Geologic Mapping Units: Fordyce 7.5 minute Quad
Qal1 Quaternary alluvium of West Bow Creek and Norwegian Bow Creek valley bottoms
(Late Holocene to modern)
Silty clay, silt, sand, sand and gravel
Qal1 includes historic sediments within the modern channel and recently-abandoned
channels of West Bow Creek and Norwegian Bow Creek. Qal1 sediment, predominantly fine
to coarse sand occurs as point bars, channel bars, and bed load deposits along many reaches
of the modern channel.
Qal2 Quaternary alluvium of small tributary valleys (Holocene to modern)
Thin deposits of silty clay, silt, very fine sandy silt, and/or sand and fine gravel.
These sediments occur in small tributary valleys draining the uplands adjacent to West Bow
Creek and Norwegian Bow Creek. The deposits may be massive or thinly bedded. The
alluvium is silt and silty fine sand where Peoria Loess is the dominant upland source
material, and sandy silt, sand, and or sand and fine gravel where glacial outwash provides
sediments to the valleys.
Qal3 Quaternary alluvium (Late Holocene to modern)
Silty clay, silt, sand, sand and gravel
Qal3 represents alluvium within the valley bottoms of West Bow Creek and Norwegian Bow
Creek. The valley bottoms are composed of gray to very dark gray silty clay to sandy silt
alluvium. Fine to coarse sand and gravel occur as point bars, channel bars, and bed load
along many reaches of the modern channels. This mapping unit includes low (< 1.5 meter
above modern floodplain) remnant alluvial deposits present throughout the tributary valleys
that were not mapped separately. The low terrace fill includes one of two formally
recognized stratigraphic units: The Roberts Creek member is composed of massive or
horizontally bedded gray to dark gray clay, silt loam and sandy loam. The Honey Creek
Member is composed of gray to grayish brown, cross bedded silt loam grading downward to
bedded sand and gravel. Although conspicuous in the field, the two fills cannot be separately
mapped at this scale. Both units include more sand than in other localities in eastern
Nebraska due to the predominantly sandy source materials for the sediments. This unit is
frequently capped with ~ 1 meter of horizontally stratified, Historic-age alluvium which
buries the soil formed in the upper portions of the underlying terrace fill.
Qsw Quaternary Slopewash (Late Pleistocene to modern)
Thin deposits of silt, very fine sandy silt, and/or sand.
These sediments occur in small, ephemeral tributary valleys and valley margins draining the
uplands adjacent to West Bow Creek and Norwegian Bow Creek. These alluvial and colluvial
sediments occupy narrow valleys and gullies and are composed of silt, silty sand and fine
sand. The sediments range from massive to thinly bedded deposits. Slopewash deposits
occur as thin mantles of sediment along small valley margins and in the headwater portions
of ephemeral drainages in the uplands. They are composed of silt, sand, and/or sand and
gravel derived locally from loess, outwash, or till.
Qaf
Quaternary Alluvial fan (Early through late Holocene)
Silt, fine sandy silt, and sand
Alluvial fans found along the valley margins. Fans are composed of massive to thin bedded
silt, fine sand, and fine gravel beds, often with buried soils that slope away from the uplands.
Qswf Quaternary slope wash /alluvial fan complex (Holocene)
Bedded to massive silt, fine sandy silt, sand, and sand and gravel
This mapping forms aprons of slope wash deposits and small alluvial fans along West Bow
Creek and Norwegian Bow Creek valley margins. Individual fans merge with aprons of
slope wash and thus cannot be meaningfully segregated due to contour interval and map
scale. This unit is composed of silt and silty sand where the source material is Peoria Loess,
and sand to sand and gravel where the source area is glacial outwash.
Qat1 Quaternary low terrace (Early to Middle Holocene and modern)
Silty clay, silt, and silty sand grading downward to bedded sand and gravel
This mapping unit includes small, unpaired, remnant terrace surfaces in the small upland
tributaries and along the West Bow Creek and Norwegian Bow Creek valley bottoms. Terrace
fills are composed of the Gunder Member of the DeForest Formation. This unit is composed
of brown to yellowish brown silt loam and sandy loam. The lower portions of exposures
include bedded sand and gravel. The Qat1 surfaces stand approximately 1 to 2 meters above
the Qal surfaces where both are present. This unit is commonly capped by a thin
(approximately 0.5 meter) mantle of horizontally stratified, recent alluvium which buries the
soil formed in the upper portions of the underlying terrace fill. This unit includes more sand
than in other localities in eastern Nebraska due to the predominantly sandy source materials
for the sediments.
Qes
Quaternary Eolian Sand (Late Pleistocene and Early Holocene)
Fine and very fine sand
Eolian sand occurs as small, discontinuous deposits throughout the mapping area. Eolian
sediments are derived from outwash and/or local alluvium. The sand occurs in a variety of
positions on the landscape and mantles a variety of mapping units. Extensive areas of Qes in
the northwestern portion of the quadrangle may include sand derived from the Missouri
River valley.
Qat2 Quaternary high terrace (Late Pleistocene)
Fine to coarse sand, sand and gravel, and gravel
This mapping unit includes alluvial sand and gravel deposits. Sand is mostly planar bedded
and approximately 10 meters thick. Thin gravel lag occurs in the lower portions and is very
fine to coarse-grained and includes scattered cobble to boulder-sized clasts. This unit occurs
as extensive remnants along the southern margins of the West Bow Creek valley. Although
this unit was not encountered by coring and auger borings we suspect that it occurs along
Norwegian Bow Creek as well. Terrace treads frequently include a thin mantle of Peoria
Loess(Qp).
Qp
Peoria Loess (Late Pleistocene)
Eolian silt to very fine and fine sandy silt
Peoria Loess is typically massive to laminated silt and very fine sandy silt. Thickness ranges
from less than 2.5 meters up to 12 meters or more in the mapping area. Peoria loess overlies
pre-Illinoian glacial outwash, glacial till, and late Pleistocene terrace deposits. The upper
portions are oxidized brown to light gray. A thick soil has developed in the upper portions
of the deposit. The lower portions range from massive silt to thin bedded silt and fine sand.
Peoria Loess in this area is subdivided into two mapping units: thick loess (Ql1, greater than
2.5 meters) composed of silt overlying a variety of older deposits, and thin loess (Ql2, less
than 2.5 meters) which is composed of fine sandy silt overlying fine to medium sand or till.
Ql2 includes eolian sand deposits which cannot be meaningfully separated at this scale. The
sand is mostly derived from glacial outwash, although valley bottoms, terraces, and till may
be locally important sources as well.
Ql1
Silty loess generally greater than 2.5 meters thick
Ql2
Fine sandy loess generally less than 2.5 meters thick
Qa U Undifferentiated Quaternary alluvium (Pleistocene)
Sand and gravel
This mapping unit consists of sand and gravel which occurs on alluvial deposits found in the
northwestern portion of the quadrangle. The lithology of the gravel is mixed, including
granitic and metamorphic clasts, Sioux Quartzite, white quartzite, blue and white opaline
silica, anorthosite, Niobrara Chalk, iron-manganese concretions, and petrified wood.
Exposures are limited to a few slumped roadcuts and as a result bedding styles have not been
observed. Fossils are present in the fills. The presence of a Mammuthus unciform bone from
the fill tentatively dates the deposit to the Pleistocene. Other fossils include tortoise shell and
horse teeth that are likely re-worked from older Ogallala Group sediments.
Qt
Quaternary Glacial till and adjacent colluvial sediments derived from till (Lower or
Middle Pleistocene)
Clay to boulder diamict
Gray to dark gray matrix-supported diamict. Weathers to light brown and orange-yellow,
with secondary carbonate (white in color) and manganese (black) and iron (yellow) oxide
coatings that are found along fractures. Small, isolated exposures occur throughout the
quadrangle. Many of the buried contacts are abrupt and erosional. Till thickness varies in
the region, but can be up to 45 m (~ 148 feet) thick. Sand and sand and gravel units that are
up to 1 m in thickness are commonly found within the till in the region. Till frequently
overlies sand, sand and gravel and silty deposits of various thicknesses in the region.
Qo
Quaternary Glacial outwash (Lower or Middle Pleistocene)
Poorly-sorted sand and cobbly gravel
Clast sizes range from sand to cobbles, with few boulders. Scattered boulders commonly
occur on the surface, presumably remnants due to removal of the finer sediments through
eolian and/or fluvial erosion. Gravel and larger clasts include a variety of igneous and
metamorphic lithologies including quartzite, granite, granodiorite, and distinctive Sioux
Quartzite which is a common hard and pink rock in the local fields. Few locally-derived
sedimentary rocks are also present. Sediment from this unit has been utilized in numerous
gravel pits in the region.
P
Pits
Bedded sand and gravel with sandy silt cover
This unit includes active pits as well as small, inactive pits which are shown by the borrow
pit symbol on the topographic base map. Sand pits occur in areas of eolian sand or sandy
outwash. Pits in glacial outwash include sand and gravel along with cobble to boulder sized
clasts.
Kn
Niobrara Formation (Upper Cretaceous)
Light gray to gray (white, pale yellow and yellowish brown) shaly chalk
The Niobrara Formation is mapped in extensive areas of the northeastern and east-central portions of
the quadrangle, although actual outcroppings of rock are limited in number and size and scattered.
Where mapped the Niobrara Formation is mostly obscured by a weathering profile which includes a
dark grayish brown and brown soil. Mapped areas also include thin (< 1 meter) and discontinuous
mantles of glacial outwash, alluvium, or eolian sand. Outcrops are thin bedded and break readily into
10-30 cm thick plates. Gypsum crystals and blades are present along bedding planes and fractures
locally.
Kp
Pierre Shale (Upper Cretaceous)
Gray to dark gray silty shale to clay shale
The Pierre Shale is mapped over a small area in the northern portion of the Quadrangle. No true
outcroppings were observed, although coring and hand-auger borings demonstrate that it occurs at or
close to the surface locally. Surficial deposits include a dark gray and dark grayish brown clay loam
soil that may contain secondary carbonates at the surface. The Pierre Shale may be locally capped by a
thin (<0.5 meter) erosional remnant of till or loess. Test hole 5-A-13 encountered 12 meters of Pierre
Shale.