorado: U.S. Geological Survey

Miscellaneous Investigations Map I-856-A, scale

1:100,000.

Cox, D.P., Barton, P.B., and Singer, D.A., 1986, Mineral

deposit models, Introduction, in: Cox, D.P., and Singer,

D.A., eds., Mineral deposit models: U. S. Geological

Survey Bulletin 1693, p. 1-10.

Jiongxin, Xu, 1996, Underlying gravel layers in a large sand

bed river and their influence on downstream-dam

channel adjustment: Geomorphology, v. 17, n. 4, p.

351-360.

Kondolf, G.M., 1998, Environmental effects of aggregate

extraction from river channels and floodplains, in

Bobrowsky, P. T., ed., Aggregate resources – A global

perspective: Rotterdam, Netherlands, Balkema, p. 113-

130.

Krumbein, W. C., 1941, Measurement and geological

significance of shape and roundness of sedimentary

particles: Journal of Sedimentary Petrology, v. 11, no.

2, p. 64-72.

Krumbein, W. C., and Graybill, F. A., 1965, An

introduction to statistical models in geology: New

York, McGraw-Hill, 475 p.

Krumbein, W. C., and Pettijohn, F. J., 1938, Manual of

sedimentary petrography: New York, AppletonCentury-Crofts, 549 p.

Langer, W.H., Arbogast, B.F., Knepper, D.H. Jr., Lindsey,

D.A., Nealey, L.D., and Roelle J.E., 1997, U.S.

Geological Survey Front Range Infrastructure

Resources Project: Proceedings, 5th Annual

Symposium, International Center for Aggregates

Research, University of Austin, Texas, p. E-2-1-1 – E2-1-8.

Figure 12.—Plot showing ratio of pebble roundness in

upper versus lower gravel unit, Cache la Poudre

River valley. Values for individual samples shown.

DOWNSTREAM DISTANCE IN MILES

0

.5

1

1.5

2

2.5

3

3.5

4

4.5

UPPER GRAVEL/LOWER GRAVEL

0 5 10 15 20 25 30 35

PCT E

PCT D

PCT C

PCT A+B

EXPLANATION

TM HIW HIG 35TH ST

W

POE

GW

SITES

20
Langer, W. H., and Knepper, D. H., Jr., 1998, Geologic

characterization of natural aggregate: a field

geologist’s guide to natural aggregate resource

assessment, in Bobrowsky, P. T., ed., Aggregate

resources, a global perspective: Rotterdam, Balkema,

p. 275-293.

Langer, W. H., Lindsey, D. A., Knepper, D. H., Jr., and

Faber, B. G., 1999, A GIS decision-support-system

demonstration of aggregate maps and data for the

Front Range Infrastructure Resources Project Area, in

Johnson, K. S., ed., Proceedings of the 34th Forum on

the geology of industrial minerals, 1998: Oklahoma

Geological Survey Circular 102, p. 139-146.

Lindsey, D.A., 1997, An introduction to sand and gravel

deposit models, Front Range Urban Corridor: U.S.

Geological Survey Open File Report 97-81, 6 p.

Lindsey, D. A., and Langer, W. H., 1998, Slide show on

gravel deposit models and measuring aggregate

quality, Front Range Urban Corridor, Colorado: U.S.

Geological Survey Open-File Report 98-322, 17 p.

Lindsey, D. A., and Langer, W. H., 1999, Defining models

and measuring aggregate quality for gravel deposits

of Front Range Urban Corridor, Colorado, in

Johnson, K. S., ed., Proceedings of the 34th Forum on

the geology of industrial minerals, 1998: Oklahoma

Geological Survey Circular 102, p. 125-137.

Lindsey, D.A., Langer, W.H., Cummings, L. S., and Shary,

J.F., 1998a, Gravel deposits of the South Platte River

Valley north of Denver, Colorado – Part A –

Stratigraphy and sedimentary structure: U.S.

Geological Survey Open File report 98-148-A, 18p.

Lindsey, D.A., Langer, W.H., and Shary, J.F., 1998b, Gravel

deposits of the South Platte River Valley north of

Denver, Colorado – Part B – Quality of gravel deposits

for aggregate: U.S. Geological Survey Open File

report 98-148-B, 24 p.

Lindsey, D. A., and Shary, J. F., 1997, Field measures of

gravel quality in the South Platte River north of

Denver, Colorado: a pilot study: U.S. Geological

Survey Open-File Report 97-451, 19 p.

Norman, D.K., Cederholm, C.J., and Lingley, W.S., Jr.,

1998, Flood plains, salmon habitat, and sand and

gravel mining: Washington Geology, v. 26, no. 2/3, p.

3-20.

Pettijohn, F. J., 1975, Sedimentary rocks (3rd ed.): New

York, Harper and Row, 628 p.

Schneider, P.A.,Jr., and Hershey, L.A., 1961, Records and

logs of selected wells and test hoiles, and chemical

analyses of ground water in the lower Cache la

Poudre River basin, Colorado: Colorado Water

Conservation Board Basic Data Report n. 8, 63 p.

Schwochow, S.D., Shroba, R.R., and Wicklein, P.C., 1974a,

Sand, gravel, and quarry aggregate resources –

Colorado Front Range counties: Colorado Geological

Survey Special Publication 5-A, 43 p.

Schwochow, S.D., Shroba, R.R., and Wicklein, P.C., 1974b

Atlas of sand, gravel, and quarry aggregate resources

– Colorado Front Range counties: Colorado Geological

Survey Special Publication 5-B, not paginated.

Scott, G.R., 1963, Quaternary geology and geomorphic

history of the Kassler quadrangle, Colorado: U.S.

Geological Survey Professional Paper 421-A, p. 1-70.

Scott, G.R., 1963, Quaternary geology and geomorphic

history of the Kassler quadrangle, Colorado: U.S.

Geological Survey Professional Paper 421-A, p. 1-70.

Shelton, D.C., and Rogers, W.P., 1987, Environmental and

engineering geology of the Windsor Study Area,

Larimer and Weld Counties, Colorado: Colorado

Geological Survey Environmental Geology, n. 6, 11

plates, 1:48,000 scale.

Wentworth, C. K., 1926, Methods of mechanical analysis

of sediments: University of Iowa Studies in Natural

History, v. 11, no. 11, 52 p.

21
Table A1.--Particle-size data, sieved gravel samples, Cache la Poudre River valley. All samples from gravel

beneath floodplain and lower (post-Piney Creek) terraces. Sample numbers refer to LaFarge Taft Hill (TH),

Port-of-Entry (POE), Timnath (TM), Weitzel (W), Hall Irwin Windsor (HIW), Hall Irwin Greeley (HIG),

LaFarge 35th Street (35TH), and Greeley West (25th Street, GW). Sample numbers ending in "T," upper gravel

unit; "DG," dirty gravel (= upper unit); "B," lower gravel unit; "C," entire interval of gravel; no letter ending,

entire interval of gravel. >, greater than. P25, first quartile size; P50, median size; P75, third quartile size; max.

(maximum) size, estimated from cumulative frequency curve. MM, millimeters.

SAMPLE CUMULATIVE WEIGHT PERCENT PERCENTILES

NUMBER >6

INCH

>3

INCH

>1.5

INCH

>3/4

INCH

>3/8

INCH

>3/16

INCH

P25

(MM)

P50

(MM)

P75

(MM)

MAX.

SIZE

(MM)

TH-01 12.5 40.7 50.2 63.0 69.2 73.8 4 40 110 200

TH-02 5.7 28.0 48.2 59.5 65.8 70.5 3 35 80 180

POE-01C 0.0 8.2 32.9 50.3 59.5 68.1 3 20 40 90

POE-01T 0.0 8.7 40.8 63.8 74.3 82.3 9 30 50 90

POE-01B 0.0 18.6 41.3 58.0 66.2 71.0 3 25 65 150

POE-02C 0.0 12.9 38.3 55.3 66.8 74.9 5 23 50 100

POE-02T 0.0 16.0 46.9 61.5 70.8 78.1 6 32 65 100

POE-02B 0.0 28.3 47.5 60.6 69.7 75.4 5 32 90 150

POE-03C 0.0 21.4 46.6 66.6 77.7 83.7 11 32 70 150

POE-03T 0.0 0.0 28.8 50.9 64.2 73.0 4 20 35 75

POE-03B 0.0 9.8 41.5 57.3 67.7 75.6 5 25 50 90

TM-01T 0.0 8.2 34.1 49.1 59.1 68.1 3 18 58 94

TM-01B 0.0 15.1 50.9 64.4 72.7 80.5 8 40 60 100

TM-02T 0.0 8.8 35.9 45.3 53.4 60.0 1 12 58 94

TM-02B 0.0 11.9 54.3 67.6 74.8 81.0 9 40 56 90

W-01B 0.0 10.7 44.1 61.2 69.2 75.9 5 30 58 92

W-02T 0.0 0.0 29.2 44.4 52.2 59.6 1 13 42 75

W-02B 0.0 0.0 30.9 48.4 58.9 67.9 3 18 42 75

W-03B 0.0 0.0 42.0 63.2 73.6 80.4 8 30 50 78

HIW-01T 0.0 0.0 18.3 38.1 47.7 58.4 2 8 30 70

HIW-01B 0.0 0.0 40.6 54.2 64.6 73.4 4 24 50 100

HIW-02T 0.0 0.0 15.9 41.1 56.1 65.4 2 12 30 55

HIW-02B 0.0 10.4 37.4 56.8 67.1 76.1 5 24 50 100

HIG-01T 0.0 0.0 20.5 52.2 62.9 70.7 3 20 35 60

HIG-01B 0.0 0.0 30.1 50.1 59.6 67.9 2 20 42 100

HIG-02T 0.0 0.0 27.4 51.4 63.0 74.0 5 20 40 80

HIG-02B 0.0 0.0 23.3 40.5 52.1 66.3 3 11 35 90

35ST-01C 0.0 0.0 26.0 48.4 62.5 73.4 4 18 40 75

35ST-01T 0.0 0.0 41.1 56.3 67.9 77.4 5 25 50 75

35ST-01B 0.0 0.0 34.1 56.7 69.4 78.2 6 22 45 75

GW-01C 0.0 0.0 23.6 45.3 57.6 68.5 3 15 38 75

GW-01T 0.0 0.0 18.4 39.3 51.7 63.7 2 10 30 65

GW-01B 0.0 0.0 30.1 50.4 61.5 70.8 3 20 40 100

GW-02C 0.0 0.0 19.3 43.4 58.8 73.7 4 15 30 65

GW-02T 0.0 0.0 27.8 48.7 66.3 82.4 6 18 40 90

GW-02B 0.0 0.0 24.4 49.8 64.0 76.0 5 20 38 75

22
Table A2.--Gravel lithology, Cache la Poudre River valley. All samples from gravel beneath floodplain and lower (post-Piney Creek) terraces.

Percentages determined from classification of 50 pebbles, 3/4-1.5 inch size class. Sample numbers refer to LaFarge Taft Hill (TH), Port-ofEntry (POE), Timnath (TM), Weitzel (W), Hall Irwin Windsor (HIW), Hall Irwin Greeley (HIG), LaFarge 35th Street (35TH), and Greeley

West (25th Street, GW). Sample numbers ending in "T," upper gravel unit; "DG," dirty gravel (= upper unit); "B," lower gravel unit; no letter

ending, entire interval of gravel.

ROCK TYPE IN PERCENT

PORPHYRY SAMPLE

NUMBER GRANITE GNEIS S PEGMA- TITE SAND- STONE QUARTZ DIABASE SCHIST QUARTZ- ITE FELSIC INTER- MEDIATE MAFICTH-01 22 10 36 4 4 10 6 4 4 0 0TH-02 26 10 54 2 0 2 4 2 0 0 0

POE-01T 28 2 64 0 4 0 0 2 0 0 2POE-01B 30 6 40 4 4 2 4 8 2 0 0POE-02T 22 6 54 0 0 8 0 6 0 0 0POE-02B 22 10 44 6 2 10 0 6 0 0 0POE-03T 20 8 54 8 2 0 2 6 0 0 0POE-03B 18 8 60 2 0 8 0 4 0 0 0

TM-01T 20 14 46 0 0 4 0 16 0 0 0TM-01B 28 10 54 0 2 2 0 4 0 0 0TM-02DG 32 8 42 0 4 0 2 10 0 0 2TM-02B 30 8 48 2 2 0 0 10 0 0 0W-02T 24 6 44 6 8 0 2 10 0 0 0W-02B 26 6 54 0 2 0 0 10 0 0 0

W-03B 24 22 42 2 2 2 0 6 0 0 0HIW-01T 28 6 44 0 20 0 0 2 0 0 0HIW-01B 30 10 34 2 6 6 0 2 2 0 8HIW-02T 34 2 52 0 6 2 2 2 0 0 0HIW-02B 28 8 24 0 14 4 4 12 0 2 4

HIG-01T 26 4 52 0 2 0 2 14 0 0 0HIG-01B 22 6 52 0 6 0 0 12 0 0 1HIG-02T 32 8 36 4 2 0 0 14 0 0 4HIG-02B 26 2 58 0 2 0 0 12 0 0 035ST-01T 22 6 42 2 8 2 2 14 0 2 035ST-01B 14 4 60 2 4 4 2 6 4 0 0

GW-01T 26 8 44 0 10 0 4 4 4 0 0GW-01B 18 6 50 6 8 4 0 4 4 0 0GW-02T 20 2 54 2 8 2 0 10 2 0 0GW-02B 26 0 56 0 10 0 0 2 6 0 0

23
Table A3.--Gravel quality, Cache la Poudre River valley. All samples from gravel beneath

floodplain and lower (post-Piney Creek) terraces. Percentages determined from classification of

50 pebbles, 3/4-1.5 inch size class. Sample numbers refer to LaFarge Taft Hill (TH), Port-ofEntry (POE), Timnath (TM), Weitzel (W), Hall-Irwin Windsor (HIW), Hall Irwin Greeley

(HIG), LaFarge 35th Street (35TH), and Greeley West (25th Street, GW). Sample numbers

ending in "T," upper gravel unit; "DG," dirty gravel (= upper unit); "B," lower gravel unit; no

letter ending, entire interval of gravel.

QUALITY IN PERCENT

SAMPLE PHYSICAL CHEMICAL

NUMBER FAIR POOR SATISFACTORY DELETERIOUS INNOCUOUS

TH-01 8 12 80 4 96

TH-02 2 16 82 0 100

POE-01T 0 16 82 0 100

POE-01B 6 24 70 2 98

POE-02T 20 8 72 0 100

POE-02B 8 10 82 0 100

POE-03T 4 18 78 0 100

POE-03B 6 24 70 0 100

TM-01T 14 16 70 0 100

TM-01B 26 2 72 0 100

TM-02DG 22 16 62 0 100

TM-02B 12 6 82 0 100

W-02T 24 2 74 0 100

W-02B 10 12 78 0 100

W-03B 24 8 68 0 100

HIW-01T 12 14 74 0 100

HIW-01B 10 8 82 2 98

HIW-02T 22 6 72 0 100

HIW-02B 16 8 76 2 98

HIG-01T 30 12 58 0 100

HIG-01B 32 8 60 0 100

HIG-02T 14 8 78 0 100

HIG-02B 20 2 78 0 100

35ST-01T 22 10 68 2 98

35ST-01B 4 18 78 4 96

GW-01T 12 12 76 4 96

GW-01B 4 12 84 4 96

GW-02T 14 14 72 2 98

GW-02B 0 14 86 6 94

24
Table A4.--Gravel shape, Cache la Poudre River valley. All samples from gravel beneath floodplain and lower (post-Piney

Creek) terraces. Percentages determined from measurement of 50 pebbles, 3/4-1.5 inch size class. A = long axis; B =

intermediate axis; C = short axis. MEAN SPHERICITY (Ψ) = 3

√(BC/A2), measures degree of equidimensional shape; when

A=B=C, Ψ = 1. Shape classified as spherical, disc-, rod-, and blade-shaped by criteria of Zingg (Pettijohn, 1975, fig. 3-18.

Sample numbers refer to LaFarge Taft Hill (TH), Port-of-Entry (POE), Timnath (TM), Weitzel (W), Hall Irwin Windsor

(HIW), Hall Irwin Greeley (HIG), LaFarge 35th Street (35TH), and Greeley West (25th Street, GW). Sample numbers ending in

"T," upper gravel unit; "DG," dirty gravel (= upper unit); "B," lower gravel unit; no letter ending, entire interval of gravel.

SAMPLE SHAPE CLASS IN PERCENT

NUMBER

MEAN

B/A

MEAN

C/B

MEAN

SPHERICITY SPHERICAL DISCSHAPED

RODSHAPEDBLADESHAPED

TH-01 .740 .630 .692 28 42 20 10

TH-02 .760 .680 .726 40 34 14 12

POE-01T .760 .660 .715 34 42 16 8

POE-01B .770 .680 .725 32 34 20 14

POE-02T .730 .680 .696 26 38 26 10

POE-02B .670 .660 .649 16 24 34 26

POE-03T .750 .690 .724 48 34 14 4

POE-03B .740 .720 .721 36 28 24 12

TM-01T .720 .660 .691 30 36 20 14

TM-01B .750 .670 .715 38 34 14 14

TM-02DG .760 .670 .721 36 36 14 14

TM-02B .750 .670 .714 26 48 20 6

W-02T .740 .690 .714 34 30 22 14

W-02B .750 .680 .719 42 32 18 8

W-03B .760 .620 .704 26 46 12 16

HIW-01T .740 .700 .723 38 32 20 10

HIW-01B .750 .650 .709 30 42 12 16

HIW-02T .750 .710 .728 32 38 26 4

HIW-02B .720 .660 .690 30 38 20 12

HIG-01T .770 .640 .715 28 42 14 16

HIG-01B .760 .620 .703 28 52 10 10

HIG-02T .710 .640 .672 14 48 24 14

HIG-02B .730 .640 .692 26 44 20 10

35ST-01T .690 .670 .676 32 28 22 18

35ST-01B .750 .650 .707 24 50 10 16

GW-01T .730 .640 .694 26 40 14 20

GW-01B .740 .670 .703 30 34 18 18

GW-02T .780 .650 .727 32 54 14 0

GW-02B .770 .690 .734 32 44 18 6

25
Table A5.--Gravel roundness, Cache la Poudre River valley. All samples from

gravel beneath floodplain and lower (post-Piney Creek) terraces. Percentages

estimated by visual comparison of 50 pebbles, 3/4-1.5 inch size class, by method of

Pettijohn (1975, fig. 3-24). Mean roundness = class abundance X mid-point values

of Pettijohn (1975, table 3-9). Classes defined as follows: A, angular; B,

subangular; C, subrounded; D, rounded; E, well rounded. Sample numbers refer to

LaFarge Taft Hill (TH), Port-of-Entry (POE), Timnath (TM), Weitzel (W), Hall

Irwin Windsor (HIW), Hall Irwin Greeley (HIG), LaFarge 35th Street (35TH), and

Greeley West (25th Street, GW). Sample numbers ending in "T," upper gravel unit;

"DG," dirty gravel (= upper unit); "B," lower gravel unit; no letter ending, entire

interval of gravel.

SAMPLE ROUNDNESS CLASS IN PERCENT

NUMBER A B C D E

AVERAGE

ROUNDNESS

TH-01 2 6 10 18 64 .648

TH-02 4 6 2 26 62 .649

POE-01T 0 12 16 22 50 .584

POE-01B 4 2 6 36 52 .624

POE-02T 6 12 20 22 40 .525

POE-02B 6 4 8 20 62 .637

POE-03T 4 12 16 26 42 .545

POE-03B 6 14 10 28 42 .543

TM-01T 10 10 14 20 46 .545

TM-01B 0 10 12 28 50 .598

TM-02DG 8 14 24 20 34 .486

TM-02B 4 8 26 32 30 .503

W-02T 6 8 16 16 54 .586

W-02B 2 12 14 28 44 .563

W-03B 2 8 24 20 46 .562

HIW-01T 10 14 18 28 30 .477

HIW-01B 4 2 18 44 32 .542

HIW-02T 6 16 30 26 22 .440

HIW-02B 0 10 16 32 42 .566

HIG-01T 4 16 20 34 26 .478

HIG-01B 2 12 16 44 26 .505

HIG-02T 2 6 32 42 18 .469

HIG-02B 2 0 10 44 44 .606

35ST-01T 0 8 16 36 40 .566

35ST-01B 4 2 12 26 56 .625

GW-01T 2 4 10 48 36 .570

GW-01B 2 4 16 28 50 .601

GW-02T 2 2 6 28 62 .661

GW-02B 2 6 10 26 56 .624

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Table A6.--Sample sites, site symbols, and miles downstream

from Taft-Hill gravel pit, Cache la Poudre River valley. All

sample sites are in gravel pits operated by Lafarge, except as

noted. Miles downstream were measured in a straight line. Five

miles were added to miles downstream to yield downstream

distance plotted in figures.

SAMPLE

SITE

SITE

SYMBOL

MILES

DOWNSTREAM

TAFT HILL TH 0.0

PORT-OF-ENTRY POE 7.1

TIMNATH TM 8.7

WEITZEL W 8.8

HALL IRWIN WINDSOR HIW 14.3

HALL IRWIN GREELEY HIG 20.4

35TH STREET 35TH ST 24.4

GREELEY WEST* GW 25.4

* 25TH Street

27