Surficial Geology of the Nebraska Portion of the
Gross Quadrangle
R. M. Joeckel, L. M. Howard, and M. S. Kuzila
Overview of the Nebraska Portion of the Gross Quadrangle
Geographic Setting and Geomorphology
The Gross Quadrangle lies almost entirely within northern Boyd County, Nebraska (a very
small part of it lies within Gregory County in southern South Dakota). Only that portion of the
quadrangle lying in Nebraska is presented as a geologic map in this document.
The Gross Quadrangle lies immediately southward and westward of the Laurentide glacial
limit in adjacent South Dakota, near the very eastern edge of the Great Plains. The western
limit of Laurentide glaciation is approximated by the present position of the Missouri River.
The central, southern, and western parts of of the Gross Quadrangle generally lie within the
United States Department of Agriculture Natural Resources Conservation Service (NRCS)
Major Land Resource Area (MLRA) 66 Dakota-Nebraska Eroded Tableland; the eastern and
northern parts of the quadrangle lie within MLRA 63B Southern Rolling Pierre Shale Plains
(United States Department of Agriculture, Natural Resources Conservation Service, 2006).
The topography of the Gross Quadrangle is varied and the landscape is scenic, particularly in
the northwestern part of the map area, near the Missouri River. The total relief on the land
surface within Nebraska portion of the Marty Quadrangle is approximately 630 ft (192 m),
from the lowest elevation at the eastern margin of the quadrangle in the Missouri Valley to the
summit of Ninemile Hill in section 2 of T. 34 N., R. 11 W. (Fig. 1), and then to the highest
summit at the western edge of section 33 of T. 35 N., R. 11 W. Topographic subdivisions
within the Gross Quadrangle are as follows:
1. The nearly flat uplands, which are slightly rolling and exhibit a few, low and rounded
hills (e.g., Ninemile Hill; Fig. 1) and very low but extensive ridges extending roughly
northwest-to-southeast. These extensive ridges are restricted to the southern two-thirds
of the map, and the best example is the one on which Rosedale Cemetery lies (SW
corner of section 23 of T. 34 N., R. 11 W.). In themselves, these uplands exhibit 100 ft
(30 m) or less of total relief, and within any given section local relief is typically much
less. The uplands are developed mostly on Neogene sediments partially covered by
very thin Late Pleistocene loess, although the Pierre Shale (Cretaceous) crops out
within deeper drainages.
2. The steep slopes of the Missouri River Valley, which cover more than a third of the map
area in its northern to western parts. These slopes include the notably steep and scenic
feature known as Ninemile Gulch (Fig. 2), which extends northeastward from section 2
of T. 34 N, R. 11 W. to the Missouri River. In themselves, these steep valley slopes
exhibit a total relief of more than 500 ft (152 m). These slopes have been eroded
chiefly from the Cretaceous Pierre Shale (Fig. 3).
The roughly northwest-to-southeast-oriented, streamlined hills and ridges and intervening
valleys described by Joeckel et al. (2010) and interpreted as the products of Late Pleistocene
wind erosion, can be seen in both the uplands and the Missouri River Valley slopes within the
Gross Quadrangle. Ninemile Hill (Fig. 1), which is oriented roughly north-northwest to southsoutheast, is interpreted as an eolian-erosional remnant of Ogallala Group sediments. Oriented
ridgelines and the valleys of first-order drainages are particularly prominent in sections 26, 27,
and 35 of T. 35 N., R. 11W. Tone patterns on aerial images dating from 1938 to the present
day exhibit a northwest-southeast “grain” on the landscape in the nearly flat uplands as well.
Bedrock and Regolith Geology
In the enclosing area, the upper part of the Niobrara Formation (Upper Cretaceous; ConiacianCampanian) crops out in a narrow band on the lower slopes of both the northern and southern
valley walls of the Missouri River, along which it forms a distinct terracelike surface. These
walls form a straight-sided trench. The Niobrara Formation is the oldest stratigraphic unit
exposed within the boundaries of the Marty Quadrangle, but it is exposed in only a small area
near the northeastern corner of the map.
The Pierre Shale (Upper Cretaceous; Campanian-Maastrichtian), which overlies the Niobrara
Formation, is usually gray to dark gray when fresh, but it has been weathered by oxidation and
local acid rock weathering to a depth of a few to several meters under upland areas. As a
result, weathered Pierre Shale strata are typically of much lower color values (i.e., lighter) and
higher chromas (i.e., brighter) than are their unweathered equivalents. This color change can
lead a casual observer to confuse weathered Pierre Shale with Cenozoic sediments and
sedimentary rocks.
Sediments and sedimentary rocks of the Ogallala Group underlie the uplands. These materials
are mostly unconsolidated, and limited subsurface data suggest that they are dominantly sand.
Technically, any unconsolidated sediments within the Ogallala Group are part of the regolith
wherever they overlie consolidated bedrock. Weakly-consolidated siltstones are present in the
basal Ogallala Group in the southern part of the mapped area, however, and fragments of
greenish gray opaline sandstones (“green quartzites” or orthoquartzites, cf. Skinner and Taylor,
1967) can be found in multiple places on low ridges eroded from the Ogallala Group. We were
unable to identify a thick, continuous, in-place layer of this kid of sandstone anywhere in the
mapped area, but the consistent occurrence of fragments on some surfaces (e.g., the nearly flat
summit of a spur of a low ridge in the southwest corner of section 15, and its continuation into
the northwest corner of section 23 of T. 34 N., R. 11 W.) indicates that localized continuous
layers were present at the surface and have since been weathered.
In keeping with soil survey maps of the area, we interpret loamy to silty surficial materials
covering much of the mapped area as a thin increment of loess, namely the Peoria Loess,
which is also classified as regolith. Some of this loessial sediment appears to have been
amalgamated with underlying unconsolidated sands of the Ogallala Group. Weathered
materials of varying thickness at the top of the Pierre Shale and also qualify as part of the
regolith.
Groundwater Resources
Potential and realized sources of groundwater in the Gross Quadrangle are the very deep,
confined bedrock aquifers of: (1) sandstones in the mid-Cretaceous Dakota Formation, (2) the
Late Cretaceous Codell Sandstone Member (one or more sandstones) in upper part of the
Carlile Shale, as well as (3) the Ogallala Group. Sand and gravel aquifers in the Ogallala
Group in this area are strongly limited in thickness and yield, and in contrast to those in
Cretaceous strata, they are also very shallow and unconfined.
Mapping units
Knsh Smoky Hill Chalk Member of the Niobrara Formation (Upper Cretaceous,
Campanian)
Light gray to dark gray shaly chalk, weathering prominently white or brownish yellow; also,
multiple, thin bentonites.
The Niobrara Formation underlies the Pierre Shale in the enclosing region. It crops out only in
the extreme northeastern corner of the Gross Quadrangle, near the Nebraska-South Dakota
border, on the lowermost valley slopes along the Missouri River. The Smoky Hill Chalk
shows weak conchoidal fracture, splits into slabs or splinters (rather than fissile chips like the
lower part of the Pierre Shale) and it can be further distinguished by its tendency to weather
brownish yellow or even slightly reddish at the land surface. Fossil oysters (Ostrea) and fish
scales can be found in the Smoky Hill Chalk. Crystals of secondary selenite gypsum
commonly appear on bedding and fracture planes in weathered strata of the Smoky Hill Chalk.
Kp
Pierre Shale (Upper Cretaceous, Campanian-Maastrichtian)
Gray to dark gray (weathering light gray, olive gray, olive brown, grayish brown, pale brown,
and pale yellow) silt shale to clay shale, with rare, very thin to thin beds of siltstone, very fine
sandstone, and bentonite, as well as some beds of chalky limestone.
The Pierre Shale is mapped on the slopes of the Missouri River Valley (Fig. 3), which make up
nearly half of the Gross Quadrangle. It attains a maximum thickness of approximately 400 ft
(122 m) within the mapped area. The lack of large, unvegetated exposures of the Pierre Shale
in the mapped area precludes a detailed lithostratigraphic study of its constituent members.
Nevertheless, most or all of the Pierre Shale is likely to be present in the mapped area,
including the Sharon Springs, Gregory, Crow Creek, DeGrey, Verendrye, Virgin Creek,
Mobridge, and Elk Butte members, or their equivalents. Landslides are common on Pierre
Shale slopes. Striking examples of terrain developed on the Pierre Shale—particularly steep
slopes prone to mass wasting—can be seen along Ninemile Gulch in the northern third of the
mapped area.
Nov
Valentine Formation of the Ogallala Group (Neogene, Middle to Late Miocene)
Dominantly unconsolidated, well-sorted, white, light brown, and pale yellow, fine to medium
sands with a distinct “salt and pepper” aspect due to the presence of dark mineral grains, in
some places containing a few granules and pebbles; also contains thin layers of greenish gray
(weathering to white), indurated opaline sandstones.
Most of the detailed stratigraphic work identifying the Valentine Formation and differentiating
it from the overlying Ash Hollow Formation was carried out farther west in the Niobrara
Valley (Skinner and Johnson, 1984), and little, if any, attempt has yet been made to
differentiate formations within the Ogallala Group in north-central Nebraska. Herein,
however, we make a tentative distinction between two formations of the Ogallala Group
(Valentine and Fort Randall formations) on the basis of inferences about litho- and
biostratigraphy. Future mapping will shed additional light on these stratigraphic assignments.
In total, there are nine University of Nebraska State Museum vertebrate fossil localities from
Boyd County. All of these sites contain Miocene-age taxa characteristic of the Barstovian
(Ba2) North American Land Mammal Age including Untermannerix, Megasminthus,
Mylagaulus, Eucastor tortus, Aelurodon ferox, Cynarctus saxatilis, Pliohippus mirabilis,
Ustatochoerus medius, Longirostromeryx, and Procranioceras (Tedford et al., 2004). This age
assignment is compatible with our interpretation of the Fort Randall Formation.
The co-occurrence of greenish gray opaline sandstones (“green quartzites” or orthoquartzites)
with unconsolidated sands verifies our assignment of strata represented by this mapping unit to
the Ogallala Group, and very probably to the Valentine Formation as well (e.g., Skinner and
Taylor, 1967).
We interpret Ninemile Hill on the Gross Quadrangle as a wind-eroded remnant of
unconsolidated sands of the Valentine Formation, rather than as an accumulation of Quaternary
eolian sands, on the basis of the regional pattern of Neogene stratigraphy as well as the lack of
evidence for depositional sandy eolian landforms in the area. Similarly, we interpret the low
hills (1900 ft or 570 m in elevation) at the western edge of the northwestern quarter of the map
area as erosional remnants of the Ogallala Group, and that particular interpretation is borne out
by observations of outcrops in an extension of those hills to the west, outside of the mapped
area. Thus, the aggregate thickness of Ogallala Group strata in the mapped area is at least 260
ft (79 m), although under any given the point the actual thickness of such strata may be
considerably less.
Sediments at the surface of this mapping unit may have been reworked by strong winds during
the Pleistocene, but we interpret the upper surface of the Ogallala Group in the mapped area as
being primarily an erosional surface.
Nofr Fort Randall Formation of the Ogallala Group (Neogene, Middle Miocene)
Massive, laminated, and rippled light gray, yellow, pale yellow, and light yellowish brown
weakly-consolidated siltstones, which locally contain nodules of authigenic carbonate; also
includes beds of light yellowish brown and pale brown well-sorted very fine to fine sands and
lenses of pebble to cobble gravels with clasts of: (1) opal-cemented siltstone, (2)
intraformational authigenic carbonate, (3) oxidized concretionary ironstone from the Pierre
Shale, and (5) crystalline rocks.
This unit is lithologically distinct from the upper part of the Ogallala Group in the map area,
and we have mapped it as the Fort Randall Formation on the basis of a comparison with
Skinner and Taylor (1967), who described the type section of that unit in the Bijou Hills in
northeastern Charles Mix County, South Dakota. Our mapping of the Fort Randall Formation
can be considered provisional pending a complete re-examination of Neogene stratigraphy in
the enclosing region of Nebraska. Fossils from the Fort Randall Formation in its type area
place it within the span of the Barstovian North American Land Mammal Age (16.3- 13.6 Ma).
Sands and gravels near or at the base of this unit locally produce small quantities of
groundwater. Gravel clasts in these deposits are frequently stained and weakly cemented by
black manganese oxides; thin beds of black pebbly sandstone also crop out discontinuously
near the Ogallala Group-Pierre Shale contact in sections 7 and 8 of T. 34 N., R. 10 W. and
elsewhere in the mapped area.
Comparatively good exposures of this unit can be found along roads and drainages in sections
17, 18, 19, and 20 of T. 34 N., R. 10 W., in the eastern part of the map area (Fig. 4).
Qal
Quaternary alluvium of smaller streams (Holocene to modern)
Clayey silt, silt, sandy silt, and sand.
Small, low-order streams in the mapped area deposit clayey silt, silt, sandy silt, and fine to
medium sand. Streams draining basins dominated by surficial materials of either Peoria Loess
or Pierre Shale should be expected to deposit more silt and clay than do other streams. Streams
draining the Pierre Shale may include deposits dominated by coarse sand- to pebble-sized
clasts of shale.
Qp
Peoria Loess (Late Pleistocene [Late Wisconsinan])
Light brownish gray, brown, pale brown, and light yellowish brown clayey silt and medium to
coarse silt.
This mapping unit designates thin Peoria Loess where it does not appear to be underlain by
more permeable sands (compare with mapping unit Qp1).
Qp1 Peoria Loess (Late Pleistocene [Late Wisconsinan]), typically atop and
amalgamated with shallow sandy sediments of the Ogallala Group.
Light brownish gray, brown, pale brown, and light yellowish brown silt and sandy silt.
This mapping unit represents a thin increment of loess is mapped, as a general rule, over
coarser and more permeable sediments of the Ogallala Group on the uplands; this increment of
loess may be partially amalgamated with the uppermost unconsolidated sediments of the
Ogallala Group, making it difficult to discern as a separate stratigraphic unit.
F
Fill
Artificial fill from local sources.
This mapping unit is intended to portray significant accumulations of artificial fill in dams,
road embankments, and other manmade structures. Most of the fill in structures in the uplands
is derived from the excavation of the Pierre Shale.
W
Water
References
Joeckel, R.M., Dillon, J.S., and Howard, L.M., 2010, Surficial Geology of the Lynch
Quadrangle, Nebraska (STATEMAP). Conservation and Survey Division, School of
Natural Resources, institute of Agriculture and Natural Resources, University of NebraskaLincoln, Lincoln, Nebraska
http://snr.unl.edu/data/geologysoils/digitalgeologicamaps/digitalgeologicmaps.asp
Skinner, M.F., and Johnson, F.W., 1984, Tertiary stratigraphy and the Frick Collection of fossil
vertebrates from north-central Nebraska. Bulletin of the American Museum of Natural
History 178, article 3.
Skinner, M.F., and Taylor, B.E., 1967, A revision of the geology and paleontology of the Bijou
Hills, South Dakota. American Museum Novitates 2300:1-53.
Tedford, R.H., Albright, L.B., III, Barnosky, A.D., Ferrusquia-Villafranca, I., Hunt, R.M., Jr.,
Storer, J.E., Swisher, C.C., III, Voorhies, M.R., Webb, S.D., Whistler, D.P., 2004,
Mammalian biochronology of the Arikareean through Hemphillian interval (Late
Oligocene through Early Pliocene Epochs). In: Woodburne, M.O. (Ed.), Late Cretaceous
and Cenozoic Mammals of North America, Geochronology and Biostratigraphy: Columbia
University Press, New York, pp. 167-231.
United States Department of Agriculture, Natural Resources Conservation Service, 2006, Land
Resource Regions and Major Land Resource Areas of the United States, the Caribbean, and
the Pacific Basin. U.S. Department of Agriculture Handbook 296.