Geologic map of the Scottsbluff 1 degree x 2 degrees quadrangle, Nebraska and Colorado

This map is necessarily generalized. For more detailed information, consult the geologic data stored in files of the Conservation and Survey Division, University of Nebraska-Lincoln. Other geologic maps at different scales that include all or parts of the map area are by Darton (1903a,b,c), Lugn (1939), Weeks and Gutentag (1981), and Weeks and otners (1988). Detailed groundwater investigations of parts of the map area were done by Wenzel and others (1946). Babcock and Visher (1951, 1952), Bjorklund (1957), Bjorklund and Brown (1957), Smith (1966, 1969), Smith and Souders (1971,1975). Souders (1986), Gottula (1993), and Verstraeten and others (1995). INTRODUCTION The Scottsbluff 1° x 2° quadrangle, in western Nebraska, is in the northern part of the High Plains section of the Great Plains Physiographic Province (Fenneman, 1931). The map area is mostly a constructional plain that has beEn dissected principally since the Pliocene by the rivers and streams that drain the area. Most ofl the southern half of the map area contains the partly eroded remnant of the High Plains known as the Cheyenne Tableland, a region occupying about 3,600 mi2. Lodgepole Creek, and to a lesser extent Lawrence Fork, Ash Hollow, and tributaries of Pumpkin Creek. and the North Platte and South Platte Rivers have modified the topography of the tableland (Condra, 1946). The part of the High Plains north of the valley of the North Platte River is mostly in the Sand Hills but includes a small part of the Box Butte Tableland. Between the Cheyenne Tableland and the Box Butte Tableland and Sand Hills are the Pumpkin Creek valley and Wildcat Ridge, a remnant of the High Plains capping the deeply dissected divide separating Pumpkin Creek from the North Platte Valley. These parts of western Nebraska are historically important. because they were crossed by the Mormon Trail, Oregon Trail, Sidney-Deadwood Trail, the Pony Express (remnants of all of these can be seen today), and the Union Pacific Railroad along Lodgepole Creek. Famous landmarks include Ash Hollow, Chimney Rock, Jail and Courthouse Rocks, and Scotts Bluff. The geologic history of the rocks exposed in the map area began some 30 million years after the Cretaceous seas withdrew from the Western Interior Seaway, and the land was subjected to subaerial processes. [n the map area, the oldest Cenozoic rocks of the upper Eocene Chadron Formation were deposited in a paleovalley system (fig. 1) eroded into Cretaceous rocks. Upland areas during late Eocene time were mainly in the southwestern and western parts of the quadrangle. A north-trending branch of a paleovalley system along the east side of the quadrangle may have been controlled by the Rush Creek structure (Diffendal, 1980; Swinehart and others, 1985). After fluvial depOSition of the Chadron ceased, dominantly eolian volcaniclastic sediments (upper part of the Chadron Formation and the Brule Formation of the White River Group) were deposited during the late Eocene and early Oligocene. The middle part of the Brule Formation does, however, locally contain thick alluvial deposits (Blodgett, 1974; Swinehart and others, 1985). A narrow paleovalley was formed across the quadrangle before deposition in the late Oligocene of the Gering Formation (Arikaree Group). The long axis of Wildcat Ridge generally coincides with the paleovalley, which contains most of the Gering Formation in the quadrangle (Swinehart and others, 1985; Tedford and others, 1966). Although the Gering is mostly composed of fluvial sediments, most of the overlying upper Oligocene and lower Miocene Arikaree Group (Harrison and Monroe Creek Formations, undivided, and beds of the informal Camp Clarke unit) are composed of eolian volcaniclastic deposits. In contrast to the White River and Arikaree Groups, the Ogallala Group (middle and upper Miocene) is dominantly flUvial, paleovalley-fill depoSits. Most of the alluvial materiaJ was derived from erosion of rocks in the Southern Rocky Mountains (Stanley, 1976). The principal Ogallala paleovalley system extends across the southern part of the quadrangle (fig. 2). Toward the end of Ogallala Group depOSition, a southeast-trending paleovalley was eroded north of the present North Platte Valley and later filled with the informal Angora sand and gravel beds. The Broadwater Formation (Pliocene) consists principally of fluvial sand and gravel and fills a major paleovalley north of the present North Platte River (Swinehart and others. 1985; Swinehart and Diffenda), 1987). The North Platte River system probably began to form during erosion of the valley that was filled by the Angora sand and gravel beds (upper Miocene), but principal tributaries like Pumpkin Creek apparently developed later, during or after the early Pleistocene (Diffendal and Comer, 1983). From western Garden County westward across the quadrangle, the North Platte River has migrab~d generally southward and has entrenched several times, whereas Pumpkin Creek has migrated northward and has also entrenched several times. The map area is mostly covered by surficial depoSits composed chiefly of eolian sand, loess, colluvium, and alluvium (Mortlock and others, 1920; Hayes, 1921; Wolfanger and others, 1924; Clocker and others, 1962; Sautter and others, 1965; Yost and others, 1968; Helzer and others, 1985; and unpublished preliminary soil sutveys of Banner, Cheyenne, Garden, and Keith Counties being prepared by the Soil Conservation Service). Deformation has affected parts of the quadrangle (DeGraw, 1969; Diffendal, 1980; Sonnenberg and Weimer, 1981; Swinehart and others, 1985; Ahlbrandt and Groen, 1987). Surface rocks from Sidney Draw in southwestern Cheyenne County northeastward to southwestern Garden County are folded. Contour maps of the base of the Ogallala Group (fig. 2) and on the base of the Cenozoic rocks (fig. 1) show highs and lows that correspond with fold axes shown by Sonnenberg and Weimer (1981). The authors thank the many farmers and ranchers who allowed us to work on their lands. We also thank v.L. Souders and FA. Smith (Conservation and SUlVey Division), J.J. Gottula (Nebraska Department of Environmental Quality), RG. Goodwin (Woodward-Clyde Consultants), and M.R. Voorhies and R.M. Hunt, Jr. (University of Nebraska State Museum)

Geologic Map of Morrill County, Nebraska

Morrill County, Nebraska, is perhaps the most geologically interesting county in the panhandle of Nebraska. Surficial deposits of alluvium, colluvium, loess, and eolian sand of Quaternary age cover large parts of the county. Holocene and latest Pleistocene(?) eolian sand as much as 150 It thick covers much of the area north of North Platte Valley and mantles Quaternary alluvial deposits to the south. A thin veneer of loess overlies bedrock in the uplands of the southern part of the county. Alluvium underlies the valley floors and caps river and stream terraces. Two units of older Quaternary alluvium are exposed in the North Platte River and Pumpkin Creek drainages. Alluvium and lake deposits of the Broadwater Formation (Pliocene) are exposed along the north side of the North Platte River valley. Although only remnants of the formation are preserved, they define the general shape of the bedrock valley in which the formation was deposited. The south side of the ancient valley is close to the south side of the present North Platte Valley. In the northwestern part of the county, the north side .of the paleovalley has been eroded away, so its former position is unknown. From Broadwater eastward, however, the general valley shape can be inferred from outcrop and drill-hole data. The valley was at least 450 ft deep and as much as 5 mi wide in the southeastern part of the county (Swinehart and Diffendal, 1987). Loose gravel and sand, conglomerate, pebbly sandstone, sandstone, siltstone, diatomite, volcanic ash, calcrete, and silcrete of the Ogallala Group (upper and middle Miocene) crop out along the north side of the North Platte Valley from the Morrill-Garden County line westward and are mostly buried beneath the Sand Hills to the north. Remnants of the Ogallala Group rocks cap the highest parts of Wildcat Ridge in the west-central part of the county. Ogallala rocks also filled a paleovalley system in the southern part of the county, and deposits of high-gradient streams that were tributary to the main valleys are well exposed there (Diffendal, 1982; Swinehart and Diffendal, 1987). Eolian and fluvial volcaniclastic deposits of the Arikaree Group (lower Miocene and upper Oligocene) crop out along the north side of the North Platte Valley and underlie the tableland and Sand Hills area to the north. They crop out on the flanks of Wildcat Ridge and underlie the isolated erosion remnants, Jail and Courthouse Rocks, to the east. Conglomerate containing pumice clasts is present locally in the basal part of the Gering Formation (upper Oligocene). Volcaniclastic eolian sedimentary rocks of the White River Group (lower Oligocene and upper Eocene) crop out along the north side of the North Platte River valley both east and west of Broadwater, and along the south side from Bridgeport westward. Siltstone of the Brule Formation (lower Oligocene) of the White River Group also crops out along both sides of Pumpkin Creek Valley and its tributaries in the southwestern part of the county. A variety of geomorphic features are represented in the county. Several forms of sand dunes are present in the northern part of the county and also overlie Quaternary terraces along the North Platte River and Pumpkin Creek (Swinehart, 1990). Strath terraces, fill terraces, and the axial deposits of tributary and trunk streams are present in the river and creek valleys (Diffendal and Corner, 1983; Swinehart and Diffendal, 1987). Evidence for Quaternary stream piracy within the county has been documented by Dilfendal (1984). Folding has affected the rocks (both at the surface and in the subsurface) in the eastern part of the county (Diffendal, 1980). Folds developed and were reactivated periodically throughout the Phanerozoic (Sonnenberg and Weimer, 1981; Swinehart and others, 1985). This map is necessarily generalized. For more detailed information, consult the files of the Conservation and Survey Division, University of Nebraska. Previously published smaller scale geologic maps that cover Morrill County include those by Darton (1903a,b), Lugn (1939), and Weeks and Gutentag (1981). We thank V. L. Souders (Conservation and Survey Division), J.J. Gottula (Nebraska Department of Environmental Quality), R.G. Goodwin (Woodward-Clyde), and Conservation and Survey Division field assistants S.S. Goodwin, Jr. , M.E. Rebone, and K.K. Kolster for their help in the field. We thank especially the landowners in Morrill County who gave us access to their properties

Geologic map of the Scottsbluff 1 degree x 2 degrees quadrangle, Nebraska and Colorado

This map is necessarily generalized. For more detailed information, consult the geologic data stored in files of the Conservation and Survey Division, University of Nebraska-Lincoln. Other geologic maps at different scales that include all or parts of the map area are by Darton (1903a,b,c), Lugn (1939), Weeks and Gutentag (1981), and Weeks and otners (1988). Detailed groundwater investigations of parts of the map area were done by Wenzel and others (1946). Babcock and Visher (1951, 1952), Bjorklund (1957), Bjorklund and Brown (1957), Smith (1966, 1969), Smith and Souders (1971,1975). Souders (1986), Gottula (1993), and Verstraeten and others (1995). INTRODUCTION The Scottsbluff 1° x 2° quadrangle, in western Nebraska, is in the northern part of the High Plains section of the Great Plains Physiographic Province (Fenneman, 1931). The map area is mostly a constructional plain that has beEn dissected principally since the Pliocene by the rivers and streams that drain the area. Most ofl the southern half of the map area contains the partly eroded remnant of the High Plains known as the Cheyenne Tableland, a region occupying about 3,600 mi2. Lodgepole Creek, and to a lesser extent Lawrence Fork, Ash Hollow, and tributaries of Pumpkin Creek. and the North Platte and South Platte Rivers have modified the topography of the tableland (Condra, 1946). The part of the High Plains north of the valley of the North Platte River is mostly in the Sand Hills but includes a small part of the Box Butte Tableland. Between the Cheyenne Tableland and the Box Butte Tableland and Sand Hills are the Pumpkin Creek valley and Wildcat Ridge, a remnant of the High Plains capping the deeply dissected divide separating Pumpkin Creek from the North Platte Valley. These parts of western Nebraska are historically important. because they were crossed by the Mormon Trail, Oregon Trail, Sidney-Deadwood Trail, the Pony Express (remnants of all of these can be seen today), and the Union Pacific Railroad along Lodgepole Creek. Famous landmarks include Ash Hollow, Chimney Rock, Jail and Courthouse Rocks, and Scotts Bluff. The geologic history of the rocks exposed in the map area began some 30 million years after the Cretaceous seas withdrew from the Western Interior Seaway, and the land was subjected to subaerial processes. [n the map area, the oldest Cenozoic rocks of the upper Eocene Chadron Formation were deposited in a paleovalley system (fig. 1) eroded into Cretaceous rocks. Upland areas during late Eocene time were mainly in the southwestern and western parts of the quadrangle. A north-trending branch of a paleovalley system along the east side of the quadrangle may have been controlled by the Rush Creek structure (Diffendal, 1980; Swinehart and others, 1985). After fluvial depOSition of the Chadron ceased, dominantly eolian volcaniclastic sediments (upper part of the Chadron Formation and the Brule Formation of the White River Group) were deposited during the late Eocene and early Oligocene. The middle part of the Brule Formation does, however, locally contain thick alluvial deposits (Blodgett, 1974; Swinehart and others, 1985). A narrow paleovalley was formed across the quadrangle before deposition in the late Oligocene of the Gering Formation (Arikaree Group). The long axis of Wildcat Ridge generally coincides with the paleovalley, which contains most of the Gering Formation in the quadrangle (Swinehart and others, 1985; Tedford and others, 1966). Although the Gering is mostly composed of fluvial sediments, most of the overlying upper Oligocene and lower Miocene Arikaree Group (Harrison and Monroe Creek Formations, undivided, and beds of the informal Camp Clarke unit) are composed of eolian volcaniclastic deposits. In contrast to the White River and Arikaree Groups, the Ogallala Group (middle and upper Miocene) is dominantly flUvial, paleovalley-fill depoSits. Most of the alluvial materiaJ was derived from erosion of rocks in the Southern Rocky Mountains (Stanley, 1976). The principal Ogallala paleovalley system extends across the southern part of the quadrangle (fig. 2). Toward the end of Ogallala Group depOSition, a southeast-trending paleovalley was eroded north of the present North Platte Valley and later filled with the informal Angora sand and gravel beds. The Broadwater Formation (Pliocene) consists principally of fluvial sand and gravel and fills a major paleovalley north of the present North Platte River (Swinehart and others. 1985; Swinehart and Diffenda), 1987). The North Platte River system probably began to form during erosion of the valley that was filled by the Angora sand and gravel beds (upper Miocene), but principal tributaries like Pumpkin Creek apparently developed later, during or after the early Pleistocene (Diffendal and Comer, 1983). From western Garden County westward across the quadrangle, the North Platte River has migrab~d generally southward and has entrenched several times, whereas Pumpkin Creek has migrated northward and has also entrenched several times. The map area is mostly covered by surficial depoSits composed chiefly of eolian sand, loess, colluvium, and alluvium (Mortlock and others, 1920; Hayes, 1921; Wolfanger and others, 1924; Clocker and others, 1962; Sautter and others, 1965; Yost and others, 1968; Helzer and others, 1985; and unpublished preliminary soil sutveys of Banner, Cheyenne, Garden, and Keith Counties being prepared by the Soil Conservation Service). Deformation has affected parts of the quadrangle (DeGraw, 1969; Diffendal, 1980; Sonnenberg and Weimer, 1981; Swinehart and others, 1985; Ahlbrandt and Groen, 1987). Surface rocks from Sidney Draw in southwestern Cheyenne County northeastward to southwestern Garden County are folded. Contour maps of the base of the Ogallala Group (fig. 2) and on the base of the Cenozoic rocks (fig. 1) show highs and lows that correspond with fold axes shown by Sonnenberg and Weimer (1981). The authors thank the many farmers and ranchers who allowed us to work on their lands. We also thank v.L. Souders and FA. Smith (Conservation and SUlVey Division), J.J. Gottula (Nebraska Department of Environmental Quality), RG. Goodwin (Woodward-Clyde Consultants), and M.R. Voorhies and R.M. Hunt, Jr. (University of Nebraska State Museum)

Duer Ranch, Morrill County, Nebraska: Contrast between Cenozoic Fluvial and Eolian Deposition

The Duer Ranch locality contains some of the finest and most easily accessible examples of different styles of alluvial cuts and fills in the Cenozoic rocks of Nebraska. It offers a unique area in which to examine the geometries and alluvial fills of several Miocene and Pliocene age paleovalleys and paleo-gullies. Good exposures of eolian volcaniclastic siltstones and a regionally important volcanic ash of the Oligocene age Brule Formation are also present at the Duer Ranch locality. In addition, Quaternary ephemeral stream development and deposits can be studied. Location and Accessibility Significance of the Locality Locality Information Measured Sections [I-IV] Selected Reference

Duer Ranch, Morrill County, Nebraska: Contrast between Cenozoic Fluvial and Eolian Deposition

The Duer Ranch locality contains some of the finest and most easily accessible examples of different styles of alluvial cuts and fills in the Cenozoic rocks of Nebraska. It offers a unique area in which to examine the geometries and alluvial fills of several Miocene and Pliocene age paleovalleys and paleo-gullies. Good exposures of eolian volcaniclastic siltstones and a regionally important volcanic ash of the Oligocene age Brule Formation are also present at the Duer Ranch locality. In addition, Quaternary ephemeral stream development and deposits can be studied. Location and Accessibility Significance of the Locality Locality Information Measured Sections [I-IV] Selected Reference

Geology of the Pre-dune Strata

Over the last 98 million years, four general geological processes have acted to shape the ancient landscapes buried beneath the Sand Hills. Three of these affected the area directly, either depositing sediments on the land surface or eroding it, while a fourth took place west of Nebraska, but affected the region nonetheless. Shells of clams, oysters, and numerous other kinds of creatures similar to forms that live today in the seas are preserved as fossils in the chalks, limestones, and shales that form the oldest rocks beneath the Sand Hills that will be described. These deposits indicate to geologists that seas once covered the area and that marine (oceanic) processes of sediment transport and deposition were active during this time. After the seas retreated, streams played a major role in depositing the strata overlying the marine sediments. Nonmarine fossils of plants and animals occur in alluvial (stream-deposited) sands, gravels, and silts laid down within the last 37 million years. The third process that actively shaped the area was eolian (wind-blown) deposition. Winds have played a major role in the geologic evolution of the entire Great Plains. They have formed the Sand Hills and considerable volumes of older, nonmarine strata. In addition, erosion by both wind- and water-related processes has helped carve the ancient landscapes buried beneath the Sand Hills. Volcanic activity far to the west and southwest of Nebraska is the fourth process that affected the Sand Hills. Fine-grained debris from volcanic eruptions was carried by high-altitude winds and deposited on the plains. The work of rivers, winds, and, to a lesser extent, volcanoes continues today. Ash from the recent eruption of Mount St. Helens in Washington was carried eastward, and fallout from the eruption cloud was deposited far downwind, including a light dusting of the Sand Hills

Lacustrine evidence for moisture changes in the Nebraska Sand Hills during Marine Isotope Stage 3

In the central Great Plains of North America, loess stratigraphy suggests that climate during the late Pleistocene was cold and dry. However, this record is discontinuous, and there are few other records of late-Pleistocene conditions. Cobb Basin, located on the northern edge of the Nebraska Sand Hills, contains lacustrine sediments deposited during Marine Isotope Stage 3, beginning approximately 45,000 cal yr BP and continuing for at least 10,000 yr. The lake was formed by a dune dam blockage on the ancient Niobrara River, and its deposits contain a diatom record that indicates changes through time in lake depth driven by changes in effective moisture. During the earliest stages of lake formation, the climate was arid enough to mobilize dunes and emplace dune sand into a blocking position within the Niobrara stream bed. Diatom assemblages suggest that lake-level was shallow at formation, increased substantially during a wet interval, and then became shallow again, as arid conditions resumed. By about 27,000 cal yr BP the lake was filled, and a shallow ephemeral river occupied the basin