MODERN SEDIMENTS, WATER, AND WETLANDS:  Includes features where recent geologic processes have been occurring.  Recent geological 
processes related to erosion and transportation of sediment have sorted these materials and deposited them.  These map units are Holocene (10,000 
years ago until Present) in age and include lakes, streams, wetlands, sand bars, beaches, flood plains, fans, terraces, and dunes.
Water:  Open water including lakes, streams, and the Mississippi River.
Wetlands - Organic Sediments:  Partially decomposed plant material, silt, and clay, found in or around shallow lakes, marshes, and 
peatlands.  Wetlands can be observed throughout the county and are common in the low areas between the drumlins.
Alluvial Sand Bar Sediments:  Stratified sand, gravel, and small cobbles with occasional layers of silt.  The sediment is moderately well- 
to well-sorted with thin layers, lenses, and pockets of very well-sorted sands and gravels.  The sand bars are exclusively found within the 
Mississippi River and are constantly changing in size and shape.
Beach Sediments:  Fine sand, coarse sand, and fine gravel, with occasional layers of silts, clays, and coarser gravels.  The sediment is well- 
to very well-sorted, however, it occasionally contains organic debris.  These beach sediments resulted from the reworking of glaciofluvial 
delta sediments by modern day wave action.  This unit is found in the western part of the county around Little Rock Lake.
Flood Plain Sediments:  Silt, fine sand, coarse sand, gravel, and clay layers, overlying coarser sand and gravel layers and lenses.  The 
sediment is generally moderately- to well-sorted and is interpreted to be alluvial overbank and stream bottom deposits.  Within this unit, 
coarser sediment is typically found in abandoned and modern channels, point bars, and river bars.  Alluvial sediments exist throughout the 
county in valleys and low lying areas where drainage occurs as channelized flow.
Alluvial Fan Sediments:  Stratified fine sand, coarse sand, and gravel, with thin layers of silt and clay.  The sediment is moderately well- 
to very well-sorted.  Coarser material is typically found at the base of the fan (that is, the bottom of the deposit) and near the mouth of 
the fan.  The sediment is generally deposited at the end of tributary streams where they enter larger valleys, due to a change in slope and 
slowing of the water flow.  Alluvial fan sediments occur near Little Rock Lake and in the southern part of the county.
Alluvial Terrace Sediments:  Fine sand, coarse sand, and gravel, with occasional layers of silt and clay.  The sediment is moderately well- 
to well-sorted with thin layers, lenses, and pockets of very well-sorted sands and gravels.  The terraces are typically covered by 6 inches to 
2 feet of silt, clay, and/or fine sand layers derived from flood plain sediments.  Alluvial terrace sediments are typically found along the 
Mississippi River as well as along tributary streams that enter the Mississippi River.
Eolian Sand Dune Sediments:  Stratified fine sand and silt, with occasional layers and lenses of coarse sand.  The sediment found within 
these dunes is well- to very well-sorted.  These wind blown sediments form large dunes (>100 feet long), which are generally oriented in a 
northwest to southeast direction (reflecting the prominent wind direction).  Smaller scaled ripple marks are found throughout the dunes.  
These eolian sand dune sediments are found throughout the outwash plain in the northwestern part of the county.
Eolian Sediments Modifying Glacial Sediments:  Discontinuous and patchy stratified eolian fine sand and silt, with occasional layers and 
lenses of coarse sand overlying glaciolfluvial sand and gravel sediments and unsorted till sediments (which include clay, silt, sand, gravel, 
cobbles, and boulders).  These till sediments were modified by glaciofluvial outwash sands and gravels, which were later modified by wind 
blown sands.  These units are discontinuous, with till most often found on the tops of hills, and outwash and wind blown sediments in the 
lowlands.  These sediments are encountered in a large area between the large outwash plain in northwestern Benton County and the till 
sediments that dominate most of the central portion of the county.

GLACIAL SEDIMENTS:  Includes all material (clay, silt, sand, gravel, and boulders) that was transported by glaciers and deposited directly from the 
glacier, from the ice as the glacier retreated, or by running water associated with the glacier.  The glacial drift, which covers almost all of Benton County, 
is Pleistocene (2.5 million to ~10,000 years ago) in age.
Des Moines Lobe Sediments:  Consists of sediments that were transported by both the main Des Moines lobe and by a relatively small northward
extension, the Duelm bulge, that flowed north from the main lobe to the southern edge of Benton County as the glaciers were advancing.  Later, the
sediments were deposited as the glacier melted.  The sediment within these mapping units therefore came from a northwestern source.  The rock 
types typically found in this sediment include carbonates, granites, shales, and other rock sources from the Winnipeg area.  These rock types are 
typically of lesser quality for construction aggregate than the Superior lobe sediments.
Outwash Channel Sediments:  Stratified coarse and fine sands, with occasional layers, lenses, and pockets of gravel.  The sediment is 
generally a well- to very well-sorted sand, however, it does contain several layers of moderately- to well-sorted sand and gravel scattered 
throughout.  The gravel percentage in these cases is usually less than 15 or 20%.   These sediments were deposited in the southern-most 
part of the county when a retreating glacier (Des Moines lobe) from the south melted, creating a very large outwash channel (Duelm 
Channel) that drained to the east. 
Till - Moraine Sediments:  Unsorted clay, silt, sand, gravel, cobbles, and boulders deposited as the ice gradually melted.  As the glacier 
retreated to the south, it left behind a thin layer (<20 feet thick) of unsorted glacial till.  This till is discontinuous and patchy, and typically 
overlies older Superior Lobe sediments.  These unsorted till sediments are found in the southwestern portion of the county, the only part 
of Benton County that was covered by the Des Moines Lobe.

Superior Lobe Sediments:  Consists of sediments that were transported from the northeast as the glacier was advancing, and subsequently 
deposited as the glacier retreated.  The sediment within these units came from a northeastern source.  The rock types typically found in this sediment 
include volcanic rocks from the Lake Superior region, such as gabbros and basalts.  These sediments typically consist of very high quality material for 
construction aggregate.
Glaciofluvial Terrace Sediments:  Fine sand and coarse sand, with occasional layers of sand and gravel.  The sediment is generally 
well-rounded and well- to very well-sorted sand.  This sediment is interpreted to have been deposited in a very large outwash channel created 
from meltwaters from the north flowing southward.  These glaciofluvial terrace sediments are found paralleling the current Mississippi River.  
Glaciofluvial Outwash Delta Sediments:  Stratified coarse and fine sands, with occasional layers, lenses, and pockets of gravel.  The 
sediment is generally well- to very well-sorted sand that contains several layers of moderately- to well-sorted sand and gravel scattered 
throughout.  Much of the coarser material was deposited at the base (that is, the bottom of the deposit) and mouth of the delta about 10 
miles north in Morrison County.  The outwash delta sediments are found in the western part of the county.  Those sediments found in the 
northern part of this mapping unit are slightly coarser than those sediments found in the southern part.
Glaciofluvial Fan Sediments:  Fine sand, coarse sand, and gravel, with occasional layers of silt and clay.  Most of the fans consist of finer 
sediment (mainly sands).  The sediment was deposited as large fan-shaped features on, within, and/or underneath the retreating ice.  The 
sediment primarily exhibits stratified layers, with some scattered areas that were thrusted, collapsed, and faulted due to the presence of 
ice blocks that melted within the fan.  These fans formed at the ends of glaciofluvial outwash channels throughout the county where they
entered larger valleys.
Ice Contact - Esker Sediments:  Stratified silt, fine sand, coarse sand, and gravel, with occasional clay and silt layers throughout.  These 
esker features form long, narrow, sinuous ridges and are composed primarily of stratified sand and gravel, but also include areas of 
non-stratified sand and gravel that were part of ice blocks that were incorporated into the eskers and later melted in place.  They are 
interpreted to have been formed by sub-glacial streams flowing between ice walls or in ice tunnels within the stagnant glacier, and were 
left behind as the ice melted.  These esker features are found throughout the eastern part of the county.
Ice Contact - Kame Sediments:  Stratified fine sand, coarse sand, and gravel, with occasional clay and silt layers throughout.  These kame 
features are low mounds, knobs, hummocks, or short irregular ridges composed primarily of stratified sand and gravel.  They are 
interpreted  to have been deposited in low areas or holes on the surface of the ice as fans or deltas. They are typically located at the ends of 
outwash valleys in the northeastern and southern parts of the county.
Glaciolacustrine Sediments:  Silt, clay, fine sand, and medium sand.  These sediments are typically deposited as thin layers with distinctive 
summer and winter banding called varves.  The sediment is interpreted to be derived from suspended material brought by meltwater 
streams into Glacial Lake Mayhew.  The sediments are slightly coarser towards the north, becoming finer (silt/clay) towards the south, where
there was deeper, calmer water.  Glacial Lake Mayhew (which was located primarily within what is now known as Mayhew Lake Township) 
was located in the south-central part of the county.
Outwash Channel Sediments:  Stratified fine to coarse sand and gravel, with occasional layers of silt, fine sand, cobbles, and small 
boulders.  The sediment is interpreted to be derived from large meltwater channels that ran on, within, and/or underneath the retreating 
glacier.  Much of the sediment is collapsed and discontinuous due to melting, buried ice blocks, and deposition on top of the ice.  These 
outwash channels are located throughout the county, but are primarily concentrated in the eastern third.  The channels are primarily oriented 
from the northeast to southwest.
Outwash Channel Sediments Modifying Glacial Sediments:  Discontinuous, patchy, and thin stratified fine to coarse sand and gravel, with 
occasional layers of silt, fine sand, cobbles, and small boulders, typically overlying unsorted till sediments.  The outwash sediment is 
interpreted to be derived from smaller meltwater channels that ran on, within, and/or underneath the retreating glacier.  These outwash 
sediments are primarily found in the eastern third of the county.
Till - Ground Moraine Sediments:  Unsorted clay, silt, sand, gravel, cobbles, and boulders, deposited as the ice gradually melted.  As the 
ice melted, it left behind these large, thick, unsorted sub-glacial tills.  Ground moraines produced in this way commonly leave behind a 
gently rolling topography.  This type of till unit occasionally contains small, thin, discontinuous layers and lenses of sand, gravel, and 
washed till.  This till unit can be observed in the eastern part of the county.
Till - Drumlinized Moraine Sediments:  Unsorted clay, silt, sand, gravel, cobbles, and boulders representing a sandy till, with areas of fine 
silt, fine sand, and coarse sand, with occasional gravel pockets.  Drumlins are low, smoothly rounded, elongated oval hills, mounds, and/or 
ridges of compacted glacial till that were created under the ice margin and were shaped by the ice flow.  These features were formed with their 
long axis parallel to the direction of the flow of the ice.  The drumlins typically consist of unsorted sediment, however, sorted sediments, 
such as sand and occasional gravels, are found in the low areas between the drumlins.  These drumlinized till sediments are the dominant 
unit within the county.

BEDROCK  UNITS:  Primarily consisting of massive exposures of granitic rocks (granites, granodiorites, and diorites) with intrusive dikes throughout.  
These Precambrian rocks are typically covered by glacial sediments ranging in thickness from a few feet to several hundred feet.  These granitic rocks 
are Precambrian (4.65 billion to 570 million years ago) in age and more specifically Early Proterozoic (2.45 billion to 1.75 billion years ago) in age.
Granite:  Precambrian bedrock units consisting primarily of pink and gray granitic rocks.  Variations of these granitic rocks include 
granodiorites and diorites.  The outcrops of granitic rocks in the northeastern part of the county are primarily all pink to salmon colored 
granites on the Foley Batholith, which underlies most of Benton County.  Many of the outcrops in the southwestern part of the county also 
consist of these pink granites, however, as you get closer to the Mississippi River, the rocks become light gray granodiorites.  Small 
intrusive dikes occasionally occur throughout the granitic units.
