U.S. Department of the Interior

U.S. Geological Survey

Geologic Studies of the Platte River, SouthCentral Nebraska and Adjacent Areas—Geologic 

Maps, Subsurface Study, and Geologic History

Professional Paper 1706
Professional Paper 1706

U.S. Department of the Interior

U.S. Geological Survey 

Geologic Studies of the Platte River, South-Central 

Nebraska and Adjacent Areas—Geologic Maps, 

Subsurface Study, and Geologic History

By Steven M. Condon
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Version 1.0, 2005

This publication and any updates to it are available online at:

http://pubs.usgs.gov/pp/pp1706/

Manuscript approved for publication, March 3, 2005

Text edited by James W. Hendley II

Layout and design by Stephen L. Scott

Any use of trade, product, or fi rm names in this publication

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 

Cover—View from the bridge over the Platte River just south of Overton, Nebraska, 

looking downstream (to the east). (USGS photo taken in 2002 by Steven M. Condon.)
Contents

Abstract ------------------------------------------------------------------------------------------------------ 1

Introduction -------------------------------------------------------------------------------------------------- 1

Acknowledgments ------------------------------------------------------------------------------ 3

I. Geologic maps of the Elm Creek West and Newark 7.5 minute quadrangles ---------------------- 3

Introduction -------------------------------------------------------------------------------------------- 3

Methods ------------------------------------------------------------------------------------------------ 3

Elm Creek West quadrangle ------------------------------------------------------------------------- 4

Description of map units ----------------------------------------------------------------------- 4

Discussion --------------------------------------------------------------------------------------- 5

Logs of test holes in the Elm Creek West quadrangle -------------------------------------- 7

Newark quadrangle ---------------------------------------------------------------------------------- 12

Description of map units ---------------------------------------------------------------------- 12

Discussion -------------------------------------------------------------------------------------- 13

Logs of test holes in the Newark quadrangle ----------------------------------------------- 16 

II. Subsurface Cenozoic geology of south-central Nebraska ----------------------------------------- 22

Introduction ------------------------------------------------------------------------------------------- 22

Methods ----------------------------------------------------------------------------------------------- 22

Subsurface geology --------------------------------------------------------------------------------- 23

III. The development of the Cenozoic Platte River drainage system --------------------------------- 31 

Introduction ------------------------------------------------------------------------------------------- 31

Structural setting ------------------------------------------------------------------------------------- 32

Precursor events ------------------------------------------------------------------------------- 32

Laramide orogeny ------------------------------------------------------------------------------ 33

Geologic history of the Tertiary and Quaternary ------------------------------------------------- 34

Paleocene --------------------------------------------------------------------------------------- 34

Eocene ------------------------------------------------------------------------------------------ 35

Oligocene --------------------------------------------------------------------------------------- 39

Miocene ----------------------------------------------------------------------------------------- 40

Pliocene ----------------------------------------------------------------------------------------- 43

Pleistocene ------------------------------------------------------------------------------------- 45

Holocene ---------------------------------------------------------------------------------------- 49

Discussion and Summary --------------------------------------------------------------------- 51

References Cited ------------------------------------------------------------------------------------------- 54

Figures

1. Regional map of study area in parts of Wyoming, Colorado, Nebraska, and Kansas ------------ 2

2. Location map of the subsurface study area in south-central Nebraska -------------------------- 22

3. Structure contour map drawn on top of Precambrian rocks --------------------------------------- 23

4. Chart showing Cenozoic stratigraphic units in south-central Nebraska, 

 northeastern Colorado, and southeastern Wyoming ----------------------------------------------- 24

5. Unconformity surface on top of Cretaceous rocks in south-central Nebraska ------------------ 25

III
Figures

 6. Isopach map of the Ogallala Group ----------------------------------------------------------------- 26

 7. Unconformity surface on top of the Ogallala Group or Cretaceous rocks in 

 south-central Nebraska ------------------------------------------------------------------------------ 27

 8. Isopach map of fluvial Pliocene and Pleistocene sediments in south-central Nebraska ---- 28 

 9. Isopach map of loess intervals in the Pliocene and Pleistocene section in 

 south-central Nebraska ------------------------------------------------------------------------------ 29

10. Isopach map of all Pliocene and Pleistocene alluvium in south-central Nebraska ----------- 30

11. Paleogeography in Paleocene time ----------------------------------------------------------------- 33

12. Paleogeography in Eocene time --------------------------------------------------------------------- 36

13. Photo of middle-Eocene erosion surface in the Front Range. View is northeast 

 from Green Mountain --------------------------------------------------------------------------------- 37

14. Paleogeography in Oligocene time ----------------------------------------------------------------- 38

15. Paleogeography in Miocene time ------------------------------------------------------------------- 42

16. Paleogeography in Pliocene time ------------------------------------------------------------------- 44

17. Paleogeography in Pleistocene time --------------------------------------------------------------- 47

Plates

1. Geologic map and topographic profile of the Elm Creek West 7.5 minute quadrangle

2. Geologic map and topographic profile of the Newark 7.5 minute quadrangle

IV
1

Abstract

 The Platte River of south-central Nebraska was studied at 

three scales to place the river in its geological context and to 

trace its evolution through geologic time. At the largest scale 

the Elm Creek West and the Newark 7.5 minute quadrangles 

were mapped. These quadrangles are located just west and 

just east of Kearney and serve to illustrate the main geomorphic elements of the present Platte River Valley. The central 

elements of the quadrangles are the Platte River channels, 

islands, and bottomlands, which are fl anked by terraces that 

step up away from the river to the north and south. Signifi -

cant other elements of the landscape are eolian sand and loess 

deposits. The geologic maps are supplemented by topographic 

profi les of the mapped terraces and graphical representations 

of subsurface units in test wells that occur within the quadrangles. 

An intermediate-scale study consisted of examining 

descriptions of well cuttings in a 17 county area in southcentral Nebraska, which includes the Platte River Valley, and 

building a database of information about sediment lithology 

and thickness. The wells penetrated a sequence of gravel, 

sand, silt, and clay beds from the ground surface to the top 

of the subsurface Tertiary Ogallala Group or Cretaceous formations. The sequence consists of Pliocene-, Pleistocene-, 

and Holocene-age strata that document the deposition of a 

veneer of alluvium by late Tertiary and Quaternary streams 

intermixed with and overlain by wind-blown loess. Various 

isopleth and structure maps illustrate the distribution and alluvial architecture of the sedimentary sequence, and support the 

interpretation of former positions of the Platte River.

A regional-scale study consisted of documenting the geologic history of the Front Range and adjacent mountains and 

depositional areas east of the mountains in Colorado, Wyoming, Kansas, and Nebraska from the end of the Cretaceous 

Period, about 65 million years ago, to the present. The structural and sedimentary history of the region is outlined, and a 

series of paleogeographic maps shows the development of the 

drainage system in the east-central Rocky Mountains and adjacent Plains. Ancestral South Platte, North Platte, and Laramie 

Rivers are recognized as early as the late Eocene, although the 

South Platte probably fl owed to the southeast from the mountain front at that time. Deposits of the North Platte River are 

recognized on the west side of the Medicine Bow Mountains 

of Wyoming in the Miocene, and the presence of distinctive 

rock clasts indicates that the Laramie River fl owed from the 

North Park area of Colorado northeast across a fi lled Laramie 

Basin and the Laramie Range of southeastern Wyoming in the 

Miocene. The present drainage system developed in the late 

Miocene to the Pliocene and included the capture and diversion of the South Platte River into its present channel. The 

combined North and South Platte Rivers deposited gravel and 

sand across Nebraska and fl owed southeast from Kearney, 

Nebraska through the middle to late Pleistocene. Within the 

past 25,000 years the Platte River below Kearney was captured and diverted into its present course and confi ned there by 

bounding valley walls of loess.

Introduction

The Platte, South Platte, and North Platte Rivers and their 

valleys in Nebraska, Colorado, and Wyoming (fi g. 1) have 

been important to the human history of the region from the 

time of the early Native Americans to the present. In prehistoric time, the area was a site of abundant buffalo and antelope 

herds that were a main source of food for hunting and gathering Native Americans. In late prehistoric time, the Platte 

River Valley was the site of many large Native American villages. Beginning in the early 19th century, the valleys became 

main thoroughfares for military exploration expeditions, for 

fur trappers and traders, for homesteaders bound for Oregon, 

and for adventurers headed to the gold fi elds of California and 

Colorado. In the 1860s, the Platte River Valley in Nebraska 

included part of the route of the Pony Express, the coast-tocoast telegraph line, and the fi rst transcontinental railroad. 

In the 20th century, the valley maintained its importance to 

transcontinental travel by hosting parts of the Lincoln Highway (U.S. Highway 30) and then Interstate 80. Thousands of 

people a year have traveled east and west along the Platte and 

its two branches from the mid-1800s to the present. Along 

with its use as a mainstream transportation route, the Platte 

River Valley also became the site of farms and ranches early in 

its development (Mattes, 1969).

Although the fi rst inhabitants had little effect on the Platte 

River Valley, later development has had a major impact. ProbGeologic Studies of the Platte River, South-Central 

Nebraska and Adjacent Areas—Geologic Maps, 

Subsurface Study, and Geologic History

By Steven M. Condon
2 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

ably the fi rst noticable change would have been the removal 

of trees for fuel by the masses of people migrating westward, 

estimated at some 350,000 individuals between 1841 and 

1866 (Mattes, 1969). As settlers populated the valley they 

started farming, and the semiarid climate west of the 98th 

meridian requires irrigation for bountiful crops. Dams were 

built in several places, most notably on the North Platte (the 

north fork of the Platte River), which changed the fl ow conditions and sedimentation patterns. Diversion canals altered the 

fl ow of the river and changed surface run-off and groundwater 

movement. Deep irrigation wells have changed the confi guration of the water table. Farming practices, such as vegetation clearing activities, have tended to change the river bank 

stability in many places. Vegetation patterns have changed, 

from prehistoric conditions when trees were mainly limited 

to the islands, to current conditions of dense trees and brush 

along the banks in addition to those on the islands. Road 

construction has added fi ll to many areas on the fl oodplain, 

altering channel patterns and affecting the lateral migration 

of channels. Bridges cross the river, and associated roads are 

built above the grade of the fl oodplains and act as de facto 

dams, impeding the fl ow of high water in the spring, and 

causing extensive fl ooding in some lowland areas. The result 

of these changes is a river system that is perennial rather than 

intermittent, has a reduced annual discharge, is narrower and 

more deeply channeled, and is more confi ned by the stabilizing effects of bankside vegetation (Williams, 1978; Nadler 

and Schumm, 1981; Eschner and others, 1983; Karlinger and 

others, 1983; Schumm, 1999; Collier and others, 2000). In 

addition, the water is clearer and has a higher erosive capacity 

Sidney

Casper

Denver

Pueblo

Kearney Lincoln

Laramie

Cheyenne

Saratoga

Sterling

Julesburg

Torrington Scottsbluff

Castle Rock

Grand Island

North Platte

Fort Collins

Colorado Springs

North

Park

South

Park

WYOMING

NEBRASKA

COLORADO KANSAS

COLORADO

PIEDMONT

Franklin

Great Bend

Russell

El. 2150 ft

El. 1800 ft

El. 1726 ft

El. 1850 ft

 iLÀ>Ã>Ê->`ÊÃ

HIGH PLAINS

Osborne

El. 1520 ft

HIGH PLAINS

XThe Gangplank

Medicine BowMountains

Laramie Range

Front Range

Park Range

Hartville uplift

Columbus

Laramie

Basin

Limon

Arkansas River

Smoky Hill Riv er

South Fork Solomon River

Republican River

North Platte River

Little Blue River

Lou

Platte River p River

Big Bl ue River Elkhorn River

Salt C ree k

Missouri River

South Platte River

Swe etwater

Laramie River

Medice neBow

Wood River

Elm Creek West

quadrangle

Newark

quadrangle

ÎÇc

Îc

{£c

{Îc

{xc

£änc £äÈc £ä{c £äÓc £ääc nc Èc {c

0 100 200 50 Miles 0 50 100 200 300 Kilometers

Figure 1 Regional map of the study area in Wyoming, Colorado, Nebraska, and Kansas. Red cross-hatching indicates Precambrian granitic or 

metamorphic rocks, generalized from Green (1992) and Green and Drouillard (1994); the Colorado Piedmont is in green (modifi ed from Trimble, 

1980b); eolian sand areas are yellow (modifi ed from Muhs and others, 1999a). The location of the study area within the United States is shown 

in the inset map.
3

than in pioneer days, because of the sediment-trapping effect 

of dams.

A river system is complex and responds to natural phenomena as well as manmade changes. This report reviews 

structural and depositional events that shaped the fl uvial 

systems that have drained eastward from the Front Range of 

Colorado and the Laramie Range of southern Wyoming since 

the Cretaceous Period (about 65 million years ago), combined 

with new data gathered for the current project. New data 

include the surface geology of two 7.5 minute topographic 

maps, Elm Creek West and Newark (just west and just east of 

Kearney, Nebraska, respectively) that were mapped to show 

the relations and hierarchy of landforms in key areas where 

other research is being done. These maps show river terraces, 

sand dunes and sand sheets, alluvial fans, and loess hills. 

Additionally, pre-Tertiary topography and thickness maps of 

Tertiary, Pleistocene, and Holocene sedimentary units were 

constructed for a 17-county area in south-central Nebraska. 

These maps show the alluvial architecture of the Pliocene to 

Holocene sediments that underlie the region between the Platte 

and Republican Rivers.

This report is not a commentary on the changes that have 

occurred in the Platte River system since man’s infl uence has 

been in effect—its purpose is to provide a summary of (1) the 

geomorphic elements of the fl oodplain and bounding valley 

walls, (2) the physical arrangement and characteristics of the 

alluvial fi ll underlying and forming the Platte River Valley and 

adjacent areas, and (3) the chronological development of the 

modern drainage system. In essence, this report provides a 

geological foundation for biological and hydrological studies 

of the Platte River. The report is divided into three sections 

that cover the topics listed above.

Acknowledgments

 I thank Randy Parker, U.S. Geological Survey (USGS), 

for his leadership in directing the Platte River study. The staff 

of the Conservation and Survey Division, Institute of Agriculture and Natural Resources, University of Nebraska at Lincoln, (aka the Nebraska Geological Survey) provided access to 

published and unpublished drill hole cuttings descriptions. I 

thank Michael Blum, previously of the University of Nebraska 

at Lincoln Geology Department, for his interest in the project. 

Noah Daniels, University of Colorado at Boulder, provided 

his expertise and equipment to collect lake-bottom sediments 

in the Kearney, Nebraska, area for use in this study. David 

Seeland, USGS, retired, shared his knowledge of the geology 

of Tertiary rocks and of the sedimentology of the Platte River, 

assisted in the fi eld, and made important improvements to the 

manuscript and maps. Paul Kinzel, USGS, assisted with GIS 

software. Michael Starbuck, USGS, provided digital maps. I 

thank Larry Wells, Elm Creek, Nebraska, for his interest in 

the project and his thought-provoking questions and insights 

into the surface and subsurface geology of the Platte River 

Valley. Steve Holen (University of Nebraska State Museum), 

Paul Tebbel (National Audubon Society, Rowe Sanctuary), 

Jim Jenniges (Nebraska Public Power District), Bob Henszey 

(Platte River Whooping Crane Maintenance Trust), and Kirk 

Schroeder (U.S. Fish and Wildlife Service) provided information, access to study sites, or tours of selected areas in southcentral Nebraska. Daniel Muhs, USGS, and James Swinehart, 

Conservation and Survey Division-University of Nebraska, 

Lincoln, critically reviewed and improved the report.

I. Geologic Maps of the Elm Creek 

West and Newark Quadrangles

Introduction

Geologic mapping was undertaken in two quadrangles 

where biological and hydrological studies on the Platte River 

Project were also being conducted (plates 1 and 2). One map 

is the Elm Creek West quadrangle, which includes the Cottonwood Ranch property, and the other is the Newark quadrangle, 

which includes the Audubon Society’s Rowe Sanctuary headquarters. The Elm Creek West quadrangle is west of Kearney, 

Nebraska, and includes the west edge of the town of Elm 

Creek. The Newark quadrangle is east of Kearney and west of 

Shelton. Although there are similarities between the quadrangles, there are also signifi cant differences, so the descriptions 

and discussions of each map are presented separately below.

Methods

The quadrangles were mapped on a combination of digital 

orthophoto quads, aerial photographs, and topographic maps 

of the areas. After careful review and study of the orthophoto 

quads, the primary technique used to discern terraces was to 

systematically traverse the quadrangles north to south along 

county roads, noting any variations in topography, slope 

breaks, vegetation changes, or other indicators of terrace 

edges. A combination of the topographic base map, orthophoto quads, and fi eld observations were used to draw contacts 

between geologic or geomorphic units.

The digital orthophoto quads were downloaded from 

the Nebraska Department of Natural Resources web site 

(http://www.dnr.state.ne.us/databank/doqall.html). The digital 

topographic base maps were downloaded from the University 

of Nebraska at Lincoln, Institute of Agriculture and Natural 

Resources, Conservation and Survey Division web site (http://csd.

unl.edu/general/drgdownloads.asp). Topographic profi les for each 

of the maps were constructed using digital elevation model (DEM) 

fi les for the quadrangles. These fi les were also obtained from 

the Nebraska Department of Natural Resources at http://www.

dnr.state.ne.us/databank/dem.html.

Graphic representations of test wells in the quadrangles 

were also constructed, using the descriptions of well cuttings 

Methods
4 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

from the wells, and are included on the geologic map plates. 

The test wells were drilled by the Conservation and Survey 

Division (CSD) and the U.S. Geological Survey (USGS) 

starting in 1930, and descriptions of cuttings from most of 

the wells were published by county. I examined some unpublished descriptions from fi les at the CSD offi ces in Lincoln, 

Nebraska. The descriptions used are reproduced in this report 

with the discussions of each quadrangle.

The descriptions of terraces that follow include estimates 

of the average height of the units above the current fl oodplain. 

These elevations were calculated by averaging the elevations 

of the edges of the terraces closest to the river on the west 

and east extents of their occurrence in the quadrangles. The 

calculated averages do not take into account variations in the 

elevations of the terraces laterally north and south away from 

the river, which can be seen on the profi les of each quadrangle.

Elm Creek West Quadrangle

Description of Map Units

All of the mapped units in the Elm Creek West quadrangle are unconsolidated deposits. No lithifi ed bedrock 

was observed, and few exposures of parent material are visible. Scattered exposures of loess in the hills in the northeast 

corner of the map, and loess in drainages cutting terraces in 

the southern part of the map, were the only parent material 

seen. Because no coring was done for this study, the mapped 

unit descriptions are of the main soil types present on their 

surfaces, which were originally mapped by Bowman and 

others (1973), Brown and others (1978), and Buller and others 

(1974). Some soils occur in more than one map unit, but most 

map units are defi ned on the basis of their geomorphic setting 

and are therefore lithologically and pedologically distinct.

Qfp Floodplain deposits (Holocene)—Lowest fl ood 

 plain and island areas of the Platte River. 

 Deposits of this unit are mostly poorly sorted and 

 clast supported. They consist of unconsolidated 

 gravel in a sandy or silty matrix, interbedded with 

 or overlain by sandy silt and silty sand. Clasts 

 are round to subangular and are composed of plu- 

 tonic, metamorphic, and sedimentary rock types. 

 Outside the active channels, many soil types 

 are at the surface of this unit, including Gothenburg 

 soils or comparable Loamy alluvial beds, Platte loam or 

 comparable Platte soils, Gibbon loam, Lex loam, Alda 

 loam, Leshara and Gibbon silt loam, Platte-Alda com- 

 plex, and Platte-Wann complex. The Gothenburg soil 

 and corresponding Loamy alluvial beds are the most 

 abundant soil types recognized where Qfp is mapped

Qaf Alluvial fan deposits (Holocene)—An apron of allu- 

 vium derived from and deposited at the base of the

 loess hills in the northeastern part of the map. The 

 unit extends southward over the Qt1b terrace. Soil 

 types consist of Uly-Holdrege-Coly silt loams, Hobbs 

 silt loam, Cozad silt loam, and Hord silt loam

Qal Alluvium (Holocene)—Thin veneer of alluvium derived 

 mainly from stream that enters the map from the south 

 in section 34, T. 8 N., R. 19 W. Soil types consist 

 primarily of Wann loam, and lesser amounts of 

 Leshara silt loam and Hord silt loam. Overlies terrace 

 Qt1a and northern edge of dune fi eld Qd in the south- 

 eastern part of the quadrangle

Qd Dune deposits (Holocene and Pleistocene)—Main 

 dune area in southeast part of the map consists of wind-

 deposited dune sand and sheet sand intermixed with 

 alluvial material in drainages. Soil types are heteroge- 

 neous, and include Valentine loamy sand, the Anselmo 

 series, Kenesaw silt loam, and Kenesaw-Coly silt loam. 

 Small areas of Qd mapped in and south of the Platte 

 River channel are composed of small dunes and sandy 

 zones intermixed with other soil types. Dunes in the 

 main area of deposition have relief of as much as 40 

 ft. Dunes in other areas have relief of 10 ft or less. 

 Eolian deposits in the southeast part of the map overlie 

 and obscure older terrace deposits

Ql Loess (Pleistocene and Holocene)—Loess that forms 

 hills at the northeast corner of the map. Soil types are 

 mainly Uly silt loam, Coly silt loam, Holdrege silt loam, 

 and Uly and Holdrege silt loams, combined. Loess 

 probably consists of the Pleistocene Peoria Loess and 

 the Holocene Bignell Loess

Qt1a Terrace 1a deposits (Holocene)—Deposits of the fi rst 

 terrace above the Platte River fl oodplain on the south 

 side of the river. Two areas of this terrace were mapped, 

 (1) immediately adjacent to the Platte River fl oodplain 

 (Qfp), in the western and central parts of the map, and 

 (2) immediately south of the Qal unit, about 0.75 mile 

 south of the river fl oodplain in the eastern part of the 

 map. In the fi rst area, the surface is cut by numerous

 small abandoned stream channels; low areas are boggy 

 and hold standing water in the spring. The second area 

 is slightly drier and has sandier soils. In the fi rst area, 

 soil types are mainly saline Leshara silt loam, Leshara 

 silt loam, Grigston silt loam, and Wann loam. Soils in 

 the second area are Anselmo fi ne sandy loam, Meadin 

 loamy sand, Meadin silt loam, and O’Neill fi ne sandy 

 loam. This terrace stands at an average of 5 ft above the 

fl oodplain

Qt1b Terrace 1b deposits (Holocene)—Deposits of the fi rst 

 terrace above the Platte River fl oodplain on the north 

 side of the river. This terrace is somewhat higher and 

 better drained and may be slightly older than the Qt1a 

 terrace on the south side of the river. This was the only 

 terrace identifi ed between the river and the loess hills on 

 the north side of the river and thus covers a signifi cant 

 portion of the map (about 24 square miles). A large 

 number of soil types occur on this terrace, including the 

 Cozad series, Gosper series, Hord series, Rusco silt 
5

 loam, and Wood River series. This terrace stands at an 

 average of 7.5 ft above the fl oodplain

Qt2 Terrace 2 deposits (Holocene? and Pleistocene)— 

 Deposits of the second terrace above the Platte River 

fl oodplain on the south side of the river. In plan view, 

 this terrace is triangular, wedging out because of erosion 

 by the Platte River just to the west of the map boundary. 

 The terrace widens eastward to about the middle of the 

 quadrangle; in the southeast part of the map the terrace 

 is mantled by dune deposits of Qd. The northern edge 

 of the terrace is defi ned by a topographic break and by 

 a change to the soil types Kenesaw silt loam and 

 Kenesaw and Coly silt loams. Soil types over most 

 of this terrace are Hord silt loam. Kenesaw silt loam 

 occurs mainly along the eastern edge of exposure. This 

 terrace stands at an average of 45 ft above the fl oodplain

Qt3 Terrace 3 deposits (Pleistocene)—Deposits of the 

 third terrace above the Platte River fl oodplain on the 

 south side of the river. In plan view, this terrace nar- 

 rows to the west and was removed by Platte River ero- 

 sion in about the middle of the adjacent Overton quad- 

 rangle. The terrace widens to the southeast and is 

 partially mantled by dune deposits in the south-central 

 part of the map. The southern edge of the terrace trends 

 generally east-west just south of the map boundary. 

 Soils on this terrace are the same as those on the Qt2 

 terrace. A change to Kenesaw silt loam and Kenesaw 

 and Coly silt loams mark the northern edge of the 

 terrace. The majority of the terrace surface is composed 

 of Hord silt loam, with some Kenesaw silt loam along 

 the eastern-most exposures. This terrace stands at an 

 average of 50 ft above the fl oodplain

Qt4 Terrace 4 deposits (Pleistocene)—Deposits of the 

 fourth terrace above the Platte River fl oodplain. Only a 

 small part of this terrace occurs in the Elm Creek West 

 quadrangle, in the southwest corner of the map. Soil 

 type is Holdrege silt loam. This terrace stands at an 

 average of 97.5 ft above the fl oodplain

Discussion

The Elm Creek West quadrangle is located about 15 

miles west of Kearney, Nebraska, and straddles the valley of 

the Platte River (fi g. 1; plate 1). The quadrangle has representatives of all the main geomorphic and genetic elements 

of the Platte River Valley. The northeastern corner of the 

quadrangle includes the edge of the valley where it abuts an 

extensive, dissected loess upland. The upland consists of 

the Peoria Loess, which was deposited in western Nebraska 

between about 25,000 years before the present (B.P.) and 

about 10,500 years B.P. (Muhs and others, 1999a; Roberts 

and others, 2003), and the Bignell Loess, deposited between 

about 9,000 and 3,000 years B.P. (Pye and others (1995). An 

apron of alluvium, in the form of coalesced alluvial fans, 

bounds the loess hills and extends southward toward the 

Platte River. No test holes have been drilled in these units in 

this quadrangle.

The main geomorphic features of the Elm Creek West 

quadrangle are the channels and islands of the Platte River, 

bottom lands that bound the river, and terraces that fl ank the 

river on the north and south. A numbering sequence for the 

terraces was adopted for this map that partially follows that 

established by Schultz and Stout (1945) and Stout (1983). In 

this quadrangle the fl oodplain, in which the river fl ows, is designated as Qfp, and successively older terraces are numbered 

Qt1, Qt2, Qt3, and Qt4.

In this quadrangle, the channel system of the Platte is 

highly braided, having small to large islands separated by 

active channels and other channels in various stages of abandonment. One large island in this quadrangle extends almost 

the entire width of the map, near the north side of the Qfp unit. 

Its width varies along its length, and is about 0.25 mi wide at 

its widest. The Cottonwood Ranch property is on part of this 

island. There is also another large island area that is partially 

attached to the south bank of the river. On the eastern side of 

the map, this island is bounded by the South Channel of the 

Platte River on the south and by the main channel at the Elm 

Creek bridge on the north.

The Qfp fl oodplain surface ranges in elevation from about 

2,290 ft on the west side of the map to about 2,245 ft on the 

east, giving an average drop of 6.9 ft per mile. The river has 

become increasingly entrenched since dams and water diversion structures have been built upstream (Eschner and others, 

1983), which has resulted in a major reduction in channel area 

and an increase in island area. The decreased channel width 

also has led to increased accretion of the islands, both vertically and horizontally. Although much of the nonisland areas 

are fl ooded during high water in the spring, some islands are 

now high enough to escape being fl ooded. One major cause 

of fl ooding in this quadrangle is the constriction of the channel 

at the Elm Creek Bridge. The bridge approaches act as dams 

during times of high water, backing up the river and fl ooding 

the upstream lowlands.

 The Qt1 terrace was originally defi ned as the low, broad 

surface just above the fl oodplain in localities west of the 

study area (Stout, 1983). In the Elm Creek West quadrangle, 

there are differences between the Qt1 terrace on the south 

and north sides of the Platte, and this terrace is divided here 

into the Qt1a and Qt1b terraces, respectively. The Qt1a terrace is thought to be the youngest terrace above the river. It 

stands at an average of 5 ft above the fl oodplain (Qfp), and 

is characterized by low, boggy land that commonly contains 

standing water in fi elds and ditches. This terrace is just high 

enough above the fl oodplain to avoid extensive fl ooding in 

the spring. The surface of this terrace is cut by numerous, 

small abandoned channels that are readily visible on aerial 

photographs. The immaturity of the landscape suggests that 

this area was recently occupied by the Platte River, and that 

lateral migration of the river has been northward, cutting into 

the Qt1b terrace. The western areas of the Qt1a terrace stand 

at an elevation of about 2,290 ft, dropping to about 2,255 

Geologic maps of the Elm Creek West and Newark 7.5 minute quadrangles
6 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

ft at the eastern map edge, for an average drop of 5.4 ft per 

mile.

Test well number 29-B-47 was drilled in the Qt1a terrace 

a short distance south of the fl oodplain (plate 1). The upper 

units tested consist of silty and sandy soil and silt, and gravel 

within 3.5 ft of the surface. Most of the subsurface alluvium 

consists of sand and gravel, and bedrock of the Ogallala Group 

was reached at 34.5 ft (plate 1). 

The Qt1b terrace is characterized by a higher elevation, a 

better-developed drainage system, and a different soil association than that of the Qt1a terrace. This terrace stands at an 

average of 7.5 ft above the average elevation of the fl oodplain 

(Qfp). Eight north-south traverses, from the river to the north 

valley wall, were made over this terrace, to try to distinguish 

any other terraces north of the river. Although the land surface 

rises gradually to the north and northwest, no topographic or 

geomorphic breaks were found that would indicate a higher 

terrace surface in that area. Thus, only the Qt1b terrace is 

recognized from the northern fl oodplain of the Platte River to 

the alluvial fan deposits in the northeastern corner of the map 

and to the northwestern corner of the map in this quadrangle. 

This is probably due to older terraces having been removed by 

erosion by lateral migration of the river in this area. Measured 

just north of the river and parallel to it, the highest elevation 

of the Qt1b terrace is about 2,295 ft. The lowest elevation is 

along the eastern border of the map, at about 2,255 ft, for an 

average drop of 6.2 ft per mile. 

Test well number 28-B-47 was drilled in the Qt1b terrace 

a short distance north of the fl oodplain. Soil, clay, and silt 

were found to a depth of 8 ft. Mixed sand, gravel, and clay 

were encountered deeper in this well, underlain by the Ogallala Group at a depth of 56 ft (plate 1).

On the south side of the Platte, a marked topographic 

break separates the Qt1a terrace from the Qt2 terrace. The 

Qt2 terrace stands at an average of 45 ft above the average 

elevation of the fl oodplain. This terrace is mantled by a layer 

of loess of undetermined depth, indicated by the Hord association of soils on its surface. The Bignell Loess is the youngest 

loess unit in central Nebraska (Pye and others, 1995), and may 

partially mantle the Qt2 and older terraces in this area. The 

Bignell is discontinuous though, and is only identifi ed if the 

underlying Brady soil is present. The older Peoria Loess is 

also present on these terraces. The Qt2 terrace is at an elevation of about 2,325 ft on the west edge of the map and drops to 

about 2,300 ft before it is covered by younger eolian material, 

for an average drop of 6.7 ft per mile. 

Another topographic break distinguishes the Qt2 terrace 

from the Qt3 terrace. The Qt3 is also covered by a layer of 

loess, and it stands at an average of 50 ft above the average 

elevation of the fl oodplain. This terrace has the same soil 

association as the Qt2 terrace, and is probably also mantled 

by Bignell and Peoria Loesses. The Qt3 terrace ranges in 

elevation from 2,330 to 2,305 ft along a line just south of, and 

parallel to, its northern edge, for an average drop of 7.1 ft per 

mile. The Jensen Mammoth site is in Qt3 terrace deposits on 

the north side of the river northwest of the Elm Creek West 

quadrangle (May and Holen, 1994; Holen, 1995; S.R. Holen, 

written commun., 1999). Radiocarbon dating indicates the 

age of the terrace alluvium from that site is about 14,000 years 

B.P.

Test well 31-B-47, which was drilled in about the middle 

of this terrace, indicates 20 ft of soil and silt (loess) at the surface of the terrace, underlain by a mixture of sand, silt, gravel, 

and minor clay. The Ogallala Group was reached at a depth of 

114 ft.

The highest and oldest terrace in the Elm Creek West 

quadrangle is the Qt4, which is only present in a small area in 

the southwestern corner of the map. The terrace stands at an 

average of 97.5 ft above the average elevation of the fl oodplain. This surface might be considered part of the southern 

uplands of the river, however, it is surmounted by a yet higher 

surface in an area approximately 12 miles southwest of the 

Elm Creek West quadrangle. The Qt4 terrace is considered to 

be the highest (and oldest) terrace of the ancestral Platte River 

in this quadrangle. This unit has a slightly different soil association than the Qt3 and Qt2 terraces, but is still mantled with 

loess.

An important feature of the southeastern part of the map 

is the area mapped as Qd, which consists of wind-deposited sand. Some areas are fl at, and are composed of sand 

sheets, while other areas consist of sand dunes. Both units 

overlie, and partially obscure, the Qt1a, Qt2, and Qt3 terraces described above. The sand is very fi ne grained, and is 

interpreted to have been blown out of Platte River channels 

by northwest-to-southeast winds. The physical relations of 

the dune area with the terraces and the minimal degree of soil 

development in dune sand indicate that the dunes are relatively 

young.

There are other small areas of dunes also mapped, and 

which are even younger than the large area of Qd. Some of 

these other areas lie on the Qfp fl oodplain, most noteworthy 

of which is a long, linear dune area in section 2, T., 8 N., R. 

19 W. This dune area has several feet of relief and is immediately south of a former channel of the river. Other small areas 

mapped as Qd were noted as “sand spots” on the county soil 

survey maps. These youngest eolian deposits are also interpreted to have been blown out of Platte River channels.

Another depositional unit is alluvium (Qal) that was 

deposited on the Qt1a terrace, and sourced from the south by 

an unnamed creek that passes by the Williamsburg church 

(plate 1). Test hole number 30-B-47 was drilled in this unit, 

near the point at which the Qal unit was fi rst deposited on the 

Qt1a terrace. Strata in this well consist of a series of stacked 

soils, 4.5 ft thick, underlain by clay and silt to a depth of 14 ft. 

Thirty-seven feet of sand and gravel were then drilled until the 

Ogallala Group was reached at a depth of 51 ft.

No material from the Elm Creek West quadrangle was 

dated for this study, so inferences about the age of terraces 

can only be made by comparison with other areas and from 

other studies. On a geologic map that includes this quadrangle (Swinehart and others, 1994), the Qt2, Qt3, and Qt4 

terraces are mapped as part of a unit interpreted to be Illinoian 
7

and pre-Illinoian in age. In numerical age, this would be 

older than about 120,000 years B.P. but younger than about 

710,000 years B.P. (Haq and Van Eysinga, 1987). Souders 

and Dreeszen (1991) described a site about 10 miles southeast 

of the Elm Creek West quadrangle on the Qt4 terrace. At that 

site the stratigraphy consists of Peoria Loess at the surface, 

underlain by a paleosol, which is in turn underlain by a sandy 

sequence. The paleosol was radiocarbon dated at 24,260±260 

years B.P., indicating that the sands that comprise the Qt4 terrace alluvium may be equivalent to the Gilman Canyon Formation of mid-Wisconsin age.

Terrace Qt1b was mapped by Swinehart and others 

(1994) partially as a late Wisconsin unit (older than ±10,500 

years B.P.) and partially as a late Wisconsin and Holocene 

unit (younger than ±10,500 years B.P.). Terrace Qt1a and the 

fl oodplain were mapped as a part of the late Wisconsin and 

Holocene unit, younger than ±10,500 years B.P. The Bignell 

Loess, dated between about 9,000 and 3,000 B.P. (Pye and 

others, 1995) partially mantles the Qt2, Qt3, and Qt4 terraces, 

but is not found on the Qt1a or Qt1b terraces or on the fl oodplain. This absence of the Bignell Loess on the younger terraces helps establish a maximum age of about 3,000 years B.P. 

for those units, and a minimum age of 3,000 to 9,000 years 

B.P. for the older terraces.

Swinehart and others (1994) indicated that stream capture 

changed the course of the Platte River in the Wisconsin glacial 

stage to approximately the confi guration it is in today (also, 

see the discussion in section III, later in this report). Since 

establishing its new course, the river has migrated back and 

forth across its valley, alternately aggrading and then downcutting, forming the younger terraces. As noted above, the Qt1a 

terrace appears to be the youngest terrace outside the fl oodplain, based on its elevation, and the relative immaturity of the 

landscape on its surface. The river thus appears to be migrating northward in this area, eroding into the Qt1b terrace at the 

present time.

Logs of Test Holes Drilled by the Conservation and Survey Division-UNL 

in the Elm Creek West Quadrangle

(Position of the Ogallala Group and Pierre Shale in brackets [ ] were determined by the author and may 

not represent the interpretations of the Conservation and Survey Division)

Phelps County–(Anonymous, 1953h)

Location Well

number

Ground

elevation

Section 12, T. 8 N., R. 19 W. 29-B-47 2,264 ft

Depth, in feet

From To

Soil: silt, sandy, dark brownish gray; contains very fine sand............... 0 1

Silt, sandy, slightly to moderately calcareous, grayish-brown; contains

very fine to fine sand and a few calcareous nodules........................ 1 2.5

Soil: silt, sandy, moderately to very calcareous, dark brownish gray;

contains fine sand to medium gravel.......................................... 2.5 3.5

Sand, clayey, buff-gray; fine texture; contains some limonitic stain......... 3.5 4.5

Sand and gravel, grayish tan; texture grades from very fine sand to very

coarse gravel..................................................................... 4.5 32

Silt, clayey, slightly to very calcareous, very light-green to gray............ 32 34.5

[Ogallala Group] Silt, clayey to sandy, pinkish tan; contains some

interbedded pinkish to greenish gray sandstone............................. 34.5 40

Sandstone, in part calcareous, pinkish to greenish gray; contains a few

rootlets and seeds; reddish brown below 58 ft; grades into a siltstone

below 75 ft........................................................................ 40 80

Siltstone to claystone, brownish to greenish gray; blocky..................... 80 90

Sandstone, slightly to very calcareous, grayish green; contains a few

rootlets; quartzitic layer from 96 to 98.5 ft; few yellowish gray marl

beds below 110 ft................................................................ 90 120

Geologic maps of the Elm Creek West and Newark 7.5 minute quadrangles
8 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

Soil: silt, clayey, dark brownish gray................................................................... 3 4.5

Silt, clayey to sandy, buff-gray; contains fine sand.................................................... 4.5 5.5

Clay, soil-like, sandy to silty, dark buff-gray; contains very fine sand.............................. 5.5 7.5

Silt, sandy to slightly clayey, grayish buff; contains very fine sand; a few

calcareous nodules; light brownish gray below 9 ft.................................................... 7.5 10

Clay, silty to silt, sandy, grayish brown; contains very fine sand;

limonitic stain in lower foot........................................................................... 10 14

Sand and gravel, pinkish gray; texture grades from fine sand to medium

gravel with some coarse gravel; texture grades from coarse sand to

coarse gravel below 40 ft.............................................................................. 14 51

[Ogallala Group] Sandstone, brownish gray-green; fine texture..................................... 51 53.5

Sandstone, very calcareous, light-gray; contains a few seeds........................................ 53.5 56.5

Sandstone, brownish gray-green; fine texture; in part calcareous; contains

rootlets and seeds below 60 ft........................................................................ 56.5 64

Sandstone, light-gray to brownish gray-green; very calcareous from 64 to

67 ft; reddish brown below 70 ft..................................................................... 64 76

Sand and some sandstone, grayish tan; texture grades from fine to coarse

sand with some fine gravel........................................................................... 76 79.5

Sandstone, very calcareous, light greenish gray; grading to a slightly

sandy silt from 85 to 91 ft; pinkish tan below 91 ft............................................... 79.5 97

 Location Well

 number

 Ground

 elevation

 Section 14, T. 8 N., R. 19 W. 30-B-47 2,280 ft

Depth, in feet

From To

Soil and road fill: silt, slightly sandy, brownish black............................................... 0 1

Soil: silt, clayey to slightly sandy, brownish black................................................... 1 3

 Location Well

number

 Ground

 elevation

Section 12, T. 8 N., R. 19 W. 29-B-47 2,264 ft

 Depth, in feet

From To

Marl, grayish yellow........................................................................................ 120 121

Sandstone, very calcareous, greenish gray with a slight brownish tint;

contains many rootlets................................................................................. 121 133

Sand, silty, to silt, sandy, slightly calcareous, grayish green.......................................... 133 142.5

Sandstone, slightly to moderately calcareous, brownish gray-green; fine

textured; few rootlets; texture grades from fine to medium sand below

170 ft...................................................................................................... 142.5 202.5

Sandstone, very calcareous, very light grayish green; fine texture................................... 202.5 215

Silt, clayey, very calcareous, light-gray to very light greenish gray................................. 215 217.5

Clay, green; contains some limonitic stain and reworked clay fragments........................... 217.5 223

[Pierre Shale] Clay, light-gray; contains much limonitic material................................... 223 259

Clay shale, dark-gray to black............................................................................. 259 270

Phelps County–(Anonymous, 1953h)—Continued

 
9

 Depth, in feet

From To

Soil: silt, sandy to slightly clayey, brownish black; contains very fine

sand....................................................................................................... 0 1

Soil: clay, sandy to silty, slightly to moderately calcareous, brownish

gray; contains very fine sand.......................................................................... 1 3

Silt, clayey, slightly to moderately calcareous, buff-gray; few small

calcareous nodules...................................................................................... 3 4

Silt, in part sandy, slightly to moderately calcareous, tan-gray; contains

very fine sand; limonitic flecks and calcareous rootlets; slightly

coarser texture below 17 ft............................................................................ 4 20

Sand, tan to buff-gray; texture grades from fine to coarse sand; contains

some limonitic stain and small gastropod shells, some fine to medium

gravel below 45 ft....................................................................................... 20 66

Silt, clayey to slightly sandy, grayish green.............................................................. 66 69.5

Location Well

number

Ground

elevation

Section 28, T. 8 N., R. 19 W. 31-B-47 2,316 ft

Geologic maps of the Elm Creek West and Newark 7.5 minute quadrangles

Silt, clayey, very calcareous, light greenish gray; contains a few marl

layers below 100 ft..................................................................................... 97 105.5

Sandstone, slightly calcareous, greenish to brownish gray; fine texture;

few rootlets and seeds; light-gray and very calcareous from 119 to 130

ft and 145.5 to 148 ft.................................................................................. 105.5 151

Sand and gravel, light grayish brown with pinkish tint; texture grades

from fine sand to fine gravel.......................................................................... 151 155

Sandstone, moderately calcareous, brownish gray to grayish green; fine

texture; very calcareous from 174 to 175 ft; some interbedded silt

from 180 to 185 ft..................................................................................... 155 195

Silt, sandy, to sandstone, in part slightly calcareous, light grayish green.......................... 195 201

Clay, silty to silt, clayey, light-gray to light grayish green; few marl

layers..................................................................................................... 201 207

Siltstone, sandy, grayish green; contains very fine sand.............................................. 207 224.5

Gravel, consists mainly of green silt pebbles and limonitic fragments;

contains some volcanic ash............................................................................ 224.5 233

[Pierre Shale] Clay, light to medium-gray; contains some weathered

material at top.......................................................................................... 233 239

Clay shale, dark ray to black; contains some weathered material; trace of

light-gray bentonitic clay from 240 to 245 ft...................................................... 239 250

 Location Well

number

 Ground

 elevation

Section 14, T. 8 N., R. 19 W. 30-B-47 2,280 ft

 Depth, in feet

From To

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10 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

Depth, in feet

From To

Soil and road fill: clay, silty, dark brownish gray; brownish black and

granular below 1 ft..................................................................................... 0 2.5

Clay, silty, light brownish gray............................................................................ 2.5 3.5

Soil: silt, medium brownish gray; granular; contains a few limy nodules.......................... 3.5 4

Clay, silty, to silt, clayey, contains a few limy nodules................................................ 4 5

Silt, calcareous, buff-gray; contains a limy nodular zone............................................. 5 7.5

Silt, clayey to sandy, soil-like, dark grayish brown..................................................... 7.5 8

Sand and gravel, buff-gray; texture grades from fine sand to coarse

gravel..................................................................................................... 8 10

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number

 Ground

 elevation

Section 5, T. 8 N., R.18 W. 28-B-47 2,262 ft

Sand and gravel, pinkish gray; texture grades from fine sand to coarse

gravel..................................................................................................... 69.5 80

Clay, silty, light-gray with a slight greenish tint........................................................ 80 84

Clay, silty, to silt, clayey, pinkish tan; calcareous nodules in lower part............................ 84 93.5

Silt, reddish brown; contains a few rootlets............................................................. 93.5 96

Silt, clayey, moderately to very calcareous, light-gray to brownish tan;

slightly calcareous with some sandy silt below 110 ft............................................ 96 114

[Ogallala Group] Sandstone, slightly to very calcareous, brownish tan;

fine texture; contains a few rootlets and seeds; grayish green 126 to

140 ft; brownish gray to grayish brown below 140 ft.............................................. 114 142

Clay, reddish brown, and marl, grayish white, interbedded........................................... 142 147

Clay, slightly calcareous, reddish brown; blocky....................................................... 147 155

Sandstone, brownish tan to grayish green; contains a few rootlets and

seeds; moderately to very calcareous below 160 ft; some fine to coarse

sand below 190 ft....................................................................................... 155 195

Sand, greenish gray; texture grades from fine to coarse sand; slightly

finer texture and some interbedded silty sand below 205 ft...................................... 195 210

Sand, slightly silty, grayish green; texture grades from fine to medium

sand...................................................................................................... 210 220

Sandstone, slightly to moderately calcareous, light grayish green to

brownish gray-green; contains a few rootlets...................................................... 220 238

Silt, clayey, grayish green.................................................................................. 238 243

Quartzite, in part calcareous, grayish green............................................................. 243 244.3

Depth, in feet

From To

Location Well

number

Ground

elevation

Section 28, T. 8 N., R.19 W. 31-B-47 2,316 ft

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Geologic maps of the Elm Creek West and Newark 7.5 minute quadrangles 11

Depth, in feet

From To

Gravel with some sand, buff-gray with many yellow and pink grains;

texture grades from coarse sand to coarse gravel with some large

pebbles.................................................................................................... 10 27.5

Clay, silty, in part sandy, slightly calcareous, gray with slight pink tint;

contains very fine sand; moderately to very calcareous below 32.5 ft;

less silty below 37 ft.................................................................................... 27.5 40

Clay, in part silty, moderately calcareous, tan-gray; contains many limy

nodules................................................................................................... 40 47

Clay, silty, to silt, clayey, in part sandy, buff-tan with slight gray tint............................... 47 51

Gravel with some sand, pink; texture grades from coarse sand to medium

gravel with some large pebbles....................................................................... 51 56

[Ogallala Group] Sandstone, tan; grayish tint below 62.5 ft.......................................... 56 68

Sandstone, very calcareous, light grayish green........................................................ 68 69.5

Sandstone, slightly to moderately calcareous, grayish green with slight

brown tint; contains some rootlets.................................................................... 69.5 72.5

Sandstone, reddish brown.................................................................................. 72.5 76

Sand, gray; texture grades from medium to coarse..................................................... 76 80.5

Sand, silty, light-gray to light greenish-gray............................................................. 80.5 86

Sandstone, grayish green to grayish brown............................................................... 86 96

Sand and gravel, tan-gray; texture grades from fine sand to medium

gravel..................................................................................................... 96 97.5

Sandstone, grayish brown; contains some white siliceous material below

100 ft............................................................................................................................. 97.5 105

Sandstone, brownish gray to green; contains many calcareous rootlets............................. 105 110

Sandstone, very calcareous, light buff-gray; contains many calcareous

and siliceous rootlets and a few fossil seeds........................................................ 110 112

Sandstone, very calcareous, light-gray; contains fossil seeds......................................... 112 115

Sand and sandstone, slightly calcareous, grayish tan; texture grades from

fine to medium with some coarse.................................................................... 115 123

Sandstone, light greenish gray with buff tint; contains some rootlets................................ 123 130.5

Silt, sandy, light greenish gray............................................................................ 130.5 140

Sandstone, moderately calcareous, buff-gray; contains interbedded

calcareous silty sand below 143 ft.................................................................... 140 145

Sandstone, very calcareous, buff-gray; contains some interbedded reddish

brown clay and thin, limy layers..................................................................... 145 150

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number

Ground

elevation

Section 5, T. 8 N., R. 18 W. 28-B-47 2,262 ft

Sandstone, moderately calcareous, grayish brown; contains some rootlets........................ 150 151.5

Sand and some gravel, light brownish gray with green and pink grains;

texture grades from fine sand to fine gravel; slightly coarser in lower

part....................................................................................................... 151.5 162
12 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

Newark Quadrangle

Description of Map Units

All of the mapped units in the Newark quadrangle are 

unconsolidated deposits. No bedrock was seen, and few 

exposures of soil parent material are visible. Because no 

coring was done for this study, the mapped units are described 

to include the main soil types that formed on their surfaces, 

which were previously mapped by Buller and others (1974) 

and Wahl and others (1984). Some soils occur in more than 

one map unit, but most map units are defi ned on the basis of 

their geomorphic setting, and are therefore lithologically and 

pedologically distinct.

Qfp Floodplain (Holocene)— Lowest fl oodplain and island 

areas of the Platte River. Deposits of this unit are 

mostly poorly sorted and clast supported. They consist of unconsolidated gravel in a sandy or silty matrix 

interbedded with or overlain by sandy silt and silty 

sand. Clasts are round to subangular and are composed 

of plutonic, metamorphic, and sedimentary rock types. 

Outside the active channels, many soil types mantle 

this unit, including Loamy alluvial land, Platte soils, 

Sandstone, very calcareous, grayish green with a yellowish tint; lightgray with greenish tint below 164 ft................................................................ 162 167

Sandstone, grayish green with slight yellowish tint................................................... 167 168.5

Silt, sandy, in part slightly calcareous, light grayish-green............................................ 168.5 171.5

Sandstone, moderately to very calcareous, grayish green with slight

yellowish tint............................................................................................ 171.5 180

Sand, clayey to silty, light grayish green................................................................. 180 185

Sand, brownish gray; texture grades from fine to medium with some

coarse..................................................................................................... 185 192

Sandstone to sand, brownish gray with greenish tint; contains a few

rounded silty clay granules............................................................................ 192 214

Sand, in part slightly silty and consolidated, brownish gray with greenish

tint; texture grades from very fine to medium sand............................................... 214 220

Sand and gravel, greenish gray with pink grains; texture grades from fine

sand to fine gravel; contains many green clay pebbles............................................ 220 230

Sand and gravel, greenish brown with some pink grains; texture grades

from coarse sand to medium gravel.................................................................. 230 242.5

Sand, silty, to sandstone, very calcareous, light greenish gray........................................ 242.5 247.5

Sandstone, very calcareous, light greenish gray........................................................ 247.5 250

Sandstone, very calcareous, ligh-gray with buff and greenish tints................................. 250 255

Sandstone, moderately calcareous, light buff-gray..................................................... 255 260

Sandstone, very calcareous, light-buff to yellowish gray.............................................. 260 265.5

Sandstone, moderately calcareous, buff to brownish gray............................................. 265.5 273

Sand and slightly consolidated sandstone, slightly calcareous, brownish

gray; texture grades from fine to medium sand.................................................... 273 283

Siltstone, grayish green..................................................................................... 283 285

[Pierre Shale] Clay, light-gray; contains many limonitic fragments................................. 285 291

Clay shale, medium-gray to black........................................................................ 291 310

Depth, in feet

From To

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Location Well

number

Ground

elevation

Section 5, T. 8 N., R. 18 W. 28-B-47 2,262 ft
13

Platte-Alda complex, Alda loam, Alda fi ne sandy loam, 

Leshara and Gibbon silt loam, Inavale, and minor Wann 

fi ne sandy loam. The Platte soils, Platte-Alda complex, 

and Leshara and Gibbon silt loam are the most abundant 

soil types recognized where Qfp is mapped. The Elm 

Island area, between the Middle and South channels of 

the Platte River, is the topographically lowest land area 

within this unit. Other island areas between the Middle 

and North channels (Killgore Island, Fort Farm Island) 

are slightly higher topographically

Qs Sand sheet (Holocene)—The sand sheet in the south 

part of the quadrangle that borders the area of sand 

dunes consists mainly of Simeon sandy loam, and lesser 

amounts of Valentine loamy fi ne sand

Qd Dunes (Holocene)—Main area in southeast part of the 

map consists of wind-deposited dunes and sand sheets 

intermixed with alluvial material in drainages. Soil 

types are mainly Valentine loamy fi ne sand. Small areas 

of Qd mapped on the Platte River fl oodplain are composed of small eolian dunes and sandy zones intermixed 

with other soil types. The dune area on the west margin 

of the map consists mainly of Inavale fi ne sandy loam. 

Small areas of dunes in the northeast corner of the map 

are mantled with Cozad silt loam and Wood River silt 

loam. Dunes in the main area of deposition in the southeast corner of the map have relief of as much as 20 ft; 

dunes in other areas have relief of 10 ft or less. Eolian 

deposits along the south edge of the map overlie older 

terrace deposits

Qt1a Terrace 1a deposits (Holocene)—Deposits of the fi rst 

terrace above the Platte River fl oodplain on the north 

side of the river. The surface is cut by numerous small 

abandoned stream channels; low areas are boggy and 

hold standing water in the spring. Soil types are mainly 

Alda loam, Lex silt loam, Platte soils, and Gibbon silt 

loam. Cass fi ne sandy loam, Wann loam, and Wann fi ne 

sandy loam are also present in minor amounts. There 

is an elevation gradient from south to north, from lowelevation Platte and Lex soils on the south to Gibbon 

silt loam farther north, mainly in the area within T. 9 N., 

R. 14 W. (Gibbon township). This terrace stands at an 

average of 1 ft above the fl oodplain

Qt1b Terrace 1b deposits (Holocene)—Deposits of the fi rst 

terrace above the Platte River fl oodplain on the south 

side of the river. This terrace is slightly higher, somewhat better drained, and is thought to be older than the 

Qt1a terrace on the north side of the river. Soil types 

that occur on this terrace include Alda loam, Boel fi ne 

sandy loam, Wann fi ne sandy loam, Gibbon loam, and 

Lex loam. This terrace stands at an average of 5 ft 

above the fl oodplain

Qt2 Terrace 2 deposits (Pleistocene)—Deposits of the 

second terrace above the Platte River fl oodplain on the 

north side of the river. The southern edge of the terrace 

is defi ned by a marked topographic break, and by the 

soil types Wood River silt loam, Hall silt loam, and Hord 

silt loam, all on 1 to 3 percent slopes. Soil types over 

most of this terrace are Hord silt loam and a signifi cant 

area of Hall silt loam. This terrace stands at an average 

of 5 ft above the fl oodplain

Ql Loess (Pleistocene?)—Area of loess of unknown age in 

the northeast part of the quadrangle. A similar feature 

east of the quadrangle was drilled by the Conservation 

and Survey Division, UNL, and was found to be underlain by clayey silt (J.D. Swinehart, written commun., 

2001)

Qt3 Terrace 3 deposits (Pleistocene)—Deposits of the 

third terrace above the Platte River fl oodplain on the 

north side of the river. This terrace forms the drainage 

divide between the Platte River and the Wood River in 

this area. A subtle elevation break separates this terrace 

from the Qt2 terrace to the south, and the railroad tracks 

are built along the boundary between the terraces. This 

surface is distinguished from the Qt2 terrace by extensive soil mottling that is visible on aerial photographs. 

Main soil types are Wood River silt loam and Hall silt 

loam. This terrace stands at an average of 12.5 ft above 

the fl oodplain

Discussion

The Newark quadrangle is located about 5 miles east of 

Kearney, Nebraska, and straddles the valley of the Platte River 

(fi g. 1; plate 2). The quadrangle is representative of most of 

the main geomorphic and genetic elements of the Platte River 

Valley, lacking only the bounding loess uplands that are north 

and south of the quadrangle.

The main geomorphic features of the Newark quadrangle 

are the channels and islands of the Platte River, bottom lands 

that bound the river, and terraces that fl ank the river to the 

north and south. A numbering sequence for the terraces was 

adopted for this map that partially follows that established 

by Schultz and Stout (1945) and Stout (1983). In this quadrangle the fl oodplain, in which the river fl ows, is designated 

as Qfp, and successively older terraces are numbered Qt1, 

Qt2, and Qt3.

In the Newark quadrangle, the channel system of the 

Platte River is only moderately braided, compared to other 

areas, such as the Elm Creek West quadrangle west of Kearney. Three main channels of the Platte are recognized in this 

quadrangle, the North, Middle, and South channels. The 

Middle Channel is the largest, and is about 0.3 mile across 

at its widest. It has relatively few islands that interrupt the 

channel, or, in other words, the amount of open channel to 

island area is relatively large. The North Channel is the next 

largest, but is substantially smaller than the Middle Channel. The North Channel has more islands and is more highly 

braided than the Middle Channel. Single channels within it 

are only about 0.1 mile or less across, but the channel complex is about as wide as the Middle Channel (up to about 

0.3 mile across). It appears that the North Channel is being 

Geologic maps of the Elm Creek West and Newark 7.5 minute quadrangles
14 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

abandoned. The South Channel is narrow and is nearly abandoned.

Within the Qfp map unit that is defi ned by the river 

channels, there are a number of large islands. The largest 

of these is the combined Killgore and Fort Farm islands, 

which are only separated by minor drainages representing 

former Platte channels. This island complex lies between the 

North and Middle channels of the Platte. The North Channel merges with the Middle Channel a short distance to the 

east of the Newark quadrangle, and Killgore and Fort Farm 

islands are not recognized beyond that point. Elm Island 

occurs between the Middle and South channels, and is the 

lowest and wettest of the island areas in the quadrangle. The 

Audubon Society’s Rowe Sanctuary headquarters is located 

on this island. The South Channel eventually dies out to the 

west, just southeast of the city of Kearney, and Elm Island 

is no longer recognized in that area. Elm Island does extend 

eastward quite some distance east of this quadrangle.

The Qfp fl oodplain surface ranges in elevation from 

about 2,110 ft on the west side of the map to about 2,065 ft 

on the east, giving an average drop of 6.7 ft per mile. The 

river has become increasingly entrenched since dams and 

water diversion structures have been built upstream (Eschner 

and others, 1983), which has resulted in a major reduction in 

channel area and an increase in island area. The decreased 

channel width also has led to increased accretion of the 

islands, both vertically and horizontally. Although much of 

the bottom land is fl ooded during high water in the spring, 

some islands are now high enough to escape being fl ooded.

Test well number 73-B-47 was drilled along the South 

channel in the Qfp map unit (plate 2). In this well, a thin soil 

layer is underlain by about 68 ft of sand and gravel and then 

by a mixed interval of sand, gravel, and silt, 135 ft thick. 

The Ogallala Group does not seem to be present in this location, and the Pliocene sequence of sediments is underlain by 

the Pierre Shale at a depth of 204 ft.

 The Qt1 terrace is defi ned as the low, broad surface just 

above the fl oodplain. In the Newark quadrangle, there are 

differences between the Qt1 terrace on the north and south 

sides of the Platte, so the Qt1 terrace has been divided into 

the Qt1a and Qt1b terraces, respectively. The Qt1a terrace 

is thought to be the youngest terrace above the river. It is 

characterized by low, boggy land that commonly contains 

standing water in fi elds and ditches. The average elevation 

of this terrace is slightly (1ft) above the average elevation 

of the fl oodplain, just high enough to avoid extensive fl ooding in the spring. In this area Interstate 80 also forms a de 

facto levee between the fl oodplain and the Qt1a terrace. The 

surface of this terrace is cut by numerous small, abandoned 

channels that are readily visible on aerial photographs. The 

immaturity of the landscape suggests that this area was 

recently occupied by the Platte River, and that lateral migration of the river has been cutting southward into the Qt1b 

terrace, opposite to the migration direction in the Elm Creek 

West quadrangle. The western exposures of the Qt1a terrace 

stand at an elevation of about 2,112 ft, dropping to about 

2,065 ft at the eastern map edge, for an average drop of 7.1 

ft per mile. No test wells were drilled in the Qt1a unit in this 

quadrangle.

The Qt1b terrace is characterized by having a higher elevation, a better-defi ned drainage system, and development of 

a different soil association than that of the Qt1a terrace. This 

terrace stands at an average of 5 ft above the average elevation of the fl oodplain. The highest elevation of the Qt1b terrace is about 2,115 ft, and the lowest elevation is along the 

eastern border of the map is about 2,070 ft, for an average 

drop of 6.8 ft per mile.

Test well 72-B-47 was drilled in about the middle of the 

exposed area of the Qt1b terrace south of the river. Pleistocene and Pliocene sediments in this well consist mainly of 

sand and gravel from the surface to a depth of 217 ft where 

the Pierre Shale was reached (plate 2). The Ogallala Group 

is not present in this well.

A marked topographic break separates the Qt1a terrace from the Qt2 terrace. The Qt2 terrace is covered with a 

layer of loess of undetermined depth, indicated by the Hord 

association of soils on its surface. This terrace also stands at 

an average of 5 ft above the average elevation of the fl oodplain. The Qt2 terrace is at an elevation of about 2,116 ft on 

the west edge of the map and falls to about 2,069 ft on the 

east edge, for an average drop of 7.1 ft per mile. Many of 

the older farmhouses and out-buildings in this area are built 

on the southern edge of the Qt2 terrace, suggesting that the 

farmers who fi rst settled the area were aware of the fl ooding 

potential of the lower Qt1a surface. Even though the average 

elevations above the fl oodplain are the same for the Qt1b and 

Qt2 terraces, features such as soil associations and degree of 

maturity of the terrace surfaces distinguishes them.

Test wells 42-31 and 43-A-54 were drilled on the Qt2 

terrace (plate 2). Well 42-31 consists of a surface layer of 

soil, 3 ft thick, underlain by 55 ft of gravel and 22 ft of silty 

sand. Well 43-A-54 consists of a surface layer of soil underlain by sand and gravel, about 51 ft thick. Below the sand 

and gravel is an interval, 186 ft thick, which consists mainly 

of silt that lies on the Pierre Shale (plate 2).

A very slight topographic break separates the Qt2 terrace 

from the Qt3 terrace. The Qt3 terrace is also mantled by a 

layer of loess, and stands at an average of 12.5 ft above the 

average elevation of the fl oodplain. The Qt3 terrace ranges 

in elevation from 2,125 to 2,075 ft along a line parallel to the 

railroad tracks, for an average drop of 7.4 ft per mile.

Well 42-A-54 was drilled in the Qt3 terrace on the north 

side of the map. In this well a 2 ft thick surface soil layer 

is underlain by 14 ft of silt, probably loess. Below this is a 

mixture of sand, gravel, and silt, 141 ft thick, underlain by 54 

ft of the Ogallala Group. The well penetrated the upper 39 ft 

of Pierre Shale (plate 2).

Important features of the southern part of the map are 

the areas mapped as Qd and Qs, which consist of winddeposited sand. Some areas are fl at, and are composed of 

sand sheets, while other areas consist of sand dunes. Both 

units overlie, and partially obscure, the Qt1b terrace. The 
Subsurface Cenozoic Geology of South-Central Nebraska 15

sand of both units is very fi ne grained, and is interpreted to 

have been transported out of the Platte River channels by 

northwest-to-southeast winds. The physical relations of the 

dune area with the terrace and the minimal degree of soil 

development in dune sand indicate that the dunes are relatively young.

Other small areas of dunes are also mapped which are 

even younger than the mapped area of eolian sand. Some 

of these other areas lie on the Qfp fl oodplain, mainly on 

Killgore and Fort Farm islands. A small dune also was 

mapped just northeast of the Rowe Sanctuary in Sec. 10, T. 

8 N., R. 14 W. This dune has several feet of relief and is 

immediately south of the river. Other small areas mapped as 

Qd occur in the northeast corner of the quadrangle.

A large hill in the northeastern corner of the quadrangle 

was fi rst thought to be a large sand dune. The hill has about 

25 ft of relief and is elongated southwest-northeast. It is 

slightly steeper on the southeast side, resulting in a morphology of a transverse dune formed by northwest-to-southeastblowing wind. The soils mapped in this northeast area are 

not characteristic of eolian dunes (Buller and others, 1974), 

however, fi eld checking of road cuts and animal burrows 

along the north-south road that crosses the hill indicated 

the presence of very fi ne grained sand in surface deposits. 

According to J.B. Swinehart (written commun., 2001) a similar feature was drilled by the Conservation and Survey Division in an area east of the Newark quadrangle. The drilling 

indicated that the feature consisted of clayey silt, signifying 

loess, not dune sand. This feature in the Newark quadrangle 

is thus shown as loess, of questioned Pleistocene age.

No material from the Newark quadrangle was dated for 

this study, so inferences about the age of terraces can only be 

made by comparison with other areas and from other studies. 

On a geologic map that includes this quadrangle (Swinehart 

and others, 1994), the Qt3 terrace was mapped as part of a 

unit interpreted to be of late Wisconsin age, slightly older 

than about 10,500 years B.P. All the younger terraces were 

included in a unit interpreted as late Wisconsin to Holocene 

in age (Swinehart and others, 1994).

Although a similar numbering system was used for terraces in the Newark and Elm Creek West quadrangles, terraces in one area may not be entirely comparable with those 

in the other area, especially regarding the older terraces. The 

Platte River fl owed southeasterly from about Kearney toward 

the Republican River drainage as late as the late Wisconsin 

glacial stage (Souders and Dreeszen, 1991; Swinehart and 

others, 1994). In the Elm Creek West quadrangle the terraces mapped as Qt2, Qt3, and Qt4 appear to have been 

formed as part of that southeast-fl owing stream system. The 

river was diverted into its present northeasterly course by 

stream capture later, some time in the late Wisconsin (Lugn, 

1935; Souders and Dreeszen, 1991; Swinehart and others, 

1994).

The Newark quadrangle is at the transition point where 

capture and diversion of the Platte occurred. Without agedating the terraces of the Elm Creek West and Newark 

quadrangles it is diffi cult to know if the Qt2 and Qt3 terraces represent the same depositional and erosional events 

in both quadrangles. The height of the terraces above the 

Platte River fl oodplain suggests that the terraces in the 

Newark quadrangle are not comparable to those in the Elm 

Creek quadrangle. In the Elm Creek West quadrangle the 

Qt2 terrace is an average of 45 ft above the fl oodplain; in 

the Newark quadrangle the Qt2 terrace is only an average 

of 5 ft above the fl oodplain. In the Elm Creek quadrangle 

the Qt3 terrace is an average of 50 ft above the fl oodplain; 

in the Newark quadrangle it is only 12.5 ft above the fl oodplain. The signifi cantly different relative elevations of the 

terraces may indicate that those in the Newark quadrangle 

were formed after the river was diverted and established a 

new channel.

Since the Platte River began to fl ow northeastward 

toward Grand Island, Nebraska and beyond, it has migrated 

back and forth across a wide valley. The valley is considerably wider at Grand Island than at Kearney. The young landscape of the Qt1a terrace in the Newark quadrangle suggests 

that the river is now migrating southward and is eroding the 

Qt1b terrace at this locality.

There have been many attempts to divide the unconsolidated sediments into formations, fi rst by Lugn (1935) and 

Lugn and Wenzel (1938), and later by Condra and others 

(1947), Condra and Reed (1950), Schultz and others (1951), 

Reed and Dreeszen (1965), and Reed and others (1965). In 

later years, many of the older names have been abandoned, 

and some provisional names have been used (Wayne and 

others, 1991; May and others, 1995). For this report, an 

attempt was initially made to apply some of the older names, 

but the variable quality of the test hole logs and heterogeneity of the sedimentary sequence made this impossible on a 

regional basis. This report thus divides the unconsolidated 

sediments into a dominantly fl uvial sequence and a dominantly eolian sequence. The fl uvial sequence typically 

underlies the eolian sequence, but one or the other sequence 

may predominate or may be absent in any given well. After 

the initial divisions were made, the data were entered into a 

database. Various maps were constructed and certain picks 

were re-examined and modifi ed based on the preliminary 

maps. The test hole locations were downloaded from the 

University of Nebraska web site http://csd.unl.edu/general/

gis-datasets.asp.

A separate data set was used to create a structure contour 

map on top of Precambrian rocks (fi g 3). For this map, data 

for the top of Precambrian rocks from approximately 4,000 

wells were retrieved from a publicly available database (IHS 

Energy Group, 2003). This, and the other contour maps in 

this report, were constructed using Discovery R2002.1, version 7.2 (Landmark Graphics Corporation, Houston, Texas). 

The location map (fi g. 1), paleogeography maps (discussed 

in section III), and the geologic map plates were compiled 

in ESRI ArcMap, version 8.3 (Environmental Systems 

Research Institute, Inc., Redlands, Calif.), and were exported 

to Adobe Illustrator for fi nal editing.
16 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

}ÃÊvÊ/iÃÌÊiÃÊÀi`ÊLÞÊÌiÊ
ÃiÀÛ>ÌÊ>`Ê-ÕÀÛiÞÊÛÃ1 

ÊÌiÊ iÜ>ÀÊ+Õ>`À>}i

(Position of the Ogallala Group and Pierre Shale in brackets [ ] were determined by the author

and may not represent the interpretations of the Conservation and Survey Division) 

	Õvv>Ê
ÕÌÞq­ÞÕÃ]Ê£xÎLÆÊÞÕÃ]ÊÕ«ÕLÃi`ÊQ£x{R®

Location Well

number

Ground

elevation

Section 13, T. 9 N., R. 15 W. 42-A-54 2,105 ft

Depth, in feet

From To

Soil: silt, slightly clayey, dark-gray; coarse texture........................... 0 2

Silt, moderately clayey, medium yellow-gray; in part slightly calcareous,

medium brown-gray from 3 to 4 ft............................................ 2 4

Silt, slightly to moderately clayey, slightly calcareous, light yellow-gray;

contains a few limy nodules; less clayey from 4.5 to 5 ft; coarse

textured with a trace of mottled yellow-brown from 5 to 7.5 ft; slightly

to moderately clayey, light yellow-gray and contains some shell

fragments below 7.5 ft.......................................................... 4 10

Silt, slightly clayey, slightly calcareous, light yellow-gray; contains much

mottled yellow-brown; noncalcareous, light-gray below 12.5 ft......... 10 16

Sand texture grades from fine to very coarse; contains a trace of fine

gravel; principally quartz with pink silicates................................ 16 20

Sand and gravel texture grades from medium sand to fine gravel ;

contains 10 to 20 percent fine gravel......................................... 20 30

Sand and gravel texture grades from medium sand to fine gravel , with a

trace of medium gravel......................................................... 30 40

Sand and gravel texture grades from fine to very coarse sand w ith some

medium to coarse gravel; contains 50 to 60 percent gravel................ 40 59.5

Silt, very clayey, medium-brown; coarse texture silt, moderately clayey,

slightly sandy, medium yellow-gray; contains very fine sand from 60

to 61 ft; very clayey and medium-gray from 61 to 63.5 ft; slightly

clayey, slightly sandy; contains very fine to fine sand below 63.5 ft.... 59.5 64.5

Sand texture grades from very fine to fine..................................... 64.5 71

Silt, slightly clayey, slightly calcareous, light olive-gray, coarse texture;

contains limy nodular layer at 72.5 ft; contains very fine sand below

80 ft; noncalcareous from 84 to 86.2 ft; moderately clayey from 86.2

to 88 ft; contains very fine to fine sand below 90 ft......................... 71 94

Sand texture grades from very fine to coarse with a trace of f ine

gravel.............................................................................. 94 100

Sand and gravel texture grades from fine to very coarse sand; contains

15 percent fine gravel; contains 30 percent fine gravel with a trace of

medium gravel below 105 ft................................................... 100 107

Silt, slightly clayey, slightly calcareous, medium-gray; coarse texture;

contains limy nodules; medium olive-gray from 110 to 118.5 ft;

noncalcareous from 115 to 120 ft; medium yellow-gray from 118.5 to

123 ft; contains more clay below 120 ft; medium-brown and

noncalcareous below 123 ft.................................................... 107 123.5

Silt, slightly clayey, medium-brown; coarse texture; contains more clay

from 125 to 128.5 ft, slightly calcareous from 125 to 130 ft.............. 123.5 130

Silt, slightly clayey, moderately to very sandy, coarse texture silt to very

fine sand below 132 ft.......................................................... 130 140
17

Buffalo County–(Anonymous, 1953b; Anonymous, unpublished [1954])—

Continued

Silt, slightly to moderately clayey, slightly calcareous, medium-brown;

coarse texture silt; contains limy nodules; noncalcareous and slightly

sandy below 145 ft, moderately calcareous from 150 to 151.5 ft;

contains very fine to medium sand from 151.5 to 153.5 ft; slightly

calcareous below 153.5 ft; contains very fine to coarse sand from

153.5 to 154 ft; contains very fine to very coarse sand from 154 to

154.5 ft; contains very fine to coarse sand from 154.5 to 155 ft.......... 140 155

Sand and gravel, slightly clayey, very silty; texture grades from very fine

to very coarse sand; contains 10 percent gravel............................. 155 157

[Ogallala Group] Sandstone, very silty, medium-gray; poorly cemented;

texture grades from very fine to fine sand.................................... 157 160

Silt, moderately sandy, medium olive-gray; slightly cemented; contains

very fine to medium sand....................................................... 160 161.5

Silt, slightly clayey, slightly sandy, light olive-gray; contains embedded

very fine to coarse sand; moderately sandy below 165.5 ft............... 161.5 166.5

Sand, texture grades from very fine to very coarse; contains some fine

gravel.............................................................................. 166.5 168

Silt, slightly clayey, slightly sandy, light olive-gray; contains coarse

texture silt to very fine sand; contains a few rootlets below 170 ft........ 168 174

Sand and some gravel texture grades from very fine to very coarse

gravel; contains 5 to 10 percent fine gravel with some medium

gravel.............................................................................. 174 180

Sand and gravel texture grades from fine to very coarse sand; contains

30 percent fine gravel with some medium gravel........................... 180 185

Sand texture grades from very fine to coarse with some very coarse....... 185 191.5

Siltstone, marly, to sandstone, very calcareous, white; contains very fine

to fine sand....................................................................... 191.5 195

Sandstone, slightly to moderately silty, very calcareous, light olive-gray;

texture of sand grades from very fine to fine; slightly silty, slightly

calcareous and contains a trace of medium sand below 197 ft............ 195 210

Sand, slightly calcareous; texture grades from very fine to very coarse;

contains some reworked green siltstone...................................... 210 211

[Pierre Shale] Shale, clayey, moderately calcareous, light-gray; contains

some mottled yellow-brown; contains 10 percent mottled yellowbrown from 212 to 215 ft, 25 percent mottled yellow-brown from 215

to 219.5 ft; 10 percent mottled yellow-brown from 220 to 229 ft;

contains bentonitic layer at 227.5 ft; medium-gray from 229 to 230.5

ft and below 236 ft; slightly friable texture below 239 ft.................. 211 250

Location Well

number

Ground

elevation

Section 13, T. 9 N., R. 15 W. 42-A-54 2,105 ft

Depth, in feet

From To

Subsurface Cenozoic Geology of South-Central Nebraska
18 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

Location Well

number

Ground

elevation

Section 36, T. 9 N., R. 15 W. 43-A-54 2,094 ft

Depth, in feet

From To

Road fill.............................................................................. 0 1

Soil: silt, moderately clayey, slightly sandy, slightly calcareous, dark

brown-gray; contains very fine with a trace of coarse sand;

noncalcareous and contains more clay from 2.8 to 3.2 ft................... 1 3.4

Sand and gravel, quartz with pink feldspar; texture grades from medium

to fine sand with a trace of medium gravel; contains 20 to 35 percent

gravel; contains a thin sandy silt layer at 33.3 ft; contains some iron

stain at 30 to 40 ft; 40 to 50 percent gravel below 40 ft.................... 3.4 54

Silt, moderately to very clayey, yellow-brown; light-brown, slightly

calcareous from 57 to 60 ft; slightly clayey and noncalcareous,

medium brown-gray, coarse texture silt from 60 to 61.5 ft; light

brown-gray below 61.5 ft......................................................

54 63

Sand texture grades from very fine to medium with a little coarse to very

coarse; contains a trace of fine gravel, quartz with pink silicates below

70 ft............................................................................... 63 78

Silt, moderately clayey, slightly calcareous, light-gray; contains more

clay below 80 ft; mottled light-gray and light-brown below 81.5 ft;

contains coarse texture silt grading to slightly sandy below 85 ft........ 78 86.5

Silt, very sandy, slightly calcareous, light-brown; granular structure;

contains very fine sand.......................................................... 86.5 87.5

Silt, slightly clayey, slightly sandy, moderately calcareous, light-gray;

contains coarse texture silt to very fine sand; slightly calcareous below

90 ft; light-brown to olive-gray below 93 ft; moderately sandy and

contains very fine to fine sand below 101 ft; noncalcareous below 110

ft.................................................................................... 87.5 112

Sand texture grades from very fine to medium with a trace of coarse to

very coarse sand................................................................. 112 114.5

Silt, slightly clayey, slightly calcareous, medium yellow-gray; medium

gray and contains more clay below 117 ft................................... 114.5 121

Clay, very silty, slightly calcareous, medium-gray............................ 121 121.5

Silt, very clayey, moderately calcareous, light-gray; contains coarse

texture silt from 124 to 125 ft................................................. 121.5 125

Silt to sandy silt, slightly calcareous, medium-gray, interbedded; contains

many gastropods................................................................ 125 129.5

Silt, slightly to moderately clayey, slightly sandy, slightly calcareous;

contains coarse texture silt to very fine sand; moderately clayey to in

part very sandy from 131.5 to 135 ft; slightly clayey, very sandy,

contains many gastropods below 135 ft; less sandy below 139 ft........ 129.5 143.5

Sand and some gravel texture grades from very fine to very coarse sand

with some fine gravel; contains mostly quartz and silicates; contains

black staining.................................................................... 143.5 144.5

Silt, moderately clayey to in part slightly sandy, slightly calcareous, darkgray................................................................................ 144.5 145

Silt, slightly to moderately sandy, slightly calcareous, light-gray; contains

very fine to fine sand, interbedded............................................ 145 150

Silt, slightly clayey, moderately sandy, slightly calcareous, medium-gray;

contains very fine to fine sand; contains a trace of medium sand below

152 ft.............................................................................. 150 155

Buffalo County–(Anonymous, 1953b; Anonymous, unpublished [1954])—

Continued
19

Buffalo County–(Anonymous, 1953b; Anonymous, unpublished [1954])—

Continued

Silt, sandy, to sand, silty, moderately calcareous; contains very fine to

fine with a trace of medium sand............................................. 155 156

Silt, moderately clayey, slightly sandy, slightly calcareous, medium-gray;

contains coarse texture silt to very fine to fine sand; contains less clay

and more sand with a trace of fine gravel from 160 to 64 ft;

noncalcareous from 164 to 165 ft............................................. 156 170

Silt, slightly clayey, slightly calcareous, light olive-gray; contains coarse

texture silt; in part slightly sandy, contains very fine sand from 180 to

185 ft; moderately clayey from 185 to 186 ft, slightly clayey to in part

slightly sandy, medium-gray below 186 ft................................... 170 195

Silt, moderately sandy, slightly calcareous, medium-gray; contains very

fine to fine sand.................................................................. 195 200

Silt, slightly clayey, slightly calcareous, medium-gray, coarse texture silt;

moderately calcareous from 201.5 to 203 ft; in part slightly to

moderately sandy below 203 ft................................................ 200 205

Silt, slightly clayey, slightly sandy, slightly calcareous, light-gray;

contains coarse texture silt to very fine sand; moderately clayey from

208 to 209 ft; contains a trace of embedded sand from 210 to 215 ft;

contains limy nodules below 215 ft; moderately clayey from 216 to

220 ft; contains a trace of embedded fine sand and noncalcareous

below 225 ft...................................................................... 205 229

Sand texture grades from very fine to very coarse with a trace of fine

gravel.............................................................................. 229 239.5

[Pierre Shale] Shale, clayey, moderately calcareous, medium-gray;

contains a white, light-gray, and yellow-brown bentonite layer at

253.5 ft........................................................................... 239.5 290

Location Well

number

Ground

elevation

Section 2, T. 8 N., R. 15 W. 42-31* 2,110 ft

Depth, in feet

From To

Soil: silt, buff........................................................................ 0 3

Gravel, coarse texture............................................................... 3 58

Sand, silty, calcareous, light-gray................................................. 58 80

*The published log of this test hole indicates a well number of 42-31 (Anonymous,

1953b, p. iv), however, J.B.Swinehart (written commun., 2001) corrected the well

number to 42-32.

Location Well

number

Ground

elevation

Section 36, T. 9 N., R. 15 W. 43-A-54 2,094 ft

Depth, in feet

From To

Subsurface Cenozoic Geology of South-Central Nebraska
20 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

Buffalo County–(Anonymous, 1953b; Anonymous, unpublished [1954])—

Continued

Location Well

number

Ground

elevation

Section 24, T. 8 N., R. 15 W. 73-B-47 2,094 ft

Depth, in feet

From To

Soil: sand, silty, brownish-gray; texture grades from fine to coarse sand

with some gravel................................................................ 0 0.7

Sand, light buff-gray; texture grades from fine to medium with a trace of

coarse............................................................................... 0.7 0.9

Sand and gravel, brownish gray with some pink grains; texture grades

from sand to medium gravel with some coarse gravel and pebbles;

limonitic-stained; contains a thin yellowish brown silty clay layer at

15 ft................................................................................ 0.9 29

Sand and gravel, brownish gray with some pink and black grains; texture

grades from sand to coarse gravel; limonitic-stained...................... 29 69

Silt, slightly clayey, brownish gray; limonitic-stained in upper part......... 69 70

Silt, slightly clayey to sandy, brownish tan with slight grayish tint;

contains very fine sand......................................................... 70 77.5

Sand and gravel, brownish gray with pink grains; texture grades from

sand to medium gravel......................................................... 77.5 81

Silt, clayey, brownish tan with grayish tint..................................... 81 91

Sand and gravel, brownish gray with pink grains; texture grades from

sand to fine gravel with some medium gravel; contains a thin silt layer

at 98.5 ft.......................................................................... 91 109.5

Silt, in part slightly clayey to sandy, light to medium brownish gray....... 109.5 124

Sand and some gravel, brownish gray with some pink grains; texture

grades from sand to medium gravel.......................................... 124 165

Silt, clayey to clay, silty, light to medium-gray; contains yellowish to

grayish brown layer from 165 to 165.7 ft.................................... 165 179.5

Sand, slightly silty, light brownish gray; texture grades from very fine to

medium sand; contains light-gray clay pebbles, aragonite and thin

shell fragments from 195 to 200 ft............................................ 179.5 204

[Pierre Shale] Clay, light-gray to light yellowish gray........................ 204 210

i>ÀiÞÊ
ÕÌÞp­ÞÕÃ]Ê£xÎv®

Location Well

number

Ground

elevation

Section 19, T. 8 N., R. 14 W. 72-B-47 2,096 ft

Depth, in feet

From To

Soil: sand, moderately silty, dark brown-gray; texture grades from very

fine to fine sand with a trace of coarser grains............................... 0 0.8

Sand, slightly silty, light brown-gray; texture grades from very fine to

coarse with a trace of very coarse sand and fine gravel; yellow-brown

and very silty to slightly clayey below 2 ft.................................. 0.8 2.5

Sand and gravel, light brownish gray with many pink grains; texture

grades from medium sand to fine gravel with some medium gravel;

iron stained.......................................................................

2.5 10

Gravel texture grades from fine to medium with some coarse gra vel and

a trace of sand, 85 to 90 percent gravel; principally quartz and pink

feldspar; iron-stained from 10 to 20 ft....................................... 10 24.5
21

Location Well

number

Ground

elevation

Section 19, T. 8 N., R. 14 W. 72-B-47 2,096 ft

Depth, in feet

From To

Silt, moderately clayey to sandy, so il-like, medium grayish brown to

light-gray............................................................................ 24.5 25.2

Gravel, light-gray; texture grades from fine to coarse gravel with some

sand; principally quartz with some pink feldspar; 75 percent gravel;

finer below 30 ft.................................................................... 25.2 48

Silt, sandy, light-gray; very fine texture sand; iron-stained in upper part... 48 51

Sand and gravel, iron-stained; texture grades from coarse sand to medium

gravel; contains some yellow-brown clay...................................... 51 58

Clay, silty, in part sandy, ye llowish-brown; contains very fine to medium

sand................................................................................. 58 61

Sand and gravel, light brownish gray with some pink feldspar; texture

grades from medium sand to medium gravel, 50 to 60 percent gravel;

iron-stained........................................................................ 61 70

Sand and gravel, light brownish gray; texture grades from fine sand to

fine gravel with some medium to coarse gravel, 45 to 50 percent

gravel; contains 80 percent gravel below 90 ft............................... 70 107

Sand and gravel, light brownish gray; texture grades from fine to very

coarse sand with some fine to medium gravel; contains yellowish

brown silty clay layers below 111.5 ft........................................ 107 115.5

Sand and gravel, light brownish gray with iron stain; texture grades from

medium sand to medium gravel, 50 percent gravel......................... 115.5 123

Silt, sandy, light-gray; contains very fine sand.................................. 123 127.5

Sand and gravel, light brownish gray; texture grades from fine sand to

fine gravel with a trace of medium to coarse gravel........................ 127.5 140

Sand, grayish white; texture grades from fine to very coarse with a trace

of gravel; principally quartz................................................... 140 143

Silt, clayey, moderately to very calcareous, light-brown; contains a trace

of very fine to fine sand............................................................ 143 153

Silt, moderately calcareous, light brownish gray; contains limy nodules at

167 ft; clayey below 170 ft........................................................ 153 180

Silt to siltstone, very calcareous, light-brown...................................... 180 186

Sand, light brow-gray; texture grades from very fine to coarse............... 186 187

Silt to siltstone, very calcareous, light-brown..................................... 187 199

Sand texture grades from fine to very coarse sand with some fine gravel;

principally quartz with some light green-yellow feldspar; contains

limonite and bone fragments...................................................... 199 209.5

Clay, very calcareous, grayish white with yellow tint; mottled brownish

yellow below 210 ft............................................................... 209.5 217

[Pierre Shale] Clay shale, very calcareous, medium-gray........................ 217 220

Kearney County–(Anonymous, 1953f)—Continued

Subsurface Cenozoic Geology of South-Central Nebraska
22 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

II. Subsurface Cenozoic Geology of 

South-Central Nebraska

Introduction

The Conservation and Survey Division (CSD), University 

of Nebraska at Lincoln, and U.S. Geological Survey (USGS) 

began a program of systematically drilling test wells for 

groundwater studies in Nebraska in 1930. The well cuttings 

that were recovered were described by CSD geologists, and 

descriptions of the sedimentary sequence in each hole were 

published in a series of reports. Examination of the test-hole 

reports in a block of counties in south-central Nebraska was 

used in the current study to gain an understanding of the lithology, thickness, and distribution of Pliocene, Pleistocene, and 

Holocene sediments in the region. Examples of the test hole 

descriptions are shown in section I. Similar previous studies 

were published by Lugn (1935) and Lugn and Wenzel (1938).

Methods

The test hole records were examined from CSD publications and unpublished material on fi le at the CSD offi ces 

in Lincoln, Nebraska. Data from 380 wells were used for 

this study (fi g. 2). The quality of the recorded descriptions 

Figure 2. Location map of the subsurface study area in south-central Nebraska. The course of the Platte River is shown in dark blue. The 

locations of the Elm Creek West (left) and Newark (right) quadrangles are in blue. Source of the test wells is the Conservation and Survey 

Division, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln.

 

CUSTER

SHERMAN HOWARD

DAWSON BUFFALO

NANCE

MERRICK

HALL HAMILTON

CLAY ADAMS

WEBSTER

NUCKOLLS

GOSPER

FURNAS

HARLAN

PHELPS KEARNEY

FRANKLIN

10 0 10 20 30 Miles 15 0 15 30 45 Kilometers

100° 99° 98°

41°

40°

Gothenburg

Overton

Elm

Creek Kearney

Grand Island

Central

City

Minden

Franklin

Shelton

NEBRASKA

KANSAS

Subsurface study area

Subsurface

control point

Platte River

Republican River

Elm Creek West Newark
23

ranges widely—early records are very general and lump many 

lithologies together, but later descriptions are more complete, 

and some of the reports identify the top of the Ogallala Group 

or the top of Cretaceous rocks in the well cuttings. In the 

descriptions examined, there are only a few identifi cations of 

named Pliocene or Pleistocene formations within the sedimentary sequence above bedrock.

Subsurface geology

The subsurface study area extends from about Gothenburg, on the west, to about Central City on the east, and 

south to the Republican River (fi g. 2). On this map the test 

hole locations, county lines, towns, and drainage system are 

shown. The course of the Platte River is shown in dark blue, 

and is also shown on the other maps in this section as a reference. The counties from which test-hole data were studied 

include Custer (Anonymous, 1953c), Dawson (Anonymous, 

1953c), Buffalo (Anonymous, 1953b; Anonymous, unpub., 

1954), Howard (Smith, 1965), Hall (Smith, 1965), Nance 

(Anonymous, 1953g), Merrick (Anonymous, 1953g), Hamilton (Keech, 1960), Clay (Burchett and Smith, 1994), Adams 

(Anonymous, 1953a), Kearney (Anonymous, 1953f), Phelps 

(Anonymous, 1953h), Gosper (Anonymous, 1953d), Furnas 

(Waite and others, undated), Harlan (Anonymous, 1953e; 

Waite and others, undated), Franklin (Burchett and Summerside, 1997a; Waite and others, undated), Webster (Burchett and Summerside, 1997b; Waite and others, undated), 

and Nuckolls (Burchett and Smith, 1989; Waite and others, 

undated). 

Figure 3. Structure contour map drawn on top of Precambrian rocks. Heavy green lines indicate crest lines of structural highs in the Great 

Plains part of the study area. The locations of the Elm Creek West and Newark quadrangles are in magenta. Cities are indicated by black 

dots and are the same as those shown on fi gure 1. The confi guration of mountainous areas is highly generalized because of a lack of oil 

and gas exploration wells in outcropping Precambrian rocks. Well data from IHS Energy (2003).

ÊÊÊÊÊ Ê 
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Subsurface Cenozoic Geology of South-Central Nebraska
Tertiary Cretaceous Quaternary

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Wisconsin

preWisconsin

Western and South-central

Nebraska

Northeastern

Colorado

So

Chadron Formation

White River Brule Formation

Group

Arikaree

Group

Gering Formation

Harrison Formation

Monroe Creek Formation

Upper Harrison beds

Ogallala Group

Broadwater Formation Fullerton Formation

Grafton Loess

Beaver Creek Loess

Loveland Formation

Gilman Canyon Formation

Peoria Loess

Brady soil

Bignell Loess

Denver Formation (part)

Green Mountain Conglomerate

Dawson Formation (part)

Pierre Shale

Pierre Shale

Carlile Shale

Niobrara Formation

Dawson Formation (part)

Chadron Formation and

Castle Rock Conglomerate

Wall Mountain Tuff

Arikaree Formation (part)

Ogallala Formation

Nussbaum Alluvium

Rocky Flats Alluvium

Verdos Alluvium

Slocum Alluvium

Broadway Alluvium

Peoria Loess

Louviers Alluvium

pre-Piney Creek alluvium

Piney Creek Alluvium

post-Piney Creek alluvium

Oga

Arikaree 

White Riv

Wasatc

 

Sand Hills dune sands

Laramie Formation

Fox Hills Sandstone

Era Period Epoch

Stratigraphic unit

Glacialstage

Cenozoic Mesozoic

Medicin

Lew

Hann

Denver Formation (part)

dune sand

and

loess

Arikaree Formation (part) Arikaree 

Broadw

White Riv

Walnut Creek Formation

Arapahoe Formation Fox H

Table M

Eagle Ro

Airport 

Harmon

Pahlow 

Stock Far

Present f

Figure 4. Chart showing late Mesozoic and Cenozoic stratigraphic units in south-central Nebraska, northeastern Colorado, and southeastern Wyoming. CDiffendal (1989), Swinehart and others (1985, 1994), May and others (1995), Robinson (1972), Scott (1960, 1963, 1978), Knight (1953), Mears (1991), Montagne Montagne (1993). Time scale is from Berggren and others (1995), Haq and Van Eysinga (1987), and Dawson (1992).
25

As noted by Lugn in 1935, the unconsolidated Pliocene, 

Pleistocene, and Holocene sediments that overlie bedrock are 

not confi ned to the current Platte Valley. Instead, the whole 

of south-central Nebraska and areas north and south is underlain by a variable sequence of sand and gravel, silt, and clay. 

This is because the present course of the Platte River is a relatively recent phenomenon, and Pliocene and Pleistocene versions of the Platte followed different courses than the present 

river. As the river migrated across Nebraska, it left deposits 

of gravel, sand, silt, and clay behind.

A structure contour map was constructed to show the 

regional structural setting of the Platte River system (fi g. 

3). This map was drawn on the top of Precambrian basement rocks deep in the subsurface of the plains and with a 

few control points in mountain areas. The map shows that 

the elevation of this horizon gradually rises eastward out of 

the Denver-Julesburg Basin in eastern Colorado and western 

Nebraska to a broad ridge extending northwest-southeast 

through Nebraska that has been named the Chadron-Cambridge Arch. This ridge branches southwestward into the 

Las Animas Arch, and extends southeastward as the Central 

Kansas Uplift. The arch borders the west side of the study 

area; however, there is a slight structural high that extends 

eastward along the south county line of Dawson and Buffalo 

CUSTER

SHERMAN HOWARD

DAWSON BUFFALO

NANCE

MERRICK

HALL HAMILTON

ADAMS CLAY

WEBSTER

NUCKOLLS

GOSPER

FURNAS HARLAN

PHELPS KEARNEY

FRANKLIN

10 0 10 20 30 Miles

15 0 15 30 45 Kilometers

1250 1530 1800 2070 2350 ft

£ääc c nc

{äc

{£c

Present course of Platte River

KANSAS

NEBRASKA

Subsurface

control point Datum is mean sea level

Contour interval 25 ft

Figure 5. Map showing the confi guration of the erosional unconformity on top of Cretaceous rocks in south-central Nebraska. Town and 

quadrangle locations are as shown in fi gure 2.

Subsurface Cenozoic Geology of South-Central Nebraska
26 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

counties. The main part of the subsurface study area is on the 

northwestern fl ank of the Central Nebraska Basin.

The nomenclature of Tertiary units in Colorado, Wyoming, and Nebraska is complex, and there are signifi cant differences between regions. The nomenclature of Cretaceous 

through Holocene units is shown on fi gure 4. Cretaceous 

rocks are present beneath nearly all of Nebraska, including 

the study area (Carlson, 1993). On a state-wide scale these 

rocks dip to the west (interrupted by the Chadron-Cambridge 

Arch) and are truncated by an unconformity, resulting in 

progressively older Cretaceous rocks being present eastward 

in the study area (Burchett and Pabian, 1991; Carlson, 1993, 

p. 60). Most of the eastern part of the study area is underlain by either the Pierre Shale or formations of the Colorado 

Group (Niobrara Formation or Carlile Shale). A long period 

of erosion in central Nebraska followed the Cretaceous 

Period. The end of the Cretaceous is considered to be about 

65.5 million years ago (Ma) (Berggren and others, 1995), but 

deposition of the Miocene Ogallala Group is not thought to 

have begun in this area until about 14 Ma (J.B. Swinehart, 

written commun., 2001). The Eocene to Miocene-age White 

River and Arikaree Groups (fi g. 4) are only present west or 

north of the subsurface study area. Another period of erosion occurred after deposition and lithifi cation of the Ogallala between about 5.4 and 3.5 Ma (May and others, 1995). 

Renewed deposition of Pliocene, Pleistocene, and Holocene 

sediments began after about 3.5 Ma and continues to the present (fi g. 4).

CUSTER

SHERMAN HOWARD

DAWSON BUFFALO

NANCE

MERRICK

HALL HAMILTON

ADAMS CLAY

WEBSTER

NUCKOLLS

GOSPER

FURNAS HARLAN

PHELPS KERNEY

FRANKLIN

0 150 300 450 600 ft

10 0 10 20 30 Miles

15 0 15 30 45 Kilometers

£ääc c nc

{£c

{äc

Contour interval 25 ft

Present course of Platte River

NEBRASKA

KANSAS

Subsurface

control point

Figure 6. Map showing thickness of the Ogallala Group. Town and quadrangle locations are as shown in fi gure 2.
27

A contour map drawn on top of Cretaceous rocks in 

south-central Nebraska shows the confi guration of this surface 

(fi g. 5). The map shows a pattern of ridges and valleys that 

were incised into the top of Cretaceous rocks both prior to and 

after deposition of the Tertiary Ogallala Group. The Ogallala was deposited over most of the map area and then later 

partially eroded, so the top of the Cretaceous, portrayed on 

fi gure 5, mostly represents the erosion surface that developed 

in the approximately 50 million years between the end of the 

Cretaceous and the beginning of deposition of the Ogallala. 

However, the far eastern side of the map was additionally 

modifi ed in the erosion cycle that removed some of the Ogallala. The most prominent features of the map are the series 

of valleys and ridges on the eastern side of the map where the 

surface was deeply eroded. The relatively low area in northeastern Gosper and western Phelps counties is also noteworthy 

because it acted as a site of deposition and preservation of the 

overlying Ogallala Group. A similar map of this surface in 

the Sand Hills area was presented by Swinehart and Diffendal 

(1989).

The Ogallala Group, of Miocene age, about 14 - 5.4 

Ma (Swinehart and Diffendal, 1989; J.B. Swinehart, written 

commun., 2001), unconformably overlies Cretaceous rocks 

in most of the study area. Rocks of Paleocene, Eocene, Oligocene, and early Miocene age are not present in the study 

area, so the unconformity at the base of the Ogallala represents 

CUSTER

SHERMAN HOWARD

DAWSON BUFFALO

NANCE

MERRICK

HALL TON

ADAMS CLAY

WEBSTER

NUCKOLLS

GOSPER

FURNAS HARLAN

PHELPS KEARNEY

£ä ä £ä Óä ÎäÊ iÃ

£x ä £x Îä {xÊ iÌiÀÃ

£Óxä £Èää £xä ÓÎää ÓÈxäÊvÌ

£ääc c nc

{£c

{äc

*ÀiÃiÌÊVÕÀÃiÊvÊ*>ÌÌiÊ,ÛiÀ

Contour interval 25 ft

 
	,-

 --

Datum is mean sea level

Subsurface

control point

SPE

Figure 7. Map showing the confi guration of the erosional unconformity on top of either the Ogallala Group or Cretaceous rocks in southcentral Nebraska. Axes of paleovalleys are shown by red lines. Town and quadrangle locations are as shown in fi gure 2.

Subsurface Cenozoic Geology of South-Central Nebraska
28 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

a gap of about 50 million years of time. The Ogallala was 

in turn truncated by an erosional surface, and was entirely 

removed in the eastern part of the study area. Figure 6 shows 

that the Ogallala reaches a thickness of over 400 ft in one well, 

and is over 300 ft thick in much of the northwestern part of the 

area. It is erosionally truncated along an irregular line trending northeast across the eastern part of the study area. In several places on the western side of the study area, the Ogallala 

is thick or thin over corresponding low or high areas, respectively, on the post-Cretaceous unconformity (fi g. 5). This 

trend may show some topographical control of deposition and 

preservation of the Ogallala, a process that was repeated later 

in the Pliocene and Pleistocene.

Another interpretation of Ogallala distribution was presented by Condra and others (1947) and Condra and Reed 

(1950). They introduced the name ‘Seward Formation’ which 

was believed to be a fi ne-grained distal facies of the Ogallala that extended into eastern Nebraska. Subsequent studies 

found that the Seward strata were younger than the Ogallala 

(Reed and Dreeszen, 1965), and the eroded edge shown in the 

current study (fi g. 6) was accepted as the eastern extent of the 

Ogallala.

The unconformity surface that overlies the Ogallala illustrates the drainage pattern that developed in post-Ogallala time 

prior to deposition of Pliocene sediments in the area. Figure 

7 is a map that is drawn on the top of the Ogallala Group, or 

Figure 8. Map showing the thickness of predominantly fl uvial Pliocene and Pleistocene sediments in south-central Nebraska. Town and 

quadrangle locations are as shown in fi gure 2.

0 88 200 312 425 ft

CUSTER

SHERMAN HOWARD

DAWSON BUFFALO

NANCE

MERRICK

HALL HAMILTON

CLAY ADAMS

WEBSTER

NUCKOLLS

GOSPER

FURNAS HARLAN

PHELPS KEARNEY

FRANKLIN

10 0 10 20 30 Miles

15 0 15 30 45 Kilometers

100° 99° 98°

41°

40°

Contour interval 25 ft

Present course of Platte River

NEBRASKA

KANSAS

Subsurface

control point
29

on top of Cretaceous rocks where the Ogallala is not present, 

that represents the top of the bedrock surface below Pliocene 

and younger sediments. The map shows a series of valleys and 

intervening drainage divides that extend east-southeastward 

from a high area on the west side of the study area. Three of 

the valleys or “basins” were discovered early in the process of 

drilling test holes and were named by Lugn (1934). The valley 

in central Hamilton county was named the Aurora Basin, the 

valley in central Adams and Clay counties was named the Hastings Basin, and the valley in southeastern Gosper and southwestern Phelps counties was called the Holdrege Basin. After 

those areas were named, additional drilling has defi ned other 

areas of low elevation on the erosion surface, notably an area 

in east-central Franklin and southern Webster counties, and 

extension of the Hastings Basin northwestward, to southeast 

of Kearney. A small-scale map of this surface over most of the 

southern and eastern parts of Nebraska was presented by Reed 

and Dreeszen (1965) and Reed and others (1965).

The study area again became the site of deposition in the 

early Pliocene, and deposition continued into the Pleistocene 

and Holocene. A sequence of gravels, sands, and interbedded silt and clay beds, largely of fl uvial origin, was named the 

“Platte Series” by Lugn (1935). This series consisted of the 

Holdrege Formation (sand and gravel), Fullerton Formation 

(silt), Grand Island Formation (sand and gravel), and Upland 

Formation (silt). The nomenclature of this series was modiFigure 9. Map showing the thickness of loess in Pleistocene and Holocene strata in south-central Nebraska. Town and quadrangle locations are as shown in fi gure 2.

CUSTER

SHERMAN HOWARD

DAWSON BUFFALO

NANCE

MERRICK

HALL HAMILTON

CLAY ADAMS

WEBSTER

NUCKOLLS

GOSPER

FURNAS HARLAN

PHELPS KEARNEY

FRANKLIN

0 81 162 243 325 ft

10 0 10 20 30 Miles

15 0 15 30 45 Kilometers

100° 99° 98°

41°

40°

Present course of Platte River

Contour interval 25 ft

NEBRASKA

KANSAS

Subsurface

control point

Subsurface Cenozoic Geology of South-Central Nebraska
30 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

fi ed by Reed and Dreeszen (1965), and later most of the names 

fell into disuse as researchers became aware of uncertainties 

in correlation of the units. Above the alluvial part of the section is a largely loess sequence that was originally named 

the “Plains Series” by Lugn (1935). This sequence consists 

of Pleistocene and Holocene units that have been named the 

Grafton Loess, Beaver Creek Loess, Loveland Formation, 

Gilman Canyon Formation, Peoria Loess, and Bignell Loess, 

from oldest to youngest (Reed and Dreeszen, 1965; Reed and 

others, 1965; Swinehart and others, 1994). 

For this study I have divided Pliocene and younger sediments into two parts, a lower, largely fl uvial part and an upper, 

largely eolian part. The lower part consists of irregularly distributed sand and gravel beds and associated silt or clay beds 

that previously have been included in the Holdrege, Fullerton, 

and Grand Island units. The base of the interval is picked at 

the top of the underlying Ogallala Group or Cretaceous rocks. 

The lithology of Cretaceous rocks is distinctive dark shale or 

even more distinctive yellow or white limestone or chalk, and 

it is not diffi cult to distinguish these rocks from the overlying 

Pliocene and younger unconsolidated sediments. The division 

between the Ogallala Group and younger sediments is more 

problematic because of lithologic similarities between those 

units.

The lithology of the top of the Ogallala changes from 

place to place, due to the unconformity at the top exposing 

successively older beds eastward. One of the main features 

that was used to distinguish the Ogallala was the presence 

Figure 10. Map showing the thickness of all Pliocene, Pleistocene, and Holocene alluvium in south-central Nebraska. Town and quadrangle

locations are as shown in fi gure 2.

CUSTER

SHERMAN HOWARD

DAWSON BUFFALO

NANCE

MERRICK

HALL HAMILTON

CLAY ADAMS

WEBSTER

NUCKOLLS

GOSPER

FURNAS HARLAN

PHELPS KEARNEY

FRANKLIN

10 0 10 20 30 Miles

15 0 15 30 45 Kilometers

0 125 250 375 500 ft

100° 99° 98°

41°

40°

Present course of Platte River

Contour interval 25 ft

NEBRASKA

KANSAS

Subsurface

control point
31

of lithifi ed sandstone, conglomerate, or siltstone versus the 

unconsolidated sediments of Pliocene and younger strata, 

although the well logs also describe the Ogallala as unconsolidated sand and gravel in places. The color of Ogallala 

strata is a common distinguishing feature as well. The Ogallala is typically a darker brown, greenish, or reddish color 

that contrasts with the lighter tan colors of younger strata. 

Other distinguishing features of the Ogallala include extensive zones having carbonate cement, rare limestone or marl 

beds, and siliceous rootlets in the upper part of the unit. In 

many areas the sands and gravels overlying the Ogallala 

contain pink grains of feldspar that do not occur in underlying rocks. Any one of the above-mentioned features may not 

be suffi cient to separate the Ogallala from overlying beds. 

However, when examining many well records in a limited 

area a combination of features can be used to distinguish the 

units in most cases.

The alluvial sequence above the Ogallala consists of 

a mixture of gravel, sand, silt, and clay. Colors are various shades of brown, tan, greenish-gray, white, and yellow. 

White ash beds were described in logs of some wells, 

although they were not recorded in enough wells to be used as 

marker beds. Logs of some wells noted the presence of abundant bone fragments. Figure 8 shows the thickness of the 

predominantly alluvial Pliocene and Pleistocene sediments in 

the study area. Of particular note is the relative thickness of 

this interval in the valleys or basins that previously had been 

incised into underlying bedrock in central Hamilton County, 

southeastward from central Kearney County, in central to 

southern Franklin and Webster counties, and in southeastern 

Gosper and southern Phelps counties. Relatively thin areas 

are in northern Buffalo County, southern Gosper County, and 

northern Webster and southwestern Nuckolls counties. These 

areas correspond well with low and high areas on the map of 

the top of bedrock (fi g. 7).

Loess was distinguished from the underlying alluvial 

sequence by the presence of thick silt intervals, associated 

soils, snail shells, and rootlets. Some sand intervals are also 

included in this sequence, whose lithology of well-sorted, 

rounded, very fi ne to medium grained quartz clasts indicates 

a probable eolian origin. Rare marl beds, less than 2 ft thick, 

were noted in Buffalo County. Colors of this sequence are 

mostly shades of tan or gray; soil intervals are dark-gray to 

black, and the Loveland Formation is noted for its reddish 

color. An isopach map of this sequence is shown as fi gure 

9. This map shows a generally thin area along the present course of the Platte River where much of the unit was 

removed as it was deposited, and a thick area that trends 

southeastward across the southern part of the map from 

Gosper to Nuckolls counties. The thickest area is on the 

western map border.

An isopach map that shows the entire sequence of Pliocene- to Holocene-age sand, gravel, silt, and clay that is 

present above the Ogallala or Cretaceous rocks is shown as 

fi gure 10. A similar map that included much of the study 

area was by Dreeszen and others (1973), and another detailed 

map of the same stratigraphic interval, just north of the study 

area, is by Souders (2000). Noteworthy features of this map 

(fi g. 10) include a thin area extending southeastward from 

the western map boundary to about the position of Kearney. 

This area has thin deposits due to both limited deposition of 

fl uvial strata over an underlying topographic high, shown on 

fi g. 5, and to erosion or non-preservation of the Peoria Loess 

in the Platte River Valley. A relatively thick area extends 

southeastward, parallel to the Platte River from southern 

Dawson and northern Frontier counties to central Franklin 

County, then abruptly trends north-northeastward into Kearney County and again trends abruptly southeastward in western Adams County.

A combination of the paleo-topographic map (fi g. 7) and 

the isopach map of alluvial sediments (fi g. 8) suggests two 

or three possible courses of a paleo-Platte River through the 

study area. These paleo-stream courses correspond well with 

postulated drainages in the early to late Pleistocene sketched 

by Swinehart and others (1994): (1) A northern stream course 

extends southeastward from southeastern Sherman County 

through south-central Hamilton County; (2) an intermediate 

stream course extends southeastward from northern Dawson 

County through southern Clay County; and (3) a southern 

stream course trends from southwestern Dawson County, 

through central Franklin County, and to southeastern Webster 

County. The southeastward-trending thick, sandy area in 

central Hamilton County may represent a drainage outlet for 

the Loup River system, which may have also fl owed southeastward until stream capture diverted it. The chronology 

suggested by Swinehart and others (1994) was that the paleoPlatte River shifted northward during the Pleistocene, but it 

was not until the Wisconsin glacial stage that stream capture 

diverted the Loup River system and the Platte River into their 

present courses (Bentall, 1982).

III—The development of the Cenozoic 

Platte River drainage system

Introduction

This section traces the structural and depositional history 

of the east-central Rocky Mountains and adjacent plains from 

Late Cretaceous time—about 65.5 million years ago—to the 

present (fi g. 4). The purpose of this summary is to examine 

the geological processes that affected the development of the 

Platte River drainage system through this time period.

The Platte River in Nebraska is formed by the combination of the North and South Platte Rivers, whose headwaters 

are in the mountains of Colorado (fi g. 1). The North Platte has 

a drainage area of approximately 35,000 mi2

, and the South 

Platte drains approximately 24,000 mi2 (Sidle and Faanes, 

1997). The North Platte fl ows northward from the North Park 

area of northern Colorado, skirts the west side of the Medicine 

Bow Mountains in southern Wyoming, and follows a semicirThe development of the Cenozoic Platte River drainage system
32 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

cular course around the north end of the Laramie Range. It 

then fl ows southeastward in a nearly straight valley to North 

Platte, Nebraska, where it joins the South Platte. Major tributaries of the North Platte in Wyoming are the Sweetwater, 

Medicine Bow, and Laramie Rivers. Many dams have been 

built on the North Platte—from the headwaters downstream 

they form the Seminoe, Pathfi nder, Alcova, Glendo, and 

McConaughy reservoirs.

The South Platte rises in the mountains west of South 

Park in Colorado, turns and fl ows northeast through Denver, 

and then fl ows east-northeast through northeastern Colorado to 

the junction with the North Platte in Nebraska. Several perennial streams fl ow out of the Front Range and join the South 

Platte, the main ones being Bear Creek, Clear Creek, Boulder 

Creek, St. Vrain Creek, the Big Thompson River, and the 

Cache la Poudre River. Relatively smaller dams on the South 

Platte have created the Spinney, Eleven Mile Canyon, Cheeseman, Strontia Springs, Chatfi eld, and Prewitt reservoirs. Flow 

in the South Platte is augmented by several diversion tunnels 

that carry water from west of the Continental Divide to the 

east side of the mountains.

Between North Platte and Columbus, Nebraska, the Platte 

River has few tributaries—the major one being the Wood 

River on the north side of the Platte. South of the Platte, 

in this reach, most drainage is internal or fl ows south to the 

Republican River or southeast into the Blue River system. 

North of the Platte, several streams in central Nebraska combine to form the Loup River, which joins the Platte at Columbus. The smaller Elkhorn and Salt Creek drainages join the 

Platte farther downstream, just above its confl uence with the 

Missouri River. There are no dams on the Platte in Nebraska 

from the confl uence of the North and South Platte Rivers 

downstream to the Missouri River.

The role of the Platte River Valley as a main transcontinental transportation route resulted in early descriptions of 

the river and descriptions of the geology of surrounding areas. 

The earliest accounts were by military exploration surveys by 

Lt. Zebulon Pike, Major Stephen Long, Captain John Fremont, 

and others in the fi rst half of the 19th century (Muhs and Holliday, 1995). Many people passed through the Platte River 

Valley in the middle 19th century with the establishment of the 

Oregon and Mormon Trails and later with the building of the 

Union Pacifi c Railroad along the Platte River across Nebraska. 

Systematic geologic studies of Nebraska Territory were fi rst 

undertaken by F.V. Hayden and F.B. Meek in the mid-1850s 

and continued through the 1870s, when Hayden’s survey was 

combined with those of King, Powell, and Wheeler into the 

U.S. Geological Survey. Hayden and Meek’s studies were the 

fi rst to describe the geology and paleontology of the region in 

detail.

The latter half of the 19th century saw the development of the geological staff at the University of Nebraska, 

the Nebraska State Museum, and, eventually, the Nebraska 

Geological Survey, now operating as the Conservation and 

Survey Division of the Institute of Agriculture and Natural 

Resources, the University of Nebraska at Lincoln. At fi rst, 

the emphasis was on studies of the natural resources of the 

State in support of economic development, but the appointment of the vertebrate paleontologist E.H. Barbour in 1891 

heralded the broadening of studies to include the richly fossiliferous Cenozoic strata of the State. The beginning of the 

20th century saw a diverse range of geological studies conducted by State and Federal researchers in Nebraska, Kansas, 

Colorado, and Wyoming on topics such as mineral and energy 

resources, water resources, and paleontology. Popular books 

that describe some aspects of the geologic history of the 

Platte River Basin include Webb (1931), Mattes (1969), and 

McPhee (1998).

Structural setting

Precursor events 

As the Atlantic Ocean opened in Jurassic time, about 150 

million years ago, the western margin of the North American 

Craton was displaced westward and collided with several 

oceanic plates in the Pacifi c. This collision led to an actively 

subducting plate margin and the development of a series of 

shallow crustal thrust sheets that were transported eastward 

(Lawton, 1985). This tectonic event was termed the “Sevier 

Orogeny”, and lasted from approximately 119 Ma to 50 Ma. 

The thrust sheets were stacked into mountainous terrain, but 

the area east of the mountains subsided and is known as the 

Rocky Mountain Foreland Basin (Gries and others, 1992; 

Bird, 1998; Willis, 1999).

In Early Cretaceous time the Rocky Mountain Foreland 

Basin deepened, and, because of a worldwide rise in sea 

level, a shallow ocean that connected boreal and tropical 

seas across the interior of the North American continent was 

formed. At its maximum, this Western Interior epicontinental sea extended north-south from the pole to the Caribbean, 

including an arm that extended northeast through Canada to 

Greenland, and east-west from the mountain front to eastern 

Minnesota and Iowa, central Missouri, and western Arkansas 

(Gries and others, 1992; Roberts and Kirschbaum, 1995). 

In cross section, the seaway was asymmetric, deeper on 

the west and shallower on the east (McGookey and others, 

1972). Clastic sediments were shed from the western mountains into the sea, while fi ne-grained muds and chalks were 

deposited in the shallower central and eastern parts of the 

sea.

These conditions persisted throughout much of Cretaceous time, varying in the degree of uplift or erosion of the 

mountains and by low-stand or high-stand extremes of sea 

level. At irregular intervals, the amount of debris shed from 

the mountains exceeded the space available in the basin, and 

shoreline and continental deposits prograded eastward across 

parts of the sea. At times of lower erosion rates in the mountains or accelerated subsidence rates in the foreland basin, 

the seas transgressed far to the west and marine shale and 

calcareous mud were deposited across the depositional basin.
33

Laramide orogeny

Renewed opening of the North Atlantic Ocean began at 

about 75 Ma and initiated another episode of subduction and 

deformation on the western margin of the North American 

Craton (Gries and others, 1992). This event, termed the 

“Laramide Orogeny”, overlapped in time with the Sevier 

Orogeny, but was of a greater magnitude, had a different 

compressional direction, and involved deep basement rocks 

in contrast to shallow Sevier thrusting (Tweto, 1975; Gries 

and others, 1992; Bird, 1998). The dominant displacement 

of basement rocks and overlying cover rocks was vertical, 

producing fault-bounded uplifted areas without a great deal 

of horizontal dislocation of strata. This deformational style 

lasted from Late Cretaceous time (about 75 Ma) until about 

35 Ma in the Tertiary Eocene Epoch.

Laramide tectonics caused buckling of the earth’s crust 

into a series of mountain ranges and intervening basins and 

had effects far east of the Rocky Mountains. Much of the 

structure east of the present-day mountain front is a result 

of Laramide deformation or of Laramide enhancement of 

older structures (Moore and Nelson, 1974). Figure 3 is a 

structure contour map drawn on the top of Precambrian 

rocks. Major basins shown on the map include the DenverJulesburg Basin, just east of the mountain front; the Central 

Nebraska and Salina Basins, in south-central Nebraska and 

northern Kansas; and the combined Hugoton Embayment 

of the Anadarko Basin and Sedgwick Basin of southwestern 

and south-central Kansas. Intervening uplifted areas are the 

Chadron-Cambridge Arch through western Nebraska and its 

extension into Kansas as the Central Kansas Uplift. Branching westward into Colorado from the Chadron-Cambridge 

Arch is the Las Animas Arch, and branching northeastward 

from Nebraska to South Dakota and Minnesota is the Siouxana Arch (fi g. 3).

At the end of the Cretaceous Period at 65.5 Ma, the 

Rocky Mountains had been uplifted by Laramide tectonics into a series of isolated mountain ranges separated by 

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0 100 200 50 Miles

0 120 240 60 Kilometers

Fort Union Formation

Waltman

Lake

Cannonball

Sea

Coalmont

Formation

Denver and

Dawson Formations,

Green Mountain

Conglomerate

Raton and Poison Canyon

Formations

Possible eastern extent of Pa el ocene deposition

Intrusive

centers

WYOMING

COLORADO

NEBRASKA

KANSAS

Hanna

Formation

Figure 11. Paleogeography in Paleocene time. Uplifts in red cross-hatched pattern and basins in brown modifi ed from Robinson (1972), 

Waltman Lake and Cannonball sea modifi ed from Blackstone (1975), streams in Wyoming modifi ed from Seeland (1988), and streams and 

intrusive centers in Colorado modifi ed from Scott (1975). Cities and mapped quadrangles are as shown on fi gure 1.

The development of the Cenozoic Platte River drainage system
34 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

basins. Kluth and Nelson (1988) suggested that uplift of 

much of the Front Range of Colorado occurred in a relatively short time. The climate was warm and temperate 

(Parrish, 1998), and erosion of the mountains was progressing vigorously. Debris from the mountains was deposited 

in the adjacent basins, some of which were subsiding fast 

enough to capture all the sediment that they received. 

The rock record of the east-central Rocky Mountains and 

adjacent plains, from the beginning of the Tertiary to the 

present, records the interplay between erosion and deposition, between sediment capture in basins adjacent to the 

mountains or widespread dispersal of the sediments far to 

the east by streams, and the addition of signifi cant amounts 

of volcanic ash from sources west of the mountains. Subtle 

variations in uplift, climate, and subsidence had large effects 

on the preserved rock record.

Geologic history of the Tertiary and Quaternary

The following subsections summarize the geologic history of the Tertiary and Quaternary Periods in the east-central 

Rocky Mountains and adjacent plains in Colorado, Wyoming, Nebraska, and Kansas. Each part summarizes (1) the 

occurrence and distribution of formations recognized in the 

study area, (2) the lithology and depositional environments 

of the units, (3) the paleogeography of the region, (4) major 

fossil fauna and fl ora, and (5) inferences about the climate 

that existed at the time.

When reviewing the published material for this section, 

two problems became evident. First, the boundaries between 

epochs, in numerical age, have been modifi ed by various 

researchers, and this is an ongoing process. For example, the 

boundaries between the Miocene and Pliocene, and between 

the Pliocene and Pleistocene have been redefi ned. Second, 

improved dating techniques have shifted the assignment of 

some formations from one epoch to another—for example, 

the Chadron Formation was previously considered to be Oligocene, but is now considered Eocene, based on placement 

of the Eocene-Oligocene boundary with respect to dated 

parts of the Chadron. The result of these changes is that 

previously published geologic histories of the region do not 

conform to the modern accepted grouping of formations into 

certain epochs. An important purpose of this section, then, 

is to note these changes in a somewhat revised chronology 

of the Tertiary and Quaternary, using the currently accepted 

time divisions and age assignments of formations.

The ages of formations for this report were checked 

against the geologic names lexicon in the USGS National 

Geologic Map Database. This is the offi cial USGS repository for information on the correct accepted usage and age 

designation (by the USGS) for lithologic and geochronologic 

unit names. The web site for this database is http://ngmdb.

usgs.gov/Geolex/geolex_home.html. The database is a 

work in progress, however, so some given unit ages lag 

behind current research results. Disagreement between 

epoch boundaries exists among current workers too, so the 

boundaries used in this report may vary from those used in 

other reports. The time scale used for this report is that of 

Berggren and others (1995), supplemented by Haq and Van 

Eysinga (1987) and Dawson (1992).

Paleocene

The Paleocene is the oldest epoch of the Tertiary Period 

(fi g. 4), and in the Denver-Julesburg Basin Paleocene units are 

the upper part of the Denver Formation, the Green Mountain 

Conglomerate, and the lowermost part of the Dawson Formation. This package of rocks is in the upper part of the synorogenic D1 sequence of Raynolds (1997). The Paleocene and 

Eocene Coalmont Formation was deposited in North Park, 

and the Upper Cretaceous and Paleocene Raton and Poison 

Canyon Formations were deposited in southern Colorado (fi g. 

11). In Wyoming, the Fort Union Formation was deposited in 

the Powder River Basin, the Hanna Formation was deposited in 

the Hanna Basin, and the Waltman Shale Member of the Fort 

Union Formation was deposited in the Wind River Basin. The 

Cannonball Member of the Fort Union Formation was deposited 

farther north, in North and South Dakota, and may have been 

present in parts of Nebraska, Kansas, and Colorado (Blackstone, 

1975). All traces of any previously existing Paleocene rocks 

have been removed from Nebraska and Kansas.

The lithology of many of these units is conglomeratic 

arkose or volcaniclastic conglomerate, sandstone, siltstone, 

and claystone. The Coalmont, Fort Union, Hanna, and Raton 

Formations contain coal beds (Beekley, 1915; Flores and 

Bader, 1999; Pillmore, 1976), and the Denver Formation 

contains thick lignite beds on the eastern side of the DenverJulesburg Basin (Kirkham and Ladwig, 1979, Raynolds, 1997; 

Nichols, 1999). The Waltman Shale and Cannonball Members 

of the Fort Union Formation are shale units deposited in lacustrine and marine environments, respectively.

The paleogeography of the study area in Paleocene time 

is shown on fi gure 11. This map shows the major uplifted 

areas and sites of deposition in Colorado and southeastern 

Wyoming, inferred intrusive centers and stream locations in 

Colorado, inferred locations of a lake in Wyoming, and a sea, 

mainly in western Nebraska. Deposition was largely from 

streams that carried eroded material from the mountains to 

adjacent basins. Much, if not all, of the material was probably 

trapped near the mountain sources and not dispersed very far 

eastward onto the Great Plains (Raynolds, 1997). The mountains have been estimated to have risen approximately 12,000 

ft in the Paleocene (Scott, 1975), and uplift was accompanied 

by intrusive igneous and volcanic activity (Morse, 1981). 

Note that the reconstructed stream fl ow directions are oriented 

northwest to southeast, except for one stream shown fl owing 

northwest to North Park. The Cannonball sea was a Tertiary 

remnant of the once-extensive Cretaceous Western Interior sea 

that soon disappeared because of infi lling by sediment (Pye, 

1958) or because of slight uplift that drained it.
35

The mammalian fauna of the Denver Formation is 

diverse, including multituberculates, proteutheres, “condylarths,” and a marsupial (Dewar, 1996, 1997). Dewar’s (1996) 

paleoenvironmental study of the Denver Formation indicated 

a subhumid to humid environment with only moderately low 

minimum temperatures. The presence of lignite and coal 

in this sequence, not only in the Denver-Julesburg Basin 

but also in the Powder River Basin of eastern Wyoming, in 

North Park, and in southern Colorado, indicates a warm, wet 

climate that promoted the lush growth of vegetation (Robinson, 1972; Montagne, 1991). Current and ongoing research 

in the Denver-Julesburg Basin is adding substantially to our 

understanding of the Paleocene climate in this region (Barclay, 

2001; Dunn, 2001; Eberle, 2001; Ellis, 2001; Fleming, 2001; 

Johnson, 2001; Nichols, 2001). Fossils collected in southern 

Colorado in the Poison Canyon Formation include turtle and 

Coryphodon (Hills, 1888), both aquatic animals. Fossils from 

the Fort Union include invertebrate shells, crocodiles, and 

turtles (Brown, 1907), further supporting the interpretation of 

a warm, wet climate. 

Eocene

A mid- to late-Eocene unconformity has been recognized 

in much of the Front Range area and in some areas on the 

plains (Scott, 1975; Trimble, 1980a; Seeland, 1985; Swinehart 

and others, 1985). Many geologists have inferred that the 

product of the erosion was an extensive, relatively fl at surface 

that left only the highest peaks of the Front Range standing as 

high mountains (Knight, 1953; Epis and Chapin, 1975; Scott, 

1975, 1982; Scott and Taylor, 1986; Morse, 1985; Raynolds, 

1997). This interpretation has not been without critics, however, most notably Steven and others (1997).

At one time there were no Eocene-age rocks recognized 

in eastern Colorado or in Nebraska (Robinson, 1972; Swinehart and Diffendal, 1989). However, Soister and Tschudy 

(1978) recognized an early to early middle Eocene pollen 

assemblage above a paleosol marker bed at or near the base of 

the Dawson Formation in the Denver-Julesburg Basin. Additionally, the age of the Wall Mountain Tuff is now thought to 

be late Eocene, based on 40Ar/39Ar dating (McIntosh and 

others, 1992) and revision of the Eocene-Oligocene boundary 

(Swisher and Prothro, 1990; Berggren and others, 1995). In 

northeastern Colorado, southeastern Wyoming, and western 

Nebraska, the Chadron Formation of the White River Group 

was long considered Oligocene (Swinehart and others, 1985; 

Swinehart and Diffendal, 1989). The change in the EoceneOligocene boundary now places the Chadron in the late 

Eocene (Swinehart and Diffendal, 1997). Important revisions 

in White River Group nomenclature were proposed by Terry 

(1998) and LaGarry (1998). Their proposed changes mainly 

concern internal divisions of the White River Group and the 

Chamberlain Pass Formation in northwestern Nebraska. The 

Chamberlain Pass is the basal unit of the White River Group 

and was named by Evans and Terry (1994). The unit was previously included with the Chadron Formation and is included 

with the Chadron in the present report. The lower Eocene 

Wind River Formation is recognized in the Laramie Basin area 

of southeastern Wyoming (Blackstone, 1975) along with the 

Wasatch Formation in the Powder River Basin.

The Castle Rock Conglomerate, in the southern DenverJulesburg Basin, has been considered as early Oligocene by 

most previous researchers but was considered late Eocene 

by Steven and others (1997). Although the unit has not been 

radiometrically dated, Steven and others (1997) suggest that 

this age assignment has merit because of the similarity in 

paleoslope indicated by the Castle Rock Formation compared to lower Eocene White River (Chadron) streams in 

northeastern Colorado. The Castle Rock contains a Chadron 

fauna (Morse, 1985), and because the Chadron Formation is 

now considered late Eocene, a similar age assignment seems 

appropriate for the Castle Rock. Although still considered 

as early Oligocene in age by the USGS as of this writing, it 

is discussed here as a late Eocene correlative of the Chadron 

Formation.

The Eocene part of the Dawson Formation consists of 

arkosic conglomerate, feldspathic to andesitic sandstone, and 

sandy claystone (Soister and Tschudy, 1978; Morse, 1981). 

The Dawson was deposited by high-energy streams that had 

their source in the nearby Front Range Uplift. The lower 

and upper parts of the Dawson are separated by a widespread 

interval of paleosols (Farnham, 2001). Dated strata above 

and below the paleosol zone indicate a time gap of 6 to 8 million years between the lower and upper parts of the Dawson 

(Raynolds, 2001). The Wall Mountain Tuff unconformably 

overlies the Dawson Formation in the southern part of the 

Denver-Julesburg Basin. It originated as an ash-fl ow that was 

ejected west of the Front Range and fl owed eastward in paleovalleys into the Denver-Julesburg Basin (Epis and Chapin, 

1974; Morse, 1985). The Wind River and Wasatch Formations 

are composed of arkosic sandstone and variegated mudstone 

beds that have a fl uvial origin (Blackstone, 1975). They were 

deposited in environments similar to that of the Dawson.

The Chadron Formation consists mainly of tuffaceous 

claystone, siltstone, and mudstone, and relatively minor 

amounts of fossiliferous, coarse-grained sandstone and conglomerate (Scott, 1978; Swinehart and others, 1985; Swinehart 

and Diffendal, 1997). Basal coarse-grained material (now 

assigned to the Chamberlain Pass Formation) was deposited in 

shallow channels that were incised into underlying rocks, but 

much of the upper part of the Chadron is reworked volcanic 

ash that was ejected in the western United States (Stanley, 

1976; Swinehart and Diffendal, 1989). The reconstructed 

paleodrainage of the Chadron indicates a paleoslope mainly to 

the east-southeast (fi g. 12). In South Dakota, the upper part of 

the Chadron contains extensive lacustrine beds (Evans, 1995), 

indicating a depositional surface of low relief that allowed 

for ponding of groundwater. The Castle Rock Conglomerate 

overlies either the Dawson Formation or the Wall Mountain 

Tuff, and is composed of coarse conglomerate with large clasts 

of Wall Mountain Tuff, granite, quartzite, and metamorphic 

The development of the Cenozoic Platte River drainage system
36 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

rocks (Morse, 1985). Paleocurrent analysis indicates that a 

high-energy stream deposited the Castle Rock and fl owed to 

the southeast (Morse, 1985). Morse (1985) also noted that the 

projected line of Castle Rock Conglomerate outcrops intersect 

the mountain front at the point where the South Platte River 

now leaves the mountains. He believed that the Castle Rock 

represents deposits of an ancestral South Platte River that 

fl owed to the southeast. The southeastward-oriented channels of the Chadron in northeastern Colorado (Scott, 1982; 

fi g. 12) contain a pebble suite different than that of the Castle 

Rock, and are believed to have been part of a different (but 

parallel) stream system. The two channels that join east of the 

mountain uplift in southeastern Wyoming were thought to be 

the ancestral Laramie River (south channel segment) and the 

ancestral North Platte River (north channel segment) by Clark 

(1975). Although these channels were discussed by Clark 

(1975) in the context of the Oligocene, they are from basal 

strata of the White River Group (Chadron), which is now considered to be Eocene.

Soister and Tschudy (1978) reported a palynomorph 

assemblage consisting of deciduous trees and shrubs and grass 

in the Dawson Formation, and Scott (1978) mentioned bones 

and teeth of fossil vertebrates (unspecifi ed) in basal beds of 

the Chadron Formation in northeastern Colorado. Bjorklund 

and Brown (1957) reported fragmentary remains of a titanothere, a large herbivorous mammal, from the Chadron near 

Sterling, Colorado, and Morse (1985) reported fi nding tortoise 

bone, ungulate, and titanothere fragments in the Castle Rock 

Conglomerate, as well as rhinoceros and other mammals 

found by others. Lovering (1929) listed upland and aquatic 

rhinoceros, horses, peccaries, deer, and camels as fauna of the 

Chadron. Just to the north of the study area, in southwestern 

South Dakota, Retallack (1983) reported fossil root traces 

that he attributed to a woodland ecosystem. Retallack (1983) 

Figure 12. Paleogeography in Eocene time. Uplifts in red cross-hatched pattern and basins in brown modifi ed from Robinson (1972), 

streams in green in Wyoming, Nebraska, and Colorado modifi ed from Clark (1975), streams in red in Wyoming and Colorado modifi ed from 

Seeland (1985), area of Chadron Formation modifi ed from Swinehart and others (1985), streams in blue and outline of Wall Mountain Tuff 

modifi ed from Chapin and Kelley (1997), and streams in magenta in Colorado modifi ed from Steven and others (1997). Cities and mapped 

quadrangles are as shown on fi gure 1.

108° 106° 104° 102° 100° 98° 96° 94°

38°

40°

42°

0 100 200 50 Miles

0 150 300 75 Kilometers

Chadron Formation

Upper

Dawson

Formation

Castle Rock

Conglomerate

Minimum original extent of

Wall Mountain Tuff (dashed line)

WYOMING

COLORADO

NEBRASKA

KANSAS

Wind

River

Formation

Wasatch

Formation
37

interpreted the climate during deposition of the Chadron as 

humid to subtropical. Evanoff (1991) used fossil land snails 

to interpret a subhumid climate for Chadron-age rocks in eastcentral Wyoming, and Evans (1995) noted fi sh, turtle, and 

abundant invertebrates in upper Chadron beds in southwestern 

South Dakota. In southwestern Wyoming and northwestern 

Colorado fossils, such as fl amingos, crocodiles, palms, and 

magnolias, in the Eocene Green River Formation indicate a 

subtropical environment at this time (Mears, 1993). Chapin 

and Kelley (1997) noted that the early Eocene was one of the 

Earth’s warmest time intervals, and began with a substantial 

warming of the oceans and a major extinction of deep benthic 

organisms.

Discussions of Eocene climate have been part of the 

debate about the development of one or more erosion surfaces 

in the southern Rocky Mountains and adjacent Great Plains. 

As few as one and as many as eleven surfaces have been recognized in the Front Range and Laramie Range since geologists began examining those areas (summarized by Bradley, 

1987). Many of the interpretations have concluded that a 

major erosion surface developed in late Eocene time (Epis and 

Chapin, 1975; Scott, 1975). A typical view of this surface, 

west of Denver, is shown in fi gure 13. The assignment of a late 

Eocene age for this surface was in part based on the presumed 

Oligocene age of the immediately overlying Wall Mountain 

Tuff (now dated at 36.7 m.y., late Eocene) (Chapin and Kelley, 

1997), and on the age of the Florissant Lake Beds (34 m.y.) 

and the Antero Formation (33 m.y.) (Epis and Chapin, 1975). 

Given that the age of the Eocene-Oligocene boundary is now 

thought to be approximately 33.7 m.y. (Berggren and others, 

1995), the earlier chronology must be revised slightly.

Many authors (Epis and Chapin, 1975; Trimble, 1980a; 

Morse, 1985; Leonard and Langford, 1994) have noted that the 

Wall Mountain Tuff spread eastward from a source west of the 

Front Range (fi g. 12), indicating that Front Range topography 

must have been subdued during this event, at least in areas 

where the tuff was deposited. In the northern part of the Front 

Range, Bolyard (1997) described an eastward-dipping surface 

as a pediment at grade with streams that fl owed onto the Great 

Plains. In the southern Denver-Julesburg Basin, the Wall 

Mountain Tuff overlies the Dawson Formation, which has been 

dated as early to lower middle Eocene by Soister and Tschudy 

(1978). The age of the Wall Mountain Tuff is now thought to 

be late Eocene, on the basis of its isotopic age (McIntosh and 

others, 1992). These dated deposits indicate a probable middle 

Eocene age for the erosion surface in the southern DenverJulesburg Basin, an age also accepted by Chapin and Kelley 

(1997). This surface was traced over much of the Front Range, 

and was thought to be roughly contemporaneous over all of 

that area and into Wyoming (Scott, 1975). It should be noted 

that the paleosol zone within the Dawson did not form at the 

same time as the Eocene erosion surface. The paleosols lie 

between the D1 and D2 synorogenic sequences of Raynolds 

(1997), whereas the Eocene erosion surface is at the top of the 

Dawson Formation (top of D2 sequence), beneath either the 

Wall Mountain Tuff or the Castle Rock Conglomerate.

MacGinitie (1953) concluded that the erosion surface had 

formed at low altitude, on the basis of comparisons of fossils 

Eocene erosion

surface

Figure 13. Photo of middle Eocene erosion surface (tree-covered hills on left-hand side of photo) in the Front 

Range. View is northeast from Green Mountain, on west side of Denver. Snow-covered peaks in far background 

are on the present Continental Divide. Interstate 70 is in the foreground.

The development of the Cenozoic Platte River drainage system
38 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

in the Florissant Lake Beds with modern equivalents. However, he was not aware of the major warm global temperature 

that is now known to have characterized the Eocene. MacGinitie’s interpretation of low altitude led some workers to assume 

that this was evidence that the surface was essentially a peneplain at nearly sea level that was later uplifted to its present 

altitude (summarized by Bradley, 1987). Later, reinterpretation of the Florissant fossils led others (Meyer, 1992; Gregory, 

1992; Gregory and Chase, 1992) to conclude that the erosion 

surface formed at elevations higher than 8,000 ft. Morphological changes in the fl ora of the upper Dawson also suggest 

an increase in elevation (Johnson, 2001). Chapin and Kelley 

(1997) summarized the paleobotanical arguments for development of a high-altitude erosion surface. The signifi cance 

of these fi ndings is that they indicate that the climate in the 

Eocene was quite warm—warm enough that plants normally 

found at low altitudes in today’s climate were growing at 

much higher altitudes in the Eocene. Beginning in late Eocene 

time, the climate turned very much cooler, possibly refl ecting 

the increased volcanic activity and associated ash clouds that 

were spread eastward from the western United States (Robinson, 1972; Chapin and Kelley, 1997).

Various aspects of the above interpretations have been 

disputed by some. Evanoff (1991) disputed the concept of 

a single, Eocene-age, low-relief erosion surface that extends 

from central Colorado to southern Wyoming. He found that 

deep paleovalleys fi lled with White River strata, including 

coarse basal conglomerates, indicated mountainous topography in pre-White River (middle Eocene) time. Because 

the subsumit erosion surface truncates White River strata in 

the paleovalleys, Evanoff (1991) concluded that the erosion 

surface in the Laramie Range is of Miocene or even Pliocene 

age. Mears (1993) also accepted the idea that topography in 

much of Wyoming was mountainous in the Eocene, but that 

the basin areas were near sea level, in sub-tropical conditions. 

He thought that the mountains were gradually buried by airborne pyroclastic debris in the Oligocene and through much 

of the Miocene, and that the widespread erosion surface on the 

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Figure 14. Paleogeography in Oligocene time. Uplifts in red cross-hatched pattern; areas of Gering Formation, Rabbit Ears Volcanics, Thirtynine Mile Volcanics, and possible former extent of Oligocene rocks modifi ed from Robinson (1972), streams in blue modifi ed from Scott 

(1975), and streams in red modifi ed from Swinehart and others (1985). Cities and mapped quadrangles are as shown on fi gure 1.
39

Laramie Mountains is mid-Miocene in age. Montagne (1991) 

noted that deep canyons were cut in the uplands adjacent to 

the Saratoga Valley that produced several thousand feet of 

local relief. Blackstone (1975) considered the erosion surface 

to have formed during deposition of the upper Ogallala Group, 

now thought to be of late Miocene age.

Steven and others (1997) did not believe that the erosion 

surface in Colorado was of low topographic relief, that the 

surface is entirely Eocene in age, or that it formed at a high 

elevation. Steven and others (1997) maintained that the surface had variable topography, ranging from very low to mountainous. Their interpretation was that Eocene topography was 

essentially a result of Laramide uplift and was similar to that 

in existence today—ranging from plains east of the mountain 

front, through a foothills area having moderate relief, to high 

peaks along the crest of the Front Range. Steven and others 

(1997) contended that the surface recognized by Epis and 

Chapin (1975) and Scott (1975) in the foothills was just one 

element of the Eocene landscape. Additionally, the Rampart 

surface, just northwest of Colorado Springs, considered one 

of the best-preserved examples of the Eocene erosion surface 

by Scott (1975), and used as the basis for arguing against postLaramide uplift of the Front Range by Leonard and Langford 

(1994), was thought by Steven and others (1997) to be of 

middle to late Miocene age. Steven and others (1997) also 

maintained that regional base level remained stable from the 

late Eocene to the late Miocene, indicating to them that the 

region was at or near sea level for most of that time. Their 

conclusions require that extensive late Cenozoic mountain 

uplift relative to the plains, and downwarping of the Great 

Plains, has created much of the topographic relief between the 

mountains and plains that exists today.

These examples of differing views show that interpretations of Eocene paleogeography and climate cover a wide 

spectrum. Research on this topic is ongoing, but fundamentally different interpretations of the same data make resolution 

of some of the questions unlikely in the near future.

Oligocene

The close of the Eocene Epoch and the beginning of 

the Oligocene saw the study area inundated by volcanic ash. 

Volcanoes in Colorado and in a belt from central Utah to 

Washington State (but mainly from Utah) were the probable 

sources, and they ejected large volumes of ash that were carried eastward by prevailing winds and reworked by streams 

and rivers in the southern Rocky Mountains and western 

Great Plains (Stanley, 1976; Seeland, 1985; Swinehart and 

others, 1985).

The Chadron Formation of the White River Group was 

long considered to be the basal Oligocene unit on the Great 

Plains but is now considered to be late Eocene (Swinehart 

and Diffendal, 1997). The overlying Brule Formation of the 

White River Group is now the basal (early) Oligocene formation on the Plains. In parts of southeastern Wyoming and 

northeastern Colorado the White River Formation (Montagne, 

1991; Izett, 1975; Scott, 1978) is a unit of presumed Eocene 

and Oligocene age. The Arikaree Formation or Group overlies 

the White River Formation in Wyoming east of the Laramie 

Mountains, in western Nebraska, and in northeastern Colorado. In western Nebraska, the Arikaree includes both Oligocene- and Miocene-age units. The basal Gering Formation 

and overlying Monroe Creek Formation of the Arikaree Group 

are considered upper Oligocene (Swinehart and Diffendal, 

1995; 1997). Figure 14 shows that Oligocene units may have 

once been present over much of northeastern Colorado (Robinson, 1972), but were later removed by erosion. Although 

it is still considered of lower Oligocene age by the USGS, 

Steven and others (1997) considered the Castle Rock a timeequivalent to the late Eocene age Chadron Formation, and the 

Castle Rock is discussed in the Eocene section of this report.

The Brule Formation is composed of volcaniclastic silty 

sandstone, sandy siltstone, and mudstone. It contains several 

ash beds that can be correlated regionally and are useful as 

marker beds (Swinehart and Diffendal, 1995; 1997). Much 

of the Brule is composed of eolian pyroclastic strata, consisting of as much as 60 percent glass shards. Relatively minor 

lacustrine deposits are present locally. The rate of ash deposition increased through the duration of the White River Group, 

and the upper part of the group consists mostly of volcaniclastic material and only minor fl uvial strata. During this time, 

the mountains were slowly buried by onlap of sediments, 

and the thickness of Oligocene strata on the Great Plains was 

increasing as the area slowly subsided under the infl ux of 

fl uvial and air-fall sediments. Stanley (1976) noted that the 

mountains were not buried in their own erosional debris, but 

rather were buried by airfall volcanic material. Small areas of 

the mountain cores remained exposed and provided igneous 

and metamorphic clasts that were carried eastward by lowgradient streams over a fl at plain of volcaniclastic-derived 

eolian material.

A hiatus in deposition occurred between the White River 

Group and the Arikaree Group, during which time the area 

was subjected to renewed uplift and erosion and the formation of a regional unconformity (Swinehart and others, 1985). 

The Gering Formation or undivided Arikaree Formation fi lled 

channels cut into the erosion surface and are composed of 

volcaniclastics, sandstone, silty sandstone, and sandy siltstone, 

and locally are composed of conglomerate, marl, and ash beds 

(Scott, 1978; Swinehart and Diffendal, 1995; 1997). Most of 

the formation consists of volcaniclastic sediments that were 

reworked by fl uvial processes, and local lacustrine beds. The 

overlying Monroe Creek Formation is composed of volcaniclastic conglomerate, sandstone, and ash beds. The unit 

is mainly of eolian origin, but most eolian strata have been 

reworked by streams. Swinehart and Diffendal (1989) noted 

that the Arikaree Group is more areally restricted than the 

White River Group, and is largely confi ned to paleovalleys in 

the eastern part of its extent in the subsurface. They thought 

that in much of the Sand Hills area the undivided Arikaree 

Formation is equivalent to the Gering Formation.

The development of the Cenozoic Platte River drainage system
40 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

Detailed Oligocene drainage maps have been reconstructed by Scott (1975) in the Front Range of Colorado (fi g. 

14). These were augmented by a more general drainage map 

by Swinehart and others (1985) in southeastern Wyoming and 

western Nebraska. Robinson (1972) showed that there was 

a slight change in highland areas of the Rocky Mountains 

compared to the Eocene (fi g. 12), but the basic confi guration 

of source areas remained the same. In contrast to the southeast-fl owing streams of the late Eocene in Colorado, drainage 

in the Oligocene was more to the east. In the early Oligocene, headwaters of most streams were in the Front Range 

or Laramie Range, but by the late Oligocene headwaters of a 

few streams were in the North Park or western Front Range 

areas (Stanley, 1976). Drainage in the North Park area was to 

the south in the early to middle Oligocene (Montagne, 1991). 

Izett (1975) thought that much of the plains of eastern Colorado were structurally high in the late Oligocene. He reasoned 

that only the upper part (early Miocene) of the Arikaree Group 

is preserved in that area; if the whole group had been deposited and then eroded, late Oligocene rocks at the base of the 

group (Gering) would have been preserved. The absence of 

rocks of Gering age in northeastern Colorado argues for nondeposition of the lower Arikaree there.

The climate of the Oligocene remained temperate, 

although drier and cooler than during late Eocene Chadron 

time. Fossils in the Brule Formation within the study area 

include invertebrates, turtles, and mammals, of which oreodonts are the most numerous taxa (Swinehart and Diffendal, 

1997). Scott (1982) reported two genera of oreodonts in the 

Arikaree Group of northeastern Colorado. Lovering (1929) 

noted that hackberry seeds are particularly abundant in some 

beds. Lovering (1929) also noted that Oligocene fauna indicated more arid conditions than in the Eocene. Stanley (1976) 

also interpreted the climate as semiarid during this time, based 

on the abundance of eolian strata. Paleoenvironmental data 

from Oligocene land snails and sedimentological changes 

from fl uvial to eolian environments of deposition led Evanoff 

(1991; 1995) to interpret the environment as semiarid. Mears 

(1993) interpreted the ecosystem as a savanna in Oligocene 

time. Clark (1975) viewed the climate as progressively 

cooling from the Eocene onward, culminating in glaciation 

during the Pleistocene. Indeed, Prothero and Heaton (1996) 

described a profound climate “crash” in the transition from the 

Eocene to the Oligocene. They considered the change in the 

early Oligocene as the most fundamental in the last 65 million years, citing evidence for a dramatic drop in mean global 

temperatures, an increase in seasonal temperature variation, 

a change in vegetation from dense woodland to mixed woodland-grassland, and a shift from fl uvial to eolian depositional 

environments interpreted from the White River Group. Surprisingly, the Oligocene Orellan mammalian fauna showed 

remarkably little change through this climatic change (Prothero and Heaton, 1996). Retallack (1997) presented evidence 

that grassland in the early Oligocene was composed of bunch 

grasses and shrubs in some areas. This environment is similar 

to that of the current intermontane west.

Miocene

The transition from the Oligocene to Miocene Epochs 

occurs within the Arikaree Group in Nebraska (Swinehart 

and Diffendal, 1995; 1997; Swinehart and others, 1985). In 

southwestern Nebraska, the Harrison Formation is the oldest 

Miocene formation recognized; younger informal units are 

also present. Lugn (1939a,b) and Skinner and others (1977) 

described a unit above the Harrison called the Hemingford 

Group. This unit name was not used by Swinehart and 

others (1985) or by Swinehart and Diffendal (1989). The 

Hemingford, or upper Harrison beds, is included with the 

Arikaree Group in this report. In northeastern Colorado the 

undivided Arikaree Formation was mapped (Scott, 1978), 

where it was considered Miocene in age. In north-central 

Colorado, the Browns Park and North Park Formations are 

recognized Miocene units (Izett, 1975) and the Browns 

Park is also recognized in adjacent south-central Wyoming 

(Montagne, 1991). In southeastern Wyoming, the undivided 

Arikaree Formation is recognized (Flanagan and Montagne, 

1993), where it is considered transitional between the Oligocene and Miocene Epochs. Unconformably overlying the 

Arikaree is the Ogallala Formation or Group. In Nebraska, 

the Ogallala has been divided into many constituent formations, members, and informal units (Swinehart and Diffendal, 1995; 1997; Swinehart and others, 1985). In Colorado, 

Scott recognized upper and lower parts of the Ogallala Formation, which correspond, in part, to the Nebraska units. In 

southeastern Wyoming, the undivided Ogallala Formation 

is identifi ed (Flanagan and Montagne, 1993), but informal 

subdivisions are recognized that are equivalent to Nebraska 

units. The Browns Park and North Park Formations are 

partly equivalent to both the Arikaree and the Ogallala 

(Izett, 1975).

The lower Miocene Harrison Formation conformably 

overlies and is similar in lithology to the underlying Monroe 

Creek Formation in southwestern Nebraska, consisting of 

fi ne grained silty sandstones (Swinehart and others, 1985). 

Large calcium carbonate concretions are characteristic features in many areas, and silica-cemented beds are locally 

present. The upper Harrison beds of Swinehart and others 

(1985), still considered lower Miocene, unconformably 

overlie the Monroe Creek-Harrison Formation interval and 

include some local coarse grained sandstones at the base. In 

Colorado, Scott (1978) described the Arikaree Formation as 

conglomerate, sandstone, siltstone, and claystone, but some 

of that unit may be equivalent to the Oligocene part of the 

Arikaree recognized in Nebraska. In southeastern Wyoming 

the Arikaree was described as consisting of fi ne- to mediumgrained sandstone, limestone, and ash beds (Flanagan and 

Montagne, 1993). Calcareous concretions are also characteristic of the Wyoming exposures. The Arikaree units in all 

areas were deposited in mixed volcaniclastic eolian, fl uvial, 

and lacustrine environments that represent a continuation of 

most conditions that existed in late Oligocene time, although 

with a lower percentage of fl uvial environments.
41

The middle to upper Miocene Ogallala Group, which 

extends from South Dakota to Texas, is a complex unit that has 

been the subject of much study over the years, especially in 

Nebraska. Lugn (1939a) and Diffendal (1984) presented overviews of the earliest studies of the Ogallala. For this report, 

the lithology of the Ogallala is generalized, without separate 

descriptions of all the formations, members, and informal 

units that are recognized in Nebraska. In western Nebraska, 

the Ogallala is composed of conglomerate, sandstone, and 

siltstone at the base, which grades up into poorly consolidated 

gravel beds, cobble to pebble conglomerate, sandstone, and 

siltstone and some diatomite and ash beds (Swinehart and Diffendal, 1995; 1997). Farther east in Nebraska, the Ogallala 

includes a higher percentage of fi ner strata of silt, clay, and 

limestone intermixed with the coarser lithofacies (Dreeszen 

and others, 1973; Eversoll and others, 1988; Swinehart and 

Diffendal, 1989; Diffendal, 1991; Souders, 2000).

In Colorado, the lower part of the Ogallala is composed 

of semiconsolidated ashy sand and silt beds and volcanic ash 

beds that grade upward into poorly consolidated gravel beds 

containing clasts of granitic, sedimentary, and volcanic rocks 

(Scott, 1978). Coarse gravel is most abundant in the middle 

of the unit in northeastern Colorado (Bjorklund and Brown, 

1957). The upper part of the Ogallala in northeastern Colorado consists of one or more calcite-cemented caliche layers or 

limestone beds that form a caprock.

In southeastern Wyoming, the Ogallala is composed of 

boulder to gravel conglomerate just east of the Laramie Range, 

and grades eastward to sandstone, siltstone, limestone, and ash 

beds (Flanagan and Montagne, 1993). Stanley (1976) identifi ed several unique petrofacies in Tertiary beds of the High 

Plains and mapped the distribution of these petrofacies in the 

Ogallala. Most of the Ogallala east of the Laramie Range is 

composed of granitic clasts, with metamorphic and rhyolite 

clasts occurring in about equal abundance, and anorthosite 

clasts occurring in some outcrops. Over much of the area 

discussed in this report, and elsewhere, the upper part of the 

Ogallala is characterized by “mortar beds” (Haworth, 1897), 

sandstone beds that are fi rmly cemented with calcium carbonate and which stand out in bold relief capping escarpments.

The volcanic eruptions that dominated Oligocene and early 

Miocene sedimentation in the study area apparently ceased in 

the middle Miocene, and the dominant processes became erosion resulting from renewed faulting and tectonic activity in 

the Rockies (Izett, 1975; Scott, 1975; Epis and others, 1976; 

Mears, 1993; Flanagan and Montagne, 1993). The erosion 

caused an unconformable surface to develop on underlying 

rocks (Swinehart and others, 1985). As noted above, some 

authors believe that this period of erosion resulted in planation 

of some of the Front Range, Laramie Range, and Medicine 

Bow Mountains (Blackstone, 1975; Mears, 1993; Steven and 

others, 1997). Erosion was accompanied by infi lling of the 

basins adjacent to the mountains. In the middle Miocene, sediment loading on the Great Plains caused subsidence, which 

created accommodation space and led to accumulation of the 

Ogallala Group (Lugn and Lugn, 1956). The Ogallala fi rst 

backfi lled the paleochannels incised in underlying rocks, and 

then spread laterally in a series of fi ning-upward cut and fi ll 

sequences. Diffendal (1982) identifi ed at least two main episodes of erosion and deposition associated with the Ogallala 

in western Nebraska, one pre-Ogallala and one intra-Ogallala. Goodwin and Diffendal (1987) interpreted the Ogallala 

as a braided stream deposit, based on stratifi cation types and 

lithofacies. In some areas the Ogallala may have been deposited on alluvial fans or on low-relief alluvial plains (Johnson, 

1901; Scott, 1982; Swinehart and Diffendal, 1989; Flanagan 

and Montagne, 1993), and some ash beds were interpreted as 

having been deposited in lacustrine environments (Diffendal, 

1982). A variety of other depositional environments have also 

been proposed for the Ogallala, summarized by Helland and 

Diffendal (1993). The end result of the uplift, erosion, and 

sedimentation at this time was an eastward-sloping wedge 

of coarse fl uvial sediments deposited by streams that were at 

grade with the subsumit surface of the nearly buried mountainous highlands (Pearl, 1971; Blackstone, 1975).

The Browns Park Formation is widely distributed in northcentral and northwestern Colorado and in the Saratoga Valley 

of southeastern Wyoming within the area considered in this 

report. In the Saratoga Valley the Browns Park includes, at 

the top, strata equivalent to the North Park Formation that is 

recognized in Colorado (Montagne, 1991). The Browns Park 

unconformably overlies various other rock units. The lower 

part is composed of conglomerate and sandstone, with clasts 

of metamorphic, sedimentary, volcanic, and plutonic rocks. 

Overlying the basal conglomerate is a sequence of pastel sandstone, siltstone, and ash beds that are capped by white chalcedonic and algal lacustrine limestone beds. The upper part of 

the Browns Park in the Saratoga Valley consists of tuffaceous 

sandstone and siltstone beds and ash beds (Montagne, 1991).

The North Park Formation overlies the White River Formation in North Park and is composed mainly of gray shale 

and sandstone. It contains several hard white calcareous 

sandstone and marly layers and is characterized by the presence of volcanic material. Shale beds in the North Park contain pinkish and gray ashy streaks and thin laminae of black 

“cinder.” Sandstone beds are usually fi ne grained but are 

locally coarse grained. Conglomerate composed of rounded 

basalt pebbles is irregularly distributed in the North Park 

(Beekly, 1915). The North Park and Browns Park Formations 

were also deposited primarily in fl uvial environments in the 

middle and upper Miocene. Montagne (1991) identifi ed a 

“Platte Channel facies” in the upper part of the Browns Park 

that consists of conglomerate and sandstone having distinctive pebble suites of volcaniclastic rocks carried northward 

from Colorado. This unit was thought to represent deposits 

of the ancestral North Platte River. Izett (1975) noted that 

volcanic clasts from northern Colorado were carried north 

and northeastward by both ancestral North Platte and Laramie 

rivers in the Miocene.

Data have been compiled to show the paleogeography of 

the study area in Miocene time (fi g. 15). The distribution of 

the Arikaree is not shown on this map because the Harrison 

The development of the Cenozoic Platte River drainage system
42 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

and equivalent units are combined with the Oligocene part of 

the Arikaree (see fi g. 14 for the distribution of the Arikaree 

Group). As noted above, Swinehart and Diffendal (1989) 

thought that most of the Arikaree underlying the Sand Hills is 

equivalent to the Oligocene Gering Formation. The locations 

of Miocene (Ogallala) streams have been compiled from various sources, and show a general easterly trend. Drainage patterns shown by Blackstone (1975) and Swinehart and others 

(1985) suggest a northern Front Range source for the Ogallala 

in southeastern Wyoming, northeastern Colorado, northwestern Kansas, and Nebraska. The North Platte drainage was 

diverted northward in the earliest Miocene (Montagne, 1991). 

Steven and others (1997) pointed out that the east-northeast 

paleoslope in Colorado was at nearly right angles to the southeast paleoslope in the late Eocene. They took this change to 

indicate regional structural tilting to the northeast in middle 

Miocene time. The Ogallala in east-central and southeastern 

Colorado and in much of Kansas probably had a source in 

the central to southern Front Range, as illustrated by Pearl 

(1971), Scott (1975), and Morse (1985). Upland source areas 

are smaller than in previous times, refl ecting the burial that 

occurred in the Oligocene and early Miocene. The areas of 

Miocene volcanics in North Park and the western Front Range 

are important, because rhyolite clasts from these outcrops have 

been identifi ed in the Ogallala on the east side of the Front 

Range and Laramie Range (Blackstone, 1975; Stanley, 1976). 

The presence of these clasts in eastern Wyoming, northeastern 

Colorado, and western Nebraska indicates that the Laramie 

Basin was fi lled with sediment, and that the headwaters of the 

drainage system were in the North Park area for some of Ogallala time (Blackstone, 1975; Flanagan and Montagne, 1993). 

At the close of deposition of the Ogallala, the central Great 

Plains consisted of a vast tableland of gently eastward-dipping 

sediment (Pearl, 1971). Parts of this surface are preserved 

Figure 15. Paleogeography in Miocene time. Uplifts in red cross-hatched pattern modifi ed from Robinson (1972), stream in green modifi ed

from Pearl (1971), streams in black modifi ed from Blackstone (1975), streams in red modifi ed from Steven and others (1997), and streams 

in blue modifi ed from Swinehart and others (1985) and Swinehart and Diffendal (1989). Outline of the Ogallala Group is partially from 

the Groundwater Atlas of the United States (http://www.nationalatlas.gov/aquifersm.html). The eastern extent of the Ogallala Group in 

Nebraska was modifi ed by the author from the outline of the High Plains Aquifer in the National Atlas, using the bedrock geologic map of 

Nebraska (Burchett and Pabian, 1991). Cities and mapped quadrangles are as shown on fi gure 1.

108° 106° 104° 102° 100° 98° 96° 94°

38°

40°

42°

0 100 50 Miles

0 100 150 200 50 Kilometers

Ogallala Group

Ogallala Group

WYOMING

COLORADO

NEBRASKA

KANSAS

Miocene

volcanics
43

today in northeastern Colorado and southwestern Kansas 

(Johnson, 1901), and at the Gangplank, west of Cheyenne, 

Wyoming (fi g. 1). The latter part of the Miocene (from about 

10-12 m.y. ago) marks the beginning of the present-day drainage system (Izett, 1975).

Fossils are abundant in Miocene rocks in the study area. 

A comprehensive list of mammals from the Ogallala was published by Schultz and Stout (1948). Swinehart and Diffendal 

(1995; 1997) reported horses, camels, tortoises, rhinoceroses, 

carnivores, antelope, deer, rodents, invertebrates, and various 

plant fossils in southwestern Nebraska. Very fossiliferous 

beds from the upper part of the Ogallala in western Nebraska 

were described by Leite (1990) and have a diverse mammalian fauna. Leite’s analysis of the fauna indicated that the 

environment changed in the Miocene of Nebraska from lush 

savanna to more arid steppe. A possible cause of the extinction of some large mammals at this time was climatic cooling 

and drying, which changed the vegetation available to the 

animals.

Fossils from the North Park Formation are similar to those 

in the upper Arikaree and Ogallala: horse, oreodont, beaver, 

camel, and mastodon (Izett, 1975). Montagne (1991) reported 

oreodonts, prongbucks, turtles, rabbits, gophers, merycodonts, 

beaver, camel, rhinoceros, horse, carnivores, mastodon, invertebrates, wood, and pollen (ponderosa pine, pinyon pine, and 

juniper) from the Browns Park Formation. He interpreted the 

climate as warm and semiarid and characterized by violent 

showers on the basis of the fossil assemblage and the presence 

of conglomeratic beds that required high-energy stream transport. Flanagan and Montagne (1993) concluded that faunal 

evidence indicated increasing aridity throughout the Miocene 

and into the Pliocene and that a change in fl ora from deciduous trees to alpine vegetation in some areas also indicates 

that the climate was cooling. Data from paleosols (Retallack, 

1997) indicates a change from bunch grasslands in the Oligocene to sod-forming short-grass prairie in the Miocene. Retallack (1997) interpreted the Great Plains to have received less 

than 16 inches of precipitation annually in the early Miocene. 

In the late Miocene, increased precipitation led to the development of tall-grass prairie and an overall expansion of grasslands (Retallack, 1997).

Pliocene

Although the modern drainage system has its origins in 

the middle Miocene (Izett, 1975), the beginning of the Pliocene Epoch, at about 5 Ma, can be considered the approximate 

point in time from which the present landscape of the southern 

Rocky Mountains and the western Great Plains has evolved. 

The close of the Miocene saw the area foundering in its own 

sedimentary debris, with only the highest mountain cores left 

standing in relief (Knight, 1953; Flanagan and Montagne, 

1993). Deposition decreased or possibly ceased over the area, 

resulting in an unconformity at the top of the Ogallala Group 

and removal of the unit in east-central Nebraska.

In the middle Miocene, the entire area from the Colorado 

Plateau to Wyoming and east to the Great Plains slowly began 

to rise (Morrison 1987; Steven and others, 1997; McMillan 

and others, 2000). As the mountain cores gained elevation in 

the Pliocene, the erosional debris that had buried them began 

to be stripped away. Much of this material was redeposited 

on top of the Ogallala Group or along steams incised into the 

Ogallala on the Great Plains as the streams lost competency. 

As regional uplift continued, these deposits were partially 

remobilized and moved farther away from the areas of highest elevation. In the Colorado part of the study area nearly 

all Pliocene-age deposits have been removed and transported 

down the depositional slope.

In contrast, Nebraska became the site of deposition of 

much of the reworked gravels and other fi ner-grained material. 

Steven and others (1997) indicated that the region was tilted to 

the northeast, shifting the locus of deposition to western and 

central Nebraska. The Broadwater Formation (originally considered early Pleistocene age) was recognized and described 

by Schultz and Stout (1945) in western Nebraska. As a named 

unit the Broadwater is mainly restricted to western and northern 

Nebraska, but equivalent units are probably present elsewhere. 

In the North Platte 1° × 2° quadrangle, Diffendal (1991) recognized an older alluvium unit, of Pliocene and Pleistocene age, 

which was said to be partially equivalent to the Broadwater 

Formation. In the Broken Bow 1° × 2° quadrangle, Souders 

(2000) recognized Pliocene-age loess and alluvial units that 

were considered equivalent to the Fullerton and Broadwater 

Formations. The Fullerton has traditionally been considered 

early Pleistocene age (Reed and Dreeszen, 1965; Dreeszen, 

1970), but volcanic ash dated at 2.3 Ma in the upper part of the 

unit (Boellstorff, 1978) indicates that the Fullerton should now 

be considered Pliocene (May and others, 1995). An unnamed 

unit and the Belleville Formation are recognized Pliocene to 

pre-Illinoian units along the Republican River in southern 

Nebraska (Swinehart and others, 1994). Swinehart and others 

(1994) also mapped a Pliocene to lower Pleistocene alluvial 

unit in much of central Nebraska (including the study area) that 

contains equivalents of the Broadwater Formation. It should 

be noted that distinguishing upper Pliocene strata from lower 

Pleistocene strata is diffi cult to impossible in many places. The 

lithology is similar, and the age difference between units is 

commonly based on fossil assemblages, which are not everywhere present. There appears to have been a gradual transition 

from Pliocene to Pleistocene sedimentation in the study area.

In Colorado, Scott (1963) recognized the Nussbaum 

Alluvium (also originally considered early Pleistocene age), 

which was a product of Pliocene erosion of older units. This 

unit is mainly found in the southern Colorado Piedmont region 

(fi g. 1), but remnants are also in northeastern Colorado (Scott, 

1975; 1982). The distribution of the Broadwater Formation 

in western and northern Nebraska (Swinehart and Diffendal, 

1997) suggests that equivalent strata should be present along 

the North Platte drainage, or possibly other areas, in eastern 

Wyoming. Blackstone (1975) described several deposits 

in southeastern Wyoming that may be Pliocene in age, and 

The development of the Cenozoic Platte River drainage system
44 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

Flanagan and Montagne (1993) mentioned high-level terrace 

deposits (unnamed) of late Miocene to early Pleistocene age 

in several areas of Wyoming. In both Colorado and Wyoming, 

the Pliocene was a time of net erosion rather than deposition, 

resulting in a sparse sedimentary record.

In Nebraska, the Broadwater and equivalent formations 

consist of conglomerate, sandstone, siltstone, claystone, and 

mudstone. Schultz and Stout (1945) recognized a basal gravel 

member, a middle member composed of sand, peat, diatomaceous marl, and silt, and an upper gravel member. Swinehart 

and Diffendal (1995, 1997) mapped generally gravelly units 

in the panhandle of Nebraska. In south-central Nebraska, 

Pliocene sediments consist of gravel, sand, and less abundant amounts of silt and clay at the base, overlain by other 

units composed of silt, clay, and lesser amounts of sand and 

gravel (Souders, 2000). In northeastern Colorado, the Nussbaum Alluvium is composed of bouldery gravel, sand, and 

minor silt and clay (Scott, 1982). In Colorado, the Nussbaum 

unconformably overlies units as old as the Cretaceous Pierre 

Shale (Scott, 1963, 1982), and in western Nebraska the Broadwater unconformably overlies units as old as the Brule Formation (Swinehart and Diffendal, 1995). The Pliocene units are 

preserved there as remnants of terraces along the major present-day drainages. 

Data from various sources have been compiled to show 

reconstructed upland areas and stream patterns in Pliocene 

time (fi g. 16). Clasts in the Broadwater Formation indicate 

source areas similar to those of the Ogallala—the Medicine 

Bow and Laramie Ranges in Wyoming and the Front Range 

in Colorado. Stanley and Wayne (1972) identifi ed a distinctive clast suite that indicates a major trunk stream fl owed 

from the Laramie Range and swept from north to south across 

Nebraska, covering much of the State with a veneer of gravelly alluvium. In the Front Range, accelerated uplift resulted 

in signifi cant canyon cutting and incision of the modern drainage system (Scott, 1975). Faulting in the Front Range caused 

disruption of some of the earlier drainage courses, and the 

northeastward trend of the South Platte River was established 

£änc £äÈc £ä{c £äÓc £ääc nc Èc {c

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Figure 16. Paleogeography in Pliocene time. Uplifts in red cross-hatched pattern modifi ed from Robinson (1972), stream in black modifi ed 

from Pearl (1971), streams in magenta modifi ed from Stanley and Wayne (1972), streams in red modifi ed from Scott (1975), streams in blue 

modifi ed from Swinehart and Diffendal (1989), and streams in green and area of Broadwater Formation modifi ed from Swinehart and others 

(1994). Cities and mapped quadrangles are as shown on fi gure 1.
45

(Epis and others, 1976), possibly as a result of northward 

tectonic tilting and stream capture as described by Steven and 

others (1997). The sediment from northeastern Colorado has 

its own distinctive pebble suite, different than that of southeastern Wyoming (Swinehart and Diffendal, 1989). In southcentral Nebraska, a structure contour map representing the 

unconformable surface at the base of Pliocene strata (fi g. 7) 

shows an east-southeast-oriented drainage pattern. This region 

is just southeast of what Souders (2000) described as a major 

paleo-drainage divide, named the Cotesfi eld paleodivide, and 

is just east of the Cambridge Arch (Darton, 1918; Swinehart 

and others, 1985). These structures may have played a role 

in directing the fl ow of the ancestral Platte River southeast 

toward the Republican River drainage that is shown on fi gure 

16 and that is supported by the interpretation of subsurface 

data in this study (section II).

Pearl (1971) showed an ancestral Arikaree River fl owing 

eastward across the Great Plains from the Pikes Peak region of 

the Front Range in late Miocene to early Pliocene time. Later 

in the Pliocene the headwaters of this stream were diverted 

into the South Platte River system, possibly by stream capture 

(Pearl, 1971; Morse, 1985), cutting off the fl ow of mountainsourced water in eastern Colorado. This event was signifi cant, 

because it resulted in the erosion of the Ogallala Group and 

older rocks along the mountain front and led to the development of the Colorado Piedmont. The west-facing erosional 

scarp of the Ogallala has gradually moved eastward from 

Pliocene time to its present location, which defi nes the eastern 

edge of the Piedmont. Scott (1982) inferred that the scarp 

moved about 50 miles eastward in the 2 million years between 

the end of Ogallala deposition at about 5 Ma and deposition of 

the Nussbaum Alluvium at about 3 Ma. Since that time, the 

scarp has only moved an additional 10 miles eastward. Steven 

and others (1997) suggested that the present position of the 

scarp marks the eastern hingeline of late Cenozoic regional 

uplift of areas to the west. 

The only major streams in southeastern Wyoming and 

eastern Colorado fl owing across the Ogallala today are the 

North and South Platte and the Arkansas. This confi guration 

has preserved the post-Ogallala erosion surface in parts of 

eastern Colorado and western Kansas with minor stream modifi cation (Johnson, 1901). The effects of this isolation of the 

Plains is shown on fi gure 1. On the Arkansas River in central 

Kansas, the elevation at Great Bend is 1,850 ft. Along a northsouth transect, perpendicular to the depositional slope, the 

elevation along the Smoky Hill River at Russell, Kansas drops 

to 1,726 ft, and it further drops to 1,520 ft along the South 

Fork of the Solomon River at Osborne, Kansas. The elevation 

rises to 1,800 ft at Franklin, Neb. along the Republican River 

and is at 2,150 ft at Kearney, Neb., along the Platte River, a 

rise of 350 ft in just over 40 miles. The difference between the 

Arkansas and the Platte and the other listed streams that causes 

these marked variations in elevation is that the Arkansas and 

Platte head in the mountains and have been net aggradational 

streams, or have had lower rates of incision, since the Pliocene. In contrast, the Smoky Hill, Solomon, and Republican 

rivers all head on the Ogallala surface east of the Colorado 

Piedmont, carry a smaller sediment load, and are incising at a 

higher rate than the Arkansas or Platte. These differences are 

a direct result of developments in the drainage system initiated 

in Pliocene time and continued to the present.

In addition to fl uvial deposition of gravels, eolian deposition also occurred in the Pliocene. The Fullerton Formation 

of south-central Nebraska is primarily an eolian silt, or loess 

(May and others, 1995). The source of the loess was probably 

local stream channels and overbank material, in contrast to the 

eolian volcaniclastic deposits of older Tertiary units. Volcanism was still occurring, however, and dated volcanic ash beds 

(1.7 to 3.5 Ma) in the Fullerton have helped to verify its Pliocene age (Souders and Swinehart, 1996a,b).

The climate in Pliocene time is thought to have further 

cooled, but not necessarily dried, at least in the mountainous uplands. Ward and Carter (1999) noted that late Tertiary 

uplift of the mountains caused a rain shadow on the plains that 

increased aridity in that region and infl uenced rates of stream 

incision, but the drying of the plains may be a fairly recent 

phenomenon. Pliocene deposits contain a rich fauna including 

elephant (Stegomastodon), camel, and horse in northeastern 

Colorado (Scott, 1982). The Broadwater Formation is famous 

for its fossil content, including shrews, beavers, ground sloths, 

jackrabbits, elephants, horses, camels, amphibians, reptiles, 

dog or wolf, and saber-toothed tiger (Schultz and Stout, 1948; 

Swinehart and Diffendal, 1997). Lugn (1935) showed pictures 

of cedar and elm logs recovered from the Fullerton Formation 

in the eastern part of the study area, two of which were still 

upright in their growth positions.

Chapin and Kelley (1997) pointed out that several lines of 

evidence indicate increased precipitation in the Pliocene. These 

include (1) the establishment of the Colorado and Rio Grande 

River drainage systems through arid lands, (2) the opening of 

previously closed basins in the Rio Grande Rift and Basin and 

Range province by through-going streams, (3) erosion of thick 

sequences of Mesozoic and Cenozoic rocks from the Colorado 

Plateau, and (4) stable-isotope compositions of paleosol carbonates. Flanagan and Montagne (1993) also described the development of large river systems in the Pliocene that excavated 

Miocene and older strata from the basins of Wyoming. They 

attributed the presence of large perennial rivers to mountain glaciation, which Love and others (1973) interpreted as occurring 

at about 3 Ma in Wyoming. Morrison (1987) showed that the 

climate became increasingly unstable in the late Pliocene, leading to large fl uctuations in erosional-depositional cycles.

Pleistocene 

A list of those who have studied Pleistocene deposits 

in Nebraska would be a who’s who of Nebraska geologists, 

but a few early pioneers should be acknowledged. N.H. 

Darton, whose keen intellect was focused on many topics in 

the West, did some of the fi rst studies in Nebraska (Darton, 

1903). E.H. Barbour was the fi rst offi cial State Geologist 

The development of the Cenozoic Platte River drainage system
46 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

of Nebraska, and as such, had wide-ranging interests in the 

geology of the State (Souders, 1993). His primary interest 

was vertebrate paleontology, and he has dozens of publications on that and other topics pertaining to Nebraska geology 

published from the early 1890s to the 1940s. G.E. Condra 

succeeded Barbour as the head of geological studies in 

Nebraska in 1921. His fi rst specialty was invertebrate paleontology of the Pennsylvanian, but as Director his interests 

were wide-ranging. With his successor, E.C. Reed, Condra 

published a standard reference for Pleistocene geology 

(Condra and others, 1947). No discussion of the Quaternary 

geology of Nebraska would be complete without mentioning 

A.L. Lugn, a staff geologist with the Nebraska Geological 

Survey. Lugn published in great detail on the subject, two 

comprehensive reports among many being Lugn (1935) and 

Lugn and Wenzel (1938). Finally, a contemporary of the 

others, C.B. Schultz, had an important impact on Nebraska 

Quaternary geology. Schultz was long associated with the 

Nebraska State Museum and published extensively on vertebrate paleontology and stratigraphy. Many of the concepts 

of chronostratigraphy and terrace development in Nebraska 

were developed by Schultz and his associates.

In Nebraska, the nomenclature of Pleistocene units west 

of the glaciated area has undergone several revisions. Lugn 

(1934, 1935) laid out the fi rst comprehensive classifi cation of 

Pleistocene units in Nebraska and assigned names to several 

intervals of alternating gravels and fi ner-grained material, 

some of which are still used today. Lugn’s work incorporated early data from a systematic drilling program initiated 

in Nebraska by the Nebraska Geological Survey (now the 

Conservation and Survey Division), in cooperation with the 

U.S. Geological Survey, whose purpose was to delineate the 

extent of sediment types in the subsurface to better understand the State’s water resources. Condra and others (1947) 

modifi ed and refi ned some of Lugn’s correlations, based on 

new information obtained from the drilling program. Reed 

and Dreeszen (1965) made further refi nements based on 

information obtained from additional drilling. Subsequent 

reconsideration of some of the earlier correlations has led to 

more caution in assigning names to stratigraphic intervals. As 

Wayne and Aber (1991) noted, early names were sometimes 

applied to conceptual models as much as to lithostratigraphic 

units. Some names and provisional names have been retained, 

but awareness of the regional complexity of the Quaternary 

depositional system has led away from broad regional correlations and to more detailed local studies. Some of the more 

recent classifi cations have been published by Wayne and Aber 

(1991) and May and others (1995).

Without being overly concerned with names, test wells 

show that the sedimentary section in south-central Nebraska 

above the Ogallala Group consists of two gravel sequences 

separated and overlain by loess sequences. The thickness and 

relative abundance of these sequences differs from place to 

place, and it has been recognized that one lithology may grade 

laterally into another lithology (May and others, 1995), and 

that the lower loess sequence is not always present (Lugn, 

1935). Section II of the present report on the subsurface geology of the region combines the two lower gravel sequences 

and interbedded loess together, and separates the upper loess 

unit from the lower unit.

The Fullerton Formation, and correlative coarse clastics, 

corresponds to the lower gravel and loess unit, and is now 

considered Pliocene in age, as discussed in the previous section. A post-Fullerton gravel unit is recognized, and younger, 

mainly loess, units are also recognized, including the Walnut 

Creek Formation, Sappa Formation, Grafton Loess, Beaver 

Creek Loess, Loveland Formation, Crete Formation, Gilman 

Canyon Formation, and Peoria Loess (Swinehart and others, 

1994; May and others, 1995).

The gravel unit above the Fullerton consists of coarse 

gravel composed of clasts of granitic and metamorphic rocks 

interbedded with sandstone and minor silt or clay beds deposited in a fl uvial system. The grain size of the unit decreases 

upward to fi ne sand in some places. Characteristic features 

of this gravel deposit are anorthosite clasts that were carried 

from the Laramie Range of southeastern Wyoming (Stanley 

and Wayne, 1972) by the ancestral North Platte River. Souders (2000) noted that volcanic ash beds are locally present in 

this unit. Ash beds in the equivalent Sappa Formation in the 

Republican River drainage have been dated at 1.2 to 1.27 Ma 

(Swinehart and others, 1994).

A rich and varied nomenclature describes the dominantly 

loess section that overlies the gravel-bearing horizon of Pleistocene sediments (Reed and Dreeszen, 1965; May and others, 

1995). Various names have been applied in different parts of 

the region, but no study to date synthesizes them all. Volcanic 

ash beds were recognized early in the study of Pleistocene 

sediments in Nebraska, and were collectively referred to as the 

“Pearlette ash” (Condra and others, 1947). Initially this was 

thought to be a single bed, but later studies determined that 

there are at least three closely related beds, designated type 

O, type S, and type B (Naeser and others, 1973). Boellstorff 

(1976) further subdivided the ash beds and assigned different 

names to them. Recognition of different-aged ash beds greatly 

enhanced the chronostratigraphic correlations of various Pleistocene units in the study area.

The Loveland Formation is a late Illinoian loess widely 

recognized, not only in Nebraska, but throughout the midcontinent region. The Loveland is a fi ne-grained silt, but a 

distinguishing feature is its oxidized reddish color that makes 

it identifi able. The Loveland was deposited over an irregular, 

eroded topography, and parts of the Loveland are reworked 

material that includes locally derived sand concentrated in 

stream channels (Lugn and Wenzel, 1938). The underlying 

Beaver Creek and Grafton loess units also have a slightly reddish color (Reed and Dreeszen, 1965), making them diffi cult 

to distinguish from the Loveland. The Loveland (probably 

including the Beaver Creek and Grafton) has been dated at 

about 163,000 years B.P. by Maat and Johnson (1996), using 

thermoluminescence (TL) methods. Wayne and Aber (1991) 

discussed the complexity of this interval, which has been generalized in the present summary.
47

The Gilman Canyon Formation, mainly mid-Wisconsin 

in age, overlies the Loveland. The Gilman Canyon is a dark 

gray humic silt containing several paleosols that had been 

previously included with the Loveland Formation (Lugn and 

Wenzel, 1938). It was recognized as a separate unit by Reed 

and Dreeszen (1965) and has since been described in several 

areas of Nebraska (Souders and Dreeszen, 1991; Swinehart 

and others, 1994), but has not yet been noted in Colorado 

(Muhs and others, 1999a). Reported radiocarbon ages of the 

Gilman Canyon range from about 20,000 to as old as 40,000 

years B.P. (most reported dates are between 20,000 and 35,000 

years B.P.) (May and Souders, 1988; Feng and others, 1994; 

Souders and Swinehart, 1996a,b; Muhs and others, 1999a,b; 

Johnson and others, 1999).

The late Wisconsin Peoria Loess overlies the Gilman 

Canyon Formation. The Peoria is also recognized over a large 

area of the mid-continent region, including parts of eastern 

Colorado (Muhs and others, 1999a,b). The composition of the 

Peoria is light brownish-gray silt. Reported ages of the Peoria 

range from 10,000 to 25,000 years B.P. (Feng and others, 

1994; Swinehart and others, 1994, Pye and others, 1995; 

Souders and Swinehart, 1996a,b; Muhs and others, 1999a,b; 

Roberts and others, 2003). It was deposited over a dissected 

topography on top of the Loveland Formation (Lugn, 1935).

Several terrace alluviums are present along the Platte 

River in western and south-central Nebraska, of both Pleistocene and Holocene age (Schultz and Stout, 1945; Reed and 

Dreeszen, 1965; Brice, 1964; May and Holen, 1985; and May, 

1989). See the Elm Creek West and Newark quadrangles 

(Section II, plates 1 and 2) for examples of terraces of the 

Platte River in the Kearney area.

In Colorado, Bryan and Ray (1940) and Scott (1960, 

1963, 1965) defi ned several units of Pleistocene age. These 

occur as gravelly veneers on pediments along the mountain 

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Figure 17. Paleogeography in Pleistocene time. Red cross-hatched pattern indicates Precambrian granitic or metamorphic rocks generalized

from Green (1992) and Green and Drouillard (1994), stream in black and ice margins modifi ed from Stanley and Wayne (1972), streams in green, 

light blue, and pink modifi ed from Swinehart and others (1994), and loess area modifi ed from Muhs and others (1999a). Streams in dark blue are 

the Loup River system, shown fl owing south into the Big Blue River drainage. The modern North, South, and combined Platte Rivers (thin blue 

lines) are shown for reference. Cities and mapped quadrangles are as shown on fi gure 1.

The development of the Cenozoic Platte River drainage system
48 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

front and as terrace remnants along streams on the plains in 

eastern Colorado. Names in common use are the Rocky Flats 

Alluvium, Verdos Alluvium, Slocum Alluvium, Louviers Alluvium, and Broadway Alluvium. A thin loess, which overlies 

the Louviers Alluvium, was correlated by Scott (1963) with 

the Peoria Loess. Pleistocene eolian sands are also present 

on the plains of eastern Colorado (Forman and Maat, 1990; 

Madole, 1991, Muhs and others, 1999b). Additional alluvial 

units (discussed below) are of Holocene age. As could be 

expected, the Pleistocene alluvial units are very coarse grained 

bouldery gravels and sandstones near the mountains, and 

grade to somewhat fi ner gravels and sands in eastern Colorado 

(Scott, 1982, Madole, 1991). The Rocky Flats, Verdos, and 

Slocum are considered pre-Wisconsin in age, and the Louviers 

and Broadway are Wisconsin (Scott, 1965).

In the Laramie Basin of southeastern Wyoming, Mears 

(1991) described a number of terraces and terrace alluviums. 

These features were dated, in part, by volcanic ash beds and 

fossils, and range in age from Pliocene to Holocene. One terrace alluvium, the Harmony bench, contains a bed of Bishop 

Tuff, dated at about 760,000 years B.P. The Bishop Tuff is 

also found in a unit of Illinoian age in south-central to southeastern Nebraska (Swinehart and others, 1994). An alluvial 

unit of comparable age in Colorado is the Rocky Flats Alluvium (Madole, 1991).

Figure 17 shows the paleogeography of the region in 

Pleistocene time. The uplift areas are where Precambrian 

rocks are shown to outcrop on the Colorado and Wyoming 

State geologic maps (Green, 1992; Green and Drouillard, 

1994). The area shown as being covered by loess was generalized from the compilation of Muhs and others (1999a). The 

area of loess in Nebraska may have extended farther to the 

northwest in Pleistocene time, but this area is now covered by 

the Holocene-age Sand Hills, so the previous limit of loess 

deposition is not known.

Reconstructed Pleistocene stream courses are drawn 

from several sources and show that west of the confl uence, 

the North and South Platte Rivers were in generally the same 

channels as today. Scott (1982) noted slight changes in the 

South Platte Valley in Colorado as indicated by channel 

remnants. Of interest is the series of paleochannels plotted 

south of the present course of the Platte River in south-central 

Nebraska. The positions of these channels were interpreted 

by Stanley and Wayne (1972) and Swinehart and others 

(1994). It appears that until middle to late Wisconsin time the 

Platte River fl owed southeast from a point in western Dawson 

County, parallel to, but south of the present river. Lueninghoener (1947) suggested that the drainage pattern of the Loup 

River system may have formed the headwaters of the Big Blue 

River in early Pleistocene time, and that postulated confi guration is also shown on fi gure 17.

A pertinent question is, at what point did the Platte River 

in south-central Nebraska begin to occupy its present channel? 

One way to answer this question is to look for the presence or 

absence of Pleistocene units outside the current channel area. 

Figure 8, an isopach map of mainly fl uvial strata in the Pliocene-Pleistocene section, seems to indicate the presence of the 

river south of its current location. Thick sand intervals, interpreted as channel deposits, occur in central Gosper, southern 

Phelps, and central Franklin and Webster Counties, as well as 

in east-central Kearney County, and central Adams and Clay 

Counties. The area occupied by the present Platte River has 

generally thinner amounts of fl uvial sand and gravel. These 

relations are interpreted as showing that prior to loess deposition (Loveland and associated units) in this area the Platte was 

fl owing south of its current location, but that it migrated northward to its present location before deposition of the Loveland.

If the Platte River had remained in its southern location 

during much or all of the time when the Walnut Creek Formation and Grafton, Beaver Creek, or Loveland Loesses were 

being deposited, it would be likely that these units would be 

absent in some areas south of the river due to syndepositional 

removal by the stream. Further, as the Platte migrated northward to its current position it would have removed any loess 

units of that age that would have been deposited just south of 

its current position. However, examination of the well records 

in this area indicates that a reddish loess (usually identifi ed as 

Loveland Formation) is present in all areas south of the river 

except in Franklin, Webster, and Nuckolls Counties. This 

indicates that the river migrated to its current channel in the 

area west of Kearney prior to deposition of the Loveland, but 

that it may still have fl owed southeast from Kearney in middle 

Pleistocene time.

The isopach map of loess in test wells (fi g. 10) also may 

hold a clue about the previous course of the Platte. The map 

shows thickest areas of loess trending southeast from southwest Dawson County through central Gosper County, southern 

Phelps and Kearney counties, and through northern Nuckolls 

County. Although Mason (2001) mapped Peoria Loess at a 

much smaller scale than fi gure 10, his map also shows an area 

of relatively thick loess south of the river. This overall trend 

indicates that the river fl owed southeast from Kearney during 

deposition of the Loveland and that the ancestral Platte channel probably contributed to local thickening of loess downwind from the river. The relatively thin area of loess northeast 

of Kearney indicates that this was not a primary region for 

loess deposition, arguing against the presence of the river in 

that location in middle Pleistocene time.

The current Platte River is confi ned by valley walls 

composed of the late Wisconsin Peoria Loess and older units 

both upstream and downstream from Kearney, indicating 

that the river has occupied its current valley at least since 

the late Wisconsin. Lugn (1935, 1939b), Stevenson (1972), 

and Bentall (1982) outlined a chronology of stream capture 

of the lower Platte region from its mouth to about Kearney. 

Progressive capture of tributaries of the Elkhorn and Loup 

Rivers (fi g. 1) eventually diverted the Platte into its present course downstream from Kearney. An area of thin loess 

east of Minden indicates that the capture occurred after some 

loess was deposited (probably after Loveland deposition), 

but well before the end of Peoria deposition at the end of the 

Wisconsin. Souders and Dreeszen (1991) estimated the time 
49

of diversion to have been after the middle Pleistocene, but 

before 24,000 years B.P.

The position of two early Pleistocene ice margins, from 

Stanley and Wayne (1972), are shown on fi gure 17. Wayne 

and Aber (1991) pointed out that the ice caused major disruptions of the drainage network in eastern Nebraska and adjacent 

areas. A preglacial drainage divide trending northeast through 

South Dakota separated streams north of the divide, fl owing 

northeast into Hudson Bay, from streams south of the divide, 

fl owing southeast into the Gulf of Mexico. The ice diverted 

all of the northern streams southward along the western ice 

margin and probably also disturbed the downstream reaches 

of the Elkhorn, Loup, and Platte River systems in eastern 

Nebraska. The increased fl ow along the ice margin was probably accompanied by ponding and fl ooding as the channels 

readjusted to the greater fl ow. This would have had the effect 

of raising local base level and causing the rivers to backfi ll 

their channels with sand and gravel and to spread sediments 

over a wide braidplain.

The climate during the Pleistocene was characterized by 

wide temperature and precipitation variations that resulted in 

glacial ice advances from the north and the growth of mountain glaciers in the upper Platte River drainage basin (Wayne 

and Aber, 1991; Madole and others, 1998). The lithology of 

Pleistocene alluvial units is a good indication of climatic conditions—coarse stream deposits throughout the Pleistocene 

section near the mountains of Colorado and Wyoming and in 

the lower part of the Pleistocene section in Nebraska indicate 

abundant water sourced from mountain glacial meltwater. 

Widespread loess in middle and late Pleistocene-age deposits 

indicates somewhat more arid conditions.

Wright (1987) summarized the general conditions that 

prevailed beyond the limit of the continental ice sheets. Permafrost can be expected within about 120 miles of the glacial 

border, so permafrost conditions may have existed as far west 

as Kearney at the time of maximum glaciation in eastern 

Nebraska in the early Pleistocene. Beyond the permafrost 

and tundra zone, spruce forests existed at least as far south 

as northeastern Kansas, but the vegetation zones shifted considerably throughout the Pleistocene, depending on whether 

glacial or interglacial conditions existed. The Great Plains 

supported a diverse fauna during the Pleistocene. Megafauna, 

such as bears, tigers, leopards, mammoths, and bison were 

listed by Schultz and Stout (1948) and Reed and others (1965), 

as well as numerous smaller mammals. Several early Pleistocene local faunas were mentioned by Wayne and Aber (1991), 

yielding fi sh, amphibians, reptiles, birds, and mammals. The 

assemblages from the early Pleistocene indicate a relatively 

mild and humid climate (Schultz and Stout, 1948). Grass 

phytoliths from the mainly middle Pleistocene-age Loveland 

Formation indicate a climate somewhat drier than that existing today (Fredlund and others, 1985). Fauna and carbon 

isotope data suggest a grassland environment in northeastern 

Colorado in the late Pleistocene (Muhs and others, 1999b). 

According to Wayne (1991) the climate during the Pleistocene 

was marked by wide temporal and geographic variability. In 

the late Pleistocene, fossils from Kansas indicate cool summers but winter temperatures about the same as today (Wayne, 

1991). In Nebraska, megafauna such as caribou and muskox 

associated with spruce indicate cooler and wetter conditions. 

Data from two mammoth kill(?) sites in southwestern and 

central Nebraska indicate an equable, cooler, possibly drier 

climate (May and Holen, 1994). Prairie grassland began to 

replace spruce forests in the western Great Plains after about 

13,000 years B.P., and mixed hardwood-grassland fl ora developed in the eastern part of the area in the Holocene. The wide 

climatic variations in the late Wisconsin caused the aggradation-erosion cycles described by Schultz and Stout (1945, 

1948) and Morrison (1987) that resulted in the terrace alluvial 

deposits along drainages that contain much of the fossil evidence for climate. These cycles continued into the Holocene.

The end of the Pleistocene was marked by a major extinction of megafauna. Large animals reported from Pleistocene 

deposits in and adjacent to the Great Plains include mammoth, 

mastodon, camel, giant bear, horse, musk ox, saber-toothed 

cats, giant beaver, ground sloth, bison, cervids, and dire wolf 

(Mehringer and others, 1970; Schultz and Martin, 1970). 

Most of these animals became extinct in the late Pleistocene 

to Holocene, although the exact cause is not precisely known. 

The changes in the distribution of vegetation undoubtedly had 

much to do with changes in the distribution of fauna—browsing animals followed the forests and predators followed their 

prey. The infl uence of man cannot be discounted either. 

Haynes (1991) presented evidence for widespread drought at 

the end of the Pleistocene, which may have led to a concentration of animals around water holes and a greater opportunity 

for Clovis-age hunters. A combination of environmental stress 

and increased hunting pressures apparently was enough to 

upset the ecological balance of the late Pleistocene.

Holocene

The Pleistocene-Holocene boundary is generally considered to be at about 10,000 years B.P. (Morrison, 1991). Using 

this somewhat arbitrary boundary, the Bignell Loess is the 

oldest Holocene loess unit that is widely distributed in the 

study area. The Bignell overlies the Peoria Loess, and is commonly separated from the Peoria by the Brady soil, dated at 

about 9,240 to 9,750 years B.P. (Swinehart and others, 1994). 

The Bignell is a fi ne silt, similar to the Peoria Loess, and has a 

reported TL age of about 9,000 to 6,000 years B.P. (Maat and 

Johnson, 1996) but may be as young as 3,000 years B.P. (Pye 

and others, 1995). Scott (1965) correlated the Bignell with 

the late Wisconsin-age Broadway Alluvium of Colorado, but 

radiocarbon ages from the Bignell indicate that it is somewhat 

younger than the Broadway.

Besides the Bignell Loess, much of Nebraska and adjacent areas is covered with a veneer of Holocene wind- and 

stream-deposited material. The largest and most well-known 

example is the extensive area of eolian sand in the Sand Hills 

of north-central to southwestern Nebraska (fi g. 1). This dune 

The development of the Cenozoic Platte River drainage system
50 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

fi eld covers about one-quarter of the State of Nebraska and is 

the largest dune fi eld in North America. 

An early interpretation was that the Sand Hills dune sands 

and the upper Pleistocene Peoria Loess were contemporaneous and were derived from Tertiary formations and from prePeoria Pleistocene deposits (Lugn, 1935). This was a logical 

interpretation because loess deposits in Nebraska are thickest 

just to the southeast (downwind) of the Sand Hills region 

(Swinehart and others, 1994; Muhs and Bettis, 2000; Mason, 

2001). As radiocarbon dates became available, studies of dune 

and interdune strata indicated that the dune fi eld was active 

several times in the Holocene (Ahlbrandt and others, 1983), 

the most signifi cant occurring between 8,000 and 5,000 years 

B.P. (Swinehart, 1989). Other work has documented possible 

blockage of streams by dunes in the late Pleistocene (Swinehart and Loope, 1992), as early as 10,600 years B.P. (Loope 

and others, 1995) or even as early as about 12,000 years B.P. 

(Sweeney and others, 1998; Muhs and others, 2000). Most 

evidence indicates that some eolian activity occurred in 

the late Pleistocene but that the major part of the dune fi eld 

formed or was remobilized in the middle to late Holocene 

(Muhs and others, 1997).

Another area of Holocene dunes in Nebraska is south of 

the Platte River between the longitudes of Elm Creek and 

Shelton. This small dune fi eld probably originated from sand 

blown southeastward out of the channels of the Platte River 

as the river was migrating to its present position. The dunes 

mantle terraces as young as the Holocene Qt1a terrace and as 

old as the Qt3 terrace in the Elm Creek West quadrangle, and 

appear to lap onto the Qt4 surface south of the quadrangle (see 

section I of this report). Similar dune deposits are present on 

the northwestern fl oodplain of the Platte River north of Grand 

Island, downwind from the Loup River.

In Colorado, Holocene loess is fairly widespread, but 

volumetrically insignifi cant (Madole, 1991; 1995). Muhs 

and others (1999a) identifi ed two main episodes of loess 

deposition, one in the late Pleistocene and one in the late 

Pleistocene to Holocene. Dune sands form thicker and more 

easily studied deposits than loess in northeastern Colorado, 

but whether this is due to processes of deposition or preservation is unclear. Times of estimated dunefi eld activity vary 

according to the method used and the locations studied. Muhs 

(1985) concluded that dunes were active from 3,000 to 1,500 

years B.P. on the basis of a comparison of soils between dated 

sites in Nebraska and soils on dunes in northeastern Colorado. 

Forman and Maat (1990) suggested that the latest eolian activity started at 9,000 to 7,000 years B.P. and ended at less than 

3,000 years B.P. on the basis of radiocarbon and thermoluminescence analysis of soils on and within dunes. Forman and 

others (1992) did further studies and identifi ed four periods 

of dune activity—about 9,500 to 5,500 years B.P., 5,500 to 

>4,800 years B.P., 4,800 to 1,000 years B.P., and less than 

1,000 years B.P. Madole (1994) concluded that more than 

half (in surface area) of the eolian sand in northeast Colorado 

was remobilized within the past 1,000 years. Dunes in Colorado were derived mainly from stream channel and fl oodplain 

alluvium, and were deposited by southeastward-blowing wind 

(Muhs and others, 1996).

In southeastern Wyoming, Holocene eolian features 

include wind-stripped terraces and benches and defl ation hollows, and dunes composed of clay and silt particles that accumulate adjacent to defl ation hollows. Sand dune fi elds occur 

in the Casper area along the North Platte River (fi g. 1).

Features common to all streams in the study area are 

Holocene terraces and terrace alluviums. In Nebraska, early 

studies that defi ned the terrace hierarchy were by Schultz and 

Stout (1945). They numbered the terraces T0, T1, T2, etc., 

starting with the modern fl oodplain as T0. This numbering 

convention was partially used in preparation of the maps of 

the Elm Creek West and Newark quadrangles for this study 

(see section I). Bryan and Ray (1940) had previously mapped 

terraces in Colorado, and Schultz and Stout (1945) considered 

their T3 terrace equivalent to the Pleasant Valley terrace of 

Bryan and Ray in Colorado, the T2 terrace equivalent to the 

Kersey terrace, and the T1 terrace equivalent to the Kuner terrace. The T3 terrace was interpreted to have formed on the 

Brady soil, after deposition of the Peoria Loess but before the 

Bignell Loess (Schultz and Stout, 1948). The oldest terrace 

fi ll (T5) was correlated with the Pliocene Broadwater Formation.

This classifi cation system was extended to other parts 

of Nebraska (Schultz and others, 1948; Frankforter, 1950), 

and a complex chronology was developed that related terraces in central and western Nebraska with glacial events in 

eastern Nebraska (Schultz and others, 1951). In most areas 

the main terrace divisions can be further subdivided (Stout, 

1983). Studies by Brice (1964), May and Holen (1985), and 

May (1989, 1992, 1998) in the Loup River Basin, and Martin 

(1992a,b) in the Republican drainage, have further refi ned 

the stratigraphy of Holocene terraces and terrace fi lls with the 

aid of radiocarbon dating. There has been little dating of the 

terraces in the Platte River drainage anywhere in Nebraska. 

Of 65 sites and hundreds of radiocarbon analyses in the Central Plains, only one site was from the Platte River (Mandel, 

1995).

Following Bryan and Ray (1940), further studies of 

terraces in Colorado identifi ed three Holocene-age terrace 

alluviums—the pre-Piney Creek alluvium, the Piney Creek 

Alluvium, and the post-Piney Creek alluvium (Scott, 1963, 

1965). Pre-Piney Creek alluvium was radiocarbon dated at 

about 5,500 years B.P. and contains Archaic artifacts; Piney 

Creek Alluvium was inferred to be 2,800 years old, based on 

dating of a correlative alluvium in Utah ; and post-Piney Creek 

alluvium forms the lowest terrace and modern fl oodplains and 

was radiocarbon dated at about 1,500 years B.P. (Scott, 1963). 

Woodland artifacts have been recovered from the post-Piney 

Creek alluvium.

Holliday (1987) studied terraces along the South Platte 

and used artifacts to infer ages of the terraces. The highest 

terrace, the Kersey, was inferred to have been available for 

occupation no later than about 10,000 years B.P., but it could 

be older. The Kersey terrace is the same surface as the late 
51

Wisconsin Broadway terrace, mapped in the Denver area, and 

the Pleasant Valley terrace was considered to be the same as 

the Kersey terrace by Holliday (1987), on the basis of downstream convergence of the two terraces. The next lowest, and 

younger, is the Kuner terrace, believed to date from about 

3,000 years B.P. on the basis of its artifacts. The Kuner terrace-fi ll alluvium may correlate with Piney Creek Alluvium, 

although the deposits cannot be traced from one area to the 

other. The youngest and lowest terrace, the Hardin, is probably less than 1,000 years old (Holliday, 1987), similar in age 

to the youngest terraces in Nebraska. Alluvium in this terrace 

is probably equivalent, or slightly younger, than post-Piney 

Creek alluvium.

In the Laramie Basin of southeastern Wyoming only the 

lowest terrace above the Laramie River is considered Holocene in age, dated on the basis of modern bison bones and 

Native American artifacts (Mears, 1991). All higher terraces 

in that area can be dated as Wisconsin or older from included 

fossils, ash beds, or stratigraphic position.

The Holocene climatic record has been investigated 

extensively in a variety of studies. Proxy data such as sedimentation patterns, pedogenesis, changes in fl ora (mainly 

determined from palynomorphs), faunal changes, oxygen, 

hydrogen, and strontium isotopes, major-ion and trace-element geochemistry, and other data have been combined with 

radiocarbon and TL dates to establish a sequence of changes 

through the Holocene. Lakes are ideal laboratories for combining several methods of analysis, and several sites in the 

northern Great Plains have been studied (Smith and others, 

1997; Valero-Garces and others, 1997; Xia and others, 1997; 

Laird, and others, 1998). In broad terms, the climate of the 

study area gradually warmed and dried in post-glacial time 

to a maximum at about 8,000 years B.P. Relatively hot and 

dry conditions persisted until about 5,000 to 4,000 years B.P., 

during which time eolian conditions dominated. After about 

4,000 years B.P. temperatures again cooled and the area gradually stabilized to the subhumid to arid conditions that exist 

today (Barry, 1983). The climate has been quite variable in 

the last 10,000 years, swinging from hot and dry to cool and 

moist in a nonrandom, periodic manner, the most recent cycles 

being the Medieval Warm Period (about A.D. 800-1,200) 

and the Little Ice Age (about A.D. 1,400-1,850) (Stahle and 

Cleaveland, 1994; Campbell and others, 1998).

Mandel (1995) outlined a chronology of Holocene alluvial 

sedimentation and erosion in the central Plains. He noted that 

early, middle, and late Holocene alluvial fi lls are preserved 

in the larger streams, but that only late Holocene material is 

preserved along the small streams. As indicated above, the 

preserved sedimentary record in terraces is variable, even 

along the larger streams. In southeast Wyoming only one 

Holocene terrace has been recognized, whereas in Colorado 

as many as three Holocene terraces have been described, and 

in Nebraska as many as four Holocene alluvial sequences 

have been identifi ed. Regionally, warmer and drier climatic 

conditions produced net erosion and removal of material from 

uplands and small valleys in the early to middle Holocene, 

from about 8,000 to 4,000 years B.P. (Mandel, 1995). This 

material was redeposited on alluvial fans and fl oodplains in 

the larger valleys. Deposition of alluvial fans along the Platte 

River was documented by Faulkner (1997, 1998). A moderate, 

more humid climate led to net deposition in valleys of all sizes 

in the region in the late Holocene. In an area on the south 

side of the river, southeast of the city of North Platte, buried 

soils at the base of alluvial fans on the Platte fl ood plain were 

dated at 9,600 years B.P., 3,300 years B.P., and 1,500 years 

B.P. The long time span represented by these soils indicates 

that the river in this reach was not only present in the current 

valley but that it has been stable through the Holocene, neither 

aggrading or downcutting signifi cantly (Faulkner, 1998). This 

contrasts with the Republican River drainage, just to the south, 

which saw two periods of incisement in the Holocene (Martin, 

1992a,b) and with the Kearney area, as shown by the presence 

of Holocene-age terraces that fl ank the river (section I, this 

report).

Several periods of Holocene drought have been documented by sedimentological studies of eolian strata in 

Nebraska by Ahlbrandt and others (1983), Mason and others 

(1997), and Muhs and others (1997); in northeastern Colorado 

by Forman and others (1992), Madole (1994, 1995), and Muhs 

and others (1996); and in Kansas by Arbogast (1996), Olson 

and others (1997), and Arbogast and Johnson (1998). Arid to 

humid cycles on the plains of Colorado and Nebraska, determined from pedogenesis, were outlined by Blecker and others 

(1997) and Kelly and others (1998).

Changes that most impacted man in the transition from 

Pleistocene to Holocene on the Great Plains were those related 

to changes in fl ora and fauna. In the late Pleistocene a boreal 

forest, composed of spruce in the east and pine in the west, 

extended across parts of the central Great Plains (Wells, 1970; 

1983). As the glaciers retreated northward in the late Pleistocene, deciduous forests replaced the spruce forest in the eastern part of the region, and prairie grassland replaced the forest 

in the central and western parts (Wright, 1970; Wayne, 1991). 

In the dry middle Holocene grassland spread even farther east 

and northeast than it does today, but in the late Holocene the 

prairie border shifted westward to its present location (Hoffman and Jones, 1970).

Discussion and Summary 

The drainage basin of the Platte River is a complex 

system that has undergone many changes in the last 65 million years. The Laramide Orogeny, lasting from about 75 to 

35 Ma, established the fundamental structural framework of 

the east-central Rocky Mountains and adjacent plains. Mountain uplifts served as a source of both water and sediment that 

were transported eastward into a cratonic foreland basin. The 

Laramide ended at about the end of the Eocene Epoch, but 

renewed uplift in middle Miocene and Pliocene time rejuvenated source areas. Deposition of Tertiary and Quaternary 

sediments on the plains has depended on a balance between 

The development of the Cenozoic Platte River drainage system
52 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

suffi cient upland source areas and suffi cient accommodation 

space in the basin to store the debris from the mountains and 

sediment transported into the area by wind.

In the Tertiary Paleocene Epoch, coarse clastics that 

became the Denver Formation, Green Mountain Conglomerate, and Dawson Formation were shed from the mountains 

and were trapped in small basins immediately adjacent to the 

Front Range. No Paleocene rocks are known from Nebraska, 

Kansas, or far eastern Colorado, and the assumption is that this 

area was a lowland region, starved of sediment, undergoing 

minor erosion. Several Paleocene formations in Colorado and 

Wyoming contain coal or lignite deposits, suggesting that the 

region was at a low altitude. Fossils of aquatic animals and 

rainforest fl ora support the interpretation of a warm, humid climate in this area during the Paleocene. The drainage of some 

basins adjacent to the mountains was probably internal; drainage patterns in the eastern part of the region are unknown.

Uplift of the mountain areas continued into the Eocene 

Epoch, but few formations of early or middle Eocene age are 

known in the region, suggesting that sediment bypassed the 

basin areas and was transported out of the region. An unconformable surface developed on top of previously deposited 

units, and a widespread erosion surface has been recognized 

in the Front Range. Two schools of thought have developed 

regarding the erosion surface—(1) the surface developed at 

a relatively high altitude and had low topographic relief, and 

(2) erosion reduced much of the area to near sea level, but that 

signifi cant mountainous relief still existed. In late Eocene 

time the plains area again began to accumulate sediments—the 

Chadron Formation, mainly in Nebraska, and the upper 

Dawson Formation, Wall Mountain Tuff, and Castle Rock 

Conglomerate in the central and southern Denver-Julesburg 

Basin. In addition to basal conglomerates, the Chadron contains signifi cant volumes of volcaniclastic material that was 

brought into the area by wind from source areas to the west. 

Fossils from Eocene units indicate that the climate remained 

warm and humid during this time. The regional slope was to 

the southeast in the late Eocene. In Colorado, a proto-South 

Platte River may be represented by the Castle Rock Conglomerate, and other southeast-oriented channels have been 

recognized in the northeast part of the State. In Wyoming and 

Nebraska, reconstructed late Eocene channels are also oriented 

mainly to the south-southeast, one of which probably was the 

proto-North Platte River. An early Laramie River was also 

established at this time.

 Volcaniclastic material continued to be brought into the 

area during the Oligocene Epoch, and this material dominates 

the lithology of Oligocene formations. The Brule Formation 

and Arikaree Group were deposited over much of southeastern Wyoming, northeastern Colorado, and western Nebraska. 

Channels were incised into the fi ne-grained volcaniclastic 

material, and gravel and sand were carried away from mountain areas in the channels. Active uplift of the mountainous 

areas was interrupted in the Oligocene, and the mountains 

were partially buried in a combination of their own debris and 

wind-transported material from enormous volcanic centers in 

the Great Basin and Colorado. The climate in the Oligocene 

remained temperate, but some fossils suggest a slight cooling 

and drying from previous times. Reconstructed Oligocene 

drainage patterns indicate a shift to an eastward paleoslope for 

the region. In the early Oligocene the headwaters of streams 

were in the eastern Laramie Range and Front Range, but by 

the late Oligocene volcanic rock clasts from the western Front 

Range and North Park, Colorado, were transported across 

a fi lled Laramie Basin and across the Laramie Range to the 

plains. Oligocene rocks are absent over much of east-central 

and southeastern Colorado, making paleodrainage reconstructions in that area impossible.

Uninterrupted deposition continued into the Miocene 

Epoch, and sediments that formed the upper part of the Arikaree Group or Formation were deposited in southeastern 

Wyoming, northeastern Colorado, and western Nebraska. 

Wind-transported volcaniclastic material from western source 

areas was still an important constituent of lower Miocene 

rocks, although this material was reworked locally by streams. 

Volcanic eruptions diminished in middle Miocene time and 

the entire area underwent erosion, resulting in the beveling 

of previously deposited Miocene units and an erosion surface 

formed on older rocks in some mountain areas. In the middle 

to late Miocene a coarse-grained alluvial unit, the Ogallala 

Group or Formation, was deposited over a wide area from 

South Dakota to Texas and from the mountain front to eastern 

Nebraska and central Kansas. The Ogallala was deposited by a 

series of streams draining eastward from the Front Range and 

the Laramie Range. The coalesced gravel and sandy strata of 

the Ogallala form an important aquifer on the plains. Miocene 

climatic indicators suggest additional cooling and drying, 

although the Ogallala contains an abundant and diverse fauna, 

indicating generally favorable conditions for life on the plains. 

Paleo-drainage patterns in the Ogallala are complex, consisting of multiple generations of overlapping channels. Rhyolite 

clasts from volcanoes of the North Park area of Colorado were 

transported to the plains, indicating that Miocene streams 

fl owed northeastward across the northern Front Range. The 

North Platte River was also established in the Saratoga Valley 

in Wyoming at this time. Anorthosite clasts from the Laramie 

Range indicate that streams fl owed across the range from the 

west. A period of tectonic quiescence marked the end of the 

Miocene and an unconformity developed at the top of the 

Ogallala Group.

At the beginning of the Pliocene Epoch, the area east of 

the mountains consisted of a vast plain of low relief underlain 

by sediments of the Ogallala Group, which extended westward 

and lapped onto the mountains. In the Pliocene most of the 

region experienced gradual uplift, and sediments in areas adjacent to the mountains were excavated by large river systems. 

The Colorado Piedmont was carved out, and the Laramie Basin 

of southeastern Wyoming experienced erosion. The conglomeratic Nussbaum Alluvium is preserved on pediments and in 

terrace deposits in the Colorado Piedmont, but Colorado and 

southeast Wyoming have few other Pliocene units preserved. 

In contrast, Nebraska became a site of deposition in the Plio-
53

cene, as exemplifi ed by the Broadwater Formation in the west 

and the equivalent Fullerton Formation and other unnamed 

units in central and southern parts of the State. These units consist of coarse gravels and less abundant siltstones that are preserved as terrace remnants in the west but as broad tabular units 

in the central and southern regions of Nebraska. The Pliocene 

climate was cool but relatively wet with a diverse fauna. The 

late Pliocene climate probably became increasingly unstable 

with the onset of glaciation. Following capture and diversion 

of the South Platte River to its present position, the Pliocene 

drainage system was similar to that in existence today, with 

only minor local shifting of channels. An exception was in 

eastern Nebraska, where the Platte River swung from north to 

south across the state, depositing a veneer of gravel and sand. 

In the latter part of the Pliocene, the Platte fl owed in a channel 

south of its current position from about Dawson County southeastward toward the Missouri River drainage.

The transition from the Pliocene to the Pleistocene Epoch 

was marked by increasing climatic oscillations leading to 

times of colder and wetter conditions alternating with times 

of warmer and drier conditions. A result of these alternating 

conditions was a series of erosional and depositional cycles 

that are documented in stream terraces. Pleistocene deposits 

in Colorado are preserved in the Rocky Flats Alluvium, Verdos 

Alluvium, Slocum Alluvium, Louviers Alluvium, and Broadway Alluvium. Partially comparable alluviums and terraces are 

also present in southeastern Wyoming and Nebraska along the 

Platte system and other rivers. Pleistocene windborne deposits 

are present over much of the study area. Named loess units 

are the Walnut Creek Formation, Grafton and Beaver Creek 

Loesses, Loveland Formation, and Peoria Loess in Nebraska 

and partially equivalent units in Colorado and Kansas. Late 

Pleistocene-age dune sands also have been reported in some 

areas of Colorado and Nebraska. Most of the study area 

was relatively far removed from active glaciation during the 

Pleistocene and was therefore not subject to the extremes of 

local climate that existed in areas closer to the glaciers. Major 

differences between then and now were that spruce forest 

covered a good part of Nebraska and extended partially into 

Kansas, and mixed grassland and pine forest probably covered 

parts of eastern Colorado. A megafauna consisting of mammoths, mastodons, horses, camels, and giant forms of other 

animals still living occupied parts of the study area. The 

drainage pattern of the Platte system was similar to that of 

today, with the exception of the area from about Kearney to 

the mouth. On the basis of subsurface sediment distribution 

patterns, it appears that the Platte River shifted into its present position from about the western side of Dawson County to 

Kearney by early to middle Pleistocene time and that it continued fl owing southeast from Kearney until late Pleistocene 

time. After deposition of the Loveland Formation, but before 

or shortly after the beginning of deposition of the Peoria Loess 

the lower reach of the Platte was captured and diverted into its 

present position downstream from Kearney.

The Holocene Epoch has seen a continuation of the 

erosional and depositional cycles along stream courses. In 

Colorado named Holocene units are the pre-Piney Creek alluvium, the Piney Creek Alluvium, and the post-Piney Creek 

alluvium that are gravelly deposits preserved in terraces along 

valley margins. Similar terraces have been identifi ed in Wyoming and Nebraska. Much of the study area is covered with 

Holocene-age wind-blown deposits. In Nebraska, the Bignell 

Loess is present in many areas, and comparable units are scattered in Colorado and Kansas. The major Holocene eolian 

deposit is the Sand Hills of Nebraska and equivalent units in 

Colorado and Kansas. These dune fi elds probably began to 

form in the late Pleistocene, but full development of the Sand 

Hills didn’t occur until the middle Holocene. The climate of 

the Holocene warmed and dried in several cycles, culminating in late middle Holocene time, and then cooled and became 

slightly less arid to the present time. The streams in the Platte 

River system have remained in generally the same position 

through the Holocene, shifting back and forth in their valleys.

A fundamental question is, what is the nature of the present Platte River in the Great Bend region that centers on Kearney, Nebraska? The idea that the Platte was an example of a 

braided stream was established by the 1930s (Lugn, 1935). 

Rust (1978a) proposed a simple classifi cation system for river 

channels and defi ned a braided stream as having multiple, low 

sinuosity channels. Furthermore, he defi ned a braided river as 

one that is confi ned between valley walls and that covers most 

of the valley fl oor during fl oods (Rust, 1978b). Friedman and 

Sanders (1978) considered a braided river to have mainly sand 

and gravel as bedload, to have formed on surfaces of moderate 

to high slope, and to have a higher percentage of longitudinal 

bars as compared to transverse bars as bedforms. Blatt and 

others (1980) distinguished proximal and distal braided streams, 

depending on whether the dominant bedload was gravel or sand. 

Braided streams in general were said to have higher slopes than 

meandering streams, have a variable discharge that affects the 

ability of the stream to carry coarse material in times of low 

fl ow, and to have bank materials that are easily eroded (Blatt 

and others, 1980; Walker and Cant, 1984).

Ore (1964) conducted one of the earliest studies that 

attempted to quantify the nature of braided streams using the 

South Platte and Platte Rivers as examples and observed that 

braiding occurs under two conditions—(1) when a stream is 

unable to transport the coarsest fraction of bedload in a given 

reach and longitudinal bars develop or (2) in waning fl ow 

conditions, when transverse bars are dissected. Smith (1970, 

1971a,b) considered the South Platte and Platte Rivers as one 

of the best known examples of braided rivers. He found that 

grain size decreased, sorting increased, and that the proportion 

of transverse to longitudinal bars increased downstream in the 

Platte. Miall (1977) considered the Platte a distinctive type 

of braided stream. Viard and Breyer (1979) studied grain size 

distributions in sediments of the Platte. Blodgett and Stanley 

(1980) described the stratifi cation types of the river under 

high and low fl ow regimes. A historical review of the South 

Platte River by Nadler and Schumm (1981) indicated that the 

river was a classic braided river in the mid- to late 1800s but 

that climatic fl uctuations and man’s activities have drastically 

The development of the Cenozoic Platte River drainage system
54 Geologic studies of the Platte River, south-central Nebraska and adjacent areas

changed the river. Karlinger and others (1983) noted that the 

channel types of the Platte River include single broad channels, multiple well-defi ned channels, or in an intricate mix of 

small channels and numerous islands. Schumm (1999) outline a sequence of changes to the Platte in two reaches—(1) 

from North Platte to Kearney and (2) from Kearney to Grand 

Island. The river originally changed from a braided channel 

to an anastomosing channel in the respective reaches, but both 

reaches are now characterized as having single-island braided 

channels (Schumm, 1999).

Two key observations are (1) the Platte system is large, 

draining a signifi cant portion of both the mountains and plains 

of northeastern Colorado and southeastern Wyoming, and (2) 

it is only one of two rivers (the other being the Arkansas) in 

Colorado and Wyoming that breach the western escarpment of 

the Ogallala Group and fl ow above the Ogallala on the plains 

(fi g. 1). It was noted previously that both the Arkansas and 

Platte Rivers fl ow at higher elevations than rivers between 

them that head on top of the Ogallala surface. This is a result 

of the greater sediment load of these rivers, the Platte more so 

than the Arkansas because of its larger drainage basin, and this 

condition has persisted through the Pliocene and Pleistocene to 

the present. Nebraska became the locus of deposition of much 

of the material eroded from the Front Range, Laramie Range, 

and Medicine Bow Mountains through the last 5 million years, 

and the Platte built up a large aggradation plain in south-central to eastern Nebraska.

Lugn (1935) emphasized that the Platte River cannot 

be thought of in the traditional manner of a stream occupying a valley cut into bedrock. Rather, just the opposite has 

occurred—paradoxically the valley was superimposed on the 

river. Prior to deposition of the Peoria Loess the Platte was 

fl owing, generally unconfi ned by valley walls, on a relatively 

fl at alluvial plain. This alluvial plain was, in part, the result 

of disruption of the stream system in eastern Nebraska by 

glaciers that caused a rise in local base level and foundering 

of the Platte in its own bedload material. The Platte was in 

its approximate present position when the Peoria Loess and 

later eolian units were superimposed on the river, and the 

Platte was essentially confi ned by loess to its valley through 

south-central Nebraska. During deposition of the loess the 

Platte merely maintained its valley by removing loess that fell 

in the regions of active channels while the loess piled up in 

adjacent areas. Since that time, the Platte River has aggraded 

its valley at times and has incised into its valley at other times 

in response to short-term climatic cycles, migrating back and 

forth across the valley and leaving terraces as evidence of 

its activity. Movement of dunes in the Sand Hills across the 

Platte River Valley throughout the Holocene must have added 

large amounts of sand to the river, contributing to the erosion/

aggradation cycles. The underlying Pliocene and Pleistocene 

gravel beds also contribute bed load material during lateral 

migrations of the river, further complicating characterization 

of the river. This makes the Platte River, if not unique, then 

at least an unusual example of a braided river, and not easily 

comparable to other rivers.

References Cited

Ahlbrandt, T.S., Swinehart, J.B., and Maroney, D.G., 1983, 

The dynamic Holocene dune fi elds of the Great Plains 

and Rocky Mountain basins, U.S.A., in Brookfi eld, M.E., 

and Ahlbrandt, T.S., eds., Eolian sediments and processes: 

Amsterdam, Elsevier Science Publishers B.V., p. 379-406.

Anonymous, unpublished, Logs of test holes, Buffalo County, 

Nebraska (supplement, 1954): Conservation and Survey 

Division, Institute of Agriculture and Natural Resources, 

University of Nebraska-Lincoln, 63 p.

Anonymous, 1953a, Logs of test holes, Adams County, 

Nebraska: Conservation and Survey Division, Institute of 

Agriculture and Natural Resources, University of NebraskaLincoln, 27 p.

Anonymous, 1953b, Logs of test holes, Buffalo County, 

Nebraska: Conservation and Survey Division, Institute of 

Agriculture and Natural Resources, University of NebraskaLincoln, 62 p.

Anonymous, 1953c, Logs of test holes, Dawson and Custer 

Counties, Nebraska: Conservation and Survey Division, 

Institute of Agriculture and Natural Resources, University 

of Nebraska-Lincoln, 79 p.

Anonymous, 1953d, Logs of test holes, Gosper County, 

Nebraska: Conservation and Survey Division, Institute of 

Agriculture and Natural Resources, University of NebraskaLincoln, 29 p.

Anonymous, 1953e, Logs of test holes, Harlan County, 

Nebraska: Conservation and Survey Division, Institute of 

Agriculture and Natural Resources, University of NebraskaLincoln, 25 p.

Anonymous, 1953f, Logs of test holes, Kearney County, 

Conservation and Survey Division, Institute of Agriculture 

and Natural Resources, University of Nebraska-Lincoln, 

27 p.

Anonymous, 1953g, Logs of test holes, Merrick and Nance 

Counties, Nebraska: Conservation and Survey Division, 

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