Qap 1 Deposits-Alluvium of the Platte River system (late Holocene)
Sand and Gravel, with local silty sediments
Qap 1 deposits lie directly adjacent to the North and South Platte Rivers. Most land is
cottonwood/cedar forest, primarily used for recreation and gravel mining. Sedimentary deposits
are generally less than ~ 3 m (10 ft) in thickness overlying either Late Pleistocene alluvium or
sediment of the Ogallala Group.
Qap1_FCN- Flood channels of the North Platte River. Channels are only occupied by
water during flooding events. Little if any vegetation is present in channels.
Qap1_FCS-Flood channels of the South Platte River. Channels are only occupied by
water during flooding events. Little if any vegetation is present in channels.
Qap1_N-Most recent alluvium deposited by the North Platte River. Deposits have
abundant bar and swale topography, and are occasionally flooded during larger
flooding events. Areas covered by this unit are largely forested with cottonwood
trees. Many areas mapped as Qap_N1a were last flooded during the floods of 2011.
Qap1b_N-Deposits have more localized bar and swale topography, and are < 1m (1-3 ft)
higher than Qap1_N deposits. Areas covered by this unit are largely forested with
cottonwood trees.
Qap1_S-Most recent alluvium deposited by the South Platte River. Deposits have
prevalent bar and swale topography, and are occasionally flooded during large scale
flooding events. Areas covered by this unit are largely forested. Many areas mapped
as Qap1_S were last flooded during the floods of 2011.
Qap2_N- North Platte River Alluvium (middle to late Holocene).
Sand and Gravel, with local silty sediments
Older alluvium of the North Platte River that is slightly higher (~ 1m or 3 feet) above the modern
floodplain than the younger alluvial deposits. Bar and swale topography is only visible locally
and the unit is extensively farmed. The upper 0.5-1 m (1.5-3 ft) is commonly silty overbank
alluvial sediment with a poorly developed surface soil. Sand and gravel thickness varies, but
commonly pale brown or calcium carbonate cemented very fine sand of the Miocene aged
Ogallala Group are encountered between ~ 9-12 m (30-40 ft) depth.
Qap2_S-South Platte River Alluvium (middle to late Holocene).
Sand and Gravel, with local silty sediments
Older alluvium of the South Platte River that is slightly higher (~ 1 m or 3 ft) above the modern
floodplain than the younger alluvial deposits. Bar and swale topography is only visible locally.
This unit is extensively farmed. The upper 0.5-1 m (1.5-3 ft) is commonly silty overbank
alluvial sediment with a poorly developed surface soil. Sand and gravel thickness varies, but
commonly pale brown or calcium carbonate cemented very fine sand of the Miocene aged
Ogallala Group are encountered between ~ 9-12 m (30-40 ft) depth.
Qap3_N- North Platte River Alluvium (Holocene)
Sand and Gravel, with local silty sediments
Deposits are about 1 m (3-4 ft) higher than Qap2_N deposits. Bar and swale topography is
absent. Land use is predominantly irrigated farmland or pastureland. Sand and gravel thickness
varies, but commonly pale brown or calcium carbonate cemented very fine sand of the Miocene
aged Ogallala Group are encountered between ~ 9-12 m (30-40 ft) depth.
Qap3_S- South Platte River Alluvium (Holocene)
Sand and Gravel, with local silty sediments
Deposits are about 1 m (3-4 ft) higher than Qap2_N deposits. Bar and swale topography is
absent. Land use is predominantly irrigated farmland or pastureland. Sand and gravel thickness
varies, but commonly pale brown or calcium carbonate cemented very fine sand of the Miocene
aged Ogallala Group are encountered between ~ 9-12 m (30-40 ft).
Qap4_S- South Platte River Alluvium (late Pleistocene)
Sand and Gravel, with local silty sediments
Older slightly higher remnant alluvium of the South Platte River, with no alluvial features
present. Areas mapped in this unit are limited in extent, and are only present along the southern
side of the North Platte-South Platte River divide to the north of the town of Sutherland. This
unit is locally obscured by alluvial fan deposits. The top of this unit is generally about 1.2 m (4
ft) higher than areas mapped as Qap3_S. Sediments mapped in this unit are thin, generally about
1.2 m (4 ft) thick over sediment of the Ogallala Group.
Qap_U-Undifferentiated Platte River Alluvium (late Pleistocene)
Sand and Gravel, with local silty sediments
Older slightly higher alluvium of the Platte River System that cannot be directly associated with
the North or South Platte River. This unit is slightly higher than the Qap3_S (~ 0.5 m or 1.5-2
ft) and slightly lower than the Qap3_N (< 1 m or 2-3 ft). Commonly sandstone of the Miocene
aged Ogallala Group are encountered between ~ 9-12 m (30-40 ft) depth, but locally Quaternary
sand and gravel may exceed ~ 31 m (100 ft) in thickness below this unit.
Qapt_HW1- Platte River alluvial terrace (late Pleistocene to early Holocene)
Fine sand or silt over very fine sand and gravel
Alluvial terrace fill with a tread that lies approximately 18-24 m (60-80 ft) above the modern
North Plate River floodplain and 12-18 m (40-60 ft) above the present-day floodplain of
Birdwood Creek. This terrace is partially covered by eolian sand in the mapping area and is
obscured by dune sand to the east and west of the mapping area. The eolian sand cover is
variable, but may be 15-20 ft in thickness. The alluvial fill ranges from laminated silty very fine
sand to sand and gravel. The alluvial fills of this terrace may have had some limited reworking
by Birdwood Creek during the late Pleistocene or early Holocene. The thickness of the alluvial
fill is also highly variable, ranging from ~ 5-27 m (15-90 ft). The underlying Ogallala Group
sediment is typically either loose or consolidated very fine to medium sand with local clay seams
and local calcium carbonate cementation. The Oligocene Brule Formation of the White River
Group was encountered in Conservation and Survey Division Test Hole 90-HP-80 between 98110 m (322-360 ft) depth. The Brule Formation is a silt or siltstone that is the base of aquifers in
the region.
Qapt_HW2-Platte River alluvial terrace (Late Pleistocene)
Fine sand or silt over very fine sand and sand and gravel
Alluvial terrace fill covered with eolian sand. The bulk of this terrace fill lies to the north of the
Hershey West 7.5’ Quadrangle and only the slope leading up to the terrace tread is shown on this
map. The terrace tread lies ~ 31 m (100 ft) above the present level of the Birdwood Creek
floodplain.
Qap_B-Birdwood Creek alluvium (late Pleistocene to Holocene)
Sandy silt, with trace gravels
Alluvium of Birdwood Creek, sediments consist of reworked eolian sand sourced from the
surrounding Sand Hills, reworked gravels sourced from the QTal gravels upstream, and
reworked Ash Hollow Formation of the Ogallala Group (Noah). Qap_B sediments are likely
thin, less than ~ 3 m (10 ft) thick over sediment and rock of the Ogallala Group.
Qes_ss-Eolian sand sheet (Late Pleistocene/Holocene)
Fine sand
Windblown sand sheet deposits that have either less than ~ 3 m (10 ft) of topographic relief or
few recognizable dune forms. Areas mapped as sand sheet are generally used as rangeland.
Qes_d- Eolian dune sand (Late Pleistocene/Holocene)
Fine sand
Windblown dune sand deposits of the Nebraska Sand Hills, and dunes found on abandoned
alluvial deposits of the North and South Platte Rivers and Birdwood Creek. The large dunes in
the extreme northeast and northwest portions of the map have relief greater than 30 m (90 ft). In
these dunes the sand is on average 15 m (50 ft) thick and overlies Pliocene/Pleistocene age
alluvial deposits (QTal) or Ogallala Group sandstone (Noah). Recent studies have shown the
dunes in the Nebraska Sand Hills were most recently active ~800 years ago (Goble et al., 2004;
Mason et al., 2004; Loope and Swinehart, 2000). Although no numerical ages were taken from
dunes on the map, the dunes are likely late Pleistocene to early Holocene in age. Areas mapped
as dune sand are generally rangeland and to a lesser extent irrigated agricultural cropland. The
dunes found to the north of the South Platte River are much smaller, ranging from 4.6-6.1 m (1520 ft) of relief. These dunes directly overlie abandoned sediment from the South Platte River.
Qp-Peoria Loess (Quaternary)
Silt and sandy silt
Yellowish brown to yellowish gray silt and sandy silt present on uplands to the south of the
South Platte River. Peoria Loess is a wind-blown silt that lies on alluvium deposited by the
Platte River in the mapping area. The Peoria Loess was deposited between ~ 25,000 to 14,000
years ago (Bettis et al., 2003). Peoria Loess thickness in the eastern portion of the mapped unit
is on the order of ~ 2.4-6.1 m (8-20 ft), but the unit thickens on the western side of the map. The
loess locally contains or directly overlies zones that are either sandy silt or sand. These sandy
intervals are interpreted as either windblown dunes or sandsheets. The alluvium underlying the
loess and eolian sand is 4.6-12 m (15-40 ft) thick and ranges from laminated very fine sand to
sand and gravel. Below the alluvium is loose and consolidated sand and silt of the Miocene
Ogallala Group. The depth of sediment overlying the Ogallala ranges from ~ 14-50 m (45-164
ft) in the mapping area.
Noah-Ash Hollow Formation of the Ogallala Group (Neogene; Late Miocene)
Weakly cemented siltstone/sandstone with local volcanic ash beds
Primarily calcium carbonate cemented cross bedded alluvial sands and silts where exposed.
Localized volcanic ash beds are present in Conservation and Survey test holes, and in exposures
along Birdwood Creek. Two volcanic ash units about 0.6 m (2 ft) thick are present along
Birdwood Creek along the north edge of the map. Diffendal (1991) mapped similar volcanic
ashes in exposures along Birdwood Creek to the north of the mapping area. Numerous outcrops
of Ash Hollow Formation are mapped on the divide between the North and South Platte Rivers.
QTal-Older alluvium (Pleistocene and Pliocene)
Very coarse sands with fine to coarse gravels
Very coarse sands and fine to coarse gravels sourced from the Rocky Mountains. Where mapped,
these gravels are ~ 9 m (30 ft) thick. Mineralogy suggests gravels have a mixed source from
both the North and South Platte Rivers. Equivalent deposits were mapped extensively on the
divide between the North and South Platte Rivers to the west of the mapping area, and elsewhere
in western Nebraska by Diffendal (1991) and Swinehart and Diffendal (1997). These sands and
gravels have been correlated to the Broadwater Formation (see Swinehart and Diffendal, 1997).
Qaf- Alluvial fan deposits (Holocene)
Sandy silt with reworked Ogallala Group sandstone
Alluvial fan sediments deposited adjacent to the divide between the North and South Platte
Rivers. Alluvial fan sediments are deposited at the mouths of small drainages, and are generally
limited in extent. Sediments are generally thin, less than 2 m (6 ft) in thickness where mapped.
W-Water
P-Pit
References
Bettis, E.A., III, Muhs, D.R., Roberts, H.M., Wintle, A.G., 2003, Last Glacial Loess in the
Conterminous USA, Quaternary Science Reviews 22, 1907-1946.
Diffendal, R.F. Jr., 1991. Geologic Map Showing Configuration of the Bedrock Surface, North
Platte 1° X 2° Quadrangle, Nebraska. United States Geological Survey. Miscellaneous
Investigation Series Map I-2277.
Goble, R.J., Mason, J.A., Loope, D.B. and Swinehart, J.B., 2004. Optical and Radiocarbon Ages
of Stacked Paleosols and Dune Sands in the Nebraska Sand Hills, USA. Quaternary Science
Reviews 23, p. 1173-1182.
Loope, D. B., Swinehart, J. B., 2000. Thinking Like a Dune Field: Geologic History in the
Nebraska Sand Hills. Great Plains Research 10, p. 5-35.
Mason, J.A., Swinehart, J.B., Goble, R.J., and Loope, D.B., 2004. Late Holocene Dune Activity
Linked to Hydrological Drought, Nebraska Sand Hills, USA. The Holocene 14, p. 209-217.
Swinehart, J.B. and Diffendal, R.F. Jr., 1997. Geologic Map of the Scottsbluff 1° X 2°
Quadrangle, Nebraska and Colorado. United States Geological Survey. Geologic Investigations
Map I-2545.
Acknowledgements
We would like to thank Robert Diffendal Jr. for providing valuable comments and suggestions
on the creation of this map.