Crete South Mapping Units Qal Recent Alluvium Holocene Silty Clay

Crete South Mapping Units

Qal Recent alluvium

Holocene

Alluvial and floodplain deposits of rivers and streams. These sediments are directly adjacent to streams, and underlie active flood channels. The upper portion of this unit are generally fine-grained sediment (silt and silty clay) that overlies varying thicknesses of coarser sediment (sand and sand and gravel). Generally the overlying silty sediments are less than 2 m thick. Qal alluvium directly overlies glacial sediment (primarily till), loess or Cretaceous bedrock. Qal sediments may be inundated in seasonal flooding events.

Qal1 Higher alluvium of smaller streams

Holocene to late Pleistocene

Clay to coarse sand

Older alluvium deposited by smaller streams tributary to the Big Blue River and Salt Creek. The surface of these sediments are 3-7 m above the present river level. Sediments are generally 2-3 m thick and directly overlie glacial sediment, loess or older sand and gravel units. Few alluvial features are visible on the surface of these deposits.

Qalt Alluvial terraces in larger stream valleys

Mid to late Pleistocene

Silt to silty clay

Higher older terraces of the Big Blue River. Terrace treads are ~15-20 m above modern stream levels. Terrace treads are covered with ~3m of Peoria Loess or re-worked silt and clay from surrounding uplands. Terrace fills are mid to late Pleistocene in age.

Qab1 Recent alluvium of the Big Blue River

Holocene

Clay to coarse sand

Recent alluvium and deposits of the historical floodplain of the Big Blue River. The Big Blue River was entrenched historically and the active channel lies approximately 4 meters below the top of the Qab1 sediments. Most areas of Qab1 are relatively small in area surrounding the deeply entrenched Big Blue River, or are located within recently cut off meanders. Sediments on the historical flood plain are less than 3 m thick. Alluvial features are distinctly visible on areas mapped as Qab1. These deposits are routinely inundated during large-scale flood events.

Qab2 Higher alluvium of the Big Blue River

Holocene

Clay to coarse sand

Older alluvium of the Big Blue River. The surface of these deposits are 6-10 m above the present river level. Sediments are generally 2-3 m thick and directly overlie glacial sediment. Few alluvial features are visible on the surface of these deposits. Qab3 Higher alluvium of the Big Blue River

Holocene to Latest Pleistocene

Clay to coarse sand

Older higher alluvium of the Big Blue River. Deposits are 1-3 m thick. Alluvial features are not present on the Qab3 surfaces. Sediments are 11-16 m above the current river level. Some areas are eroded by smaller tributaries of the Big Blue River, or are covered by small alluvial fan deposits sourced from the adjacent uplands.

Qsw Slope wash

Holocene

Clay to silt, with local sand to boulders

Deposits located at the base of slopes. Generally, slope wash is sourced from adjacent loess or till. Slope wash sourced from till contains a concentration of large boulders mixed with clay. Slope wash deposits are generally less than 3 m thick and typically decrease in thickness further from the sediment source.

Qp Peoria Loess

Late Pleistocene

Silt to silty clay

Late Pleistocene aged loess. Loess is a wind-blown silt and clay deposit. Peoria Loess is tan-yellow-gray with frequent accumulations of secondary carbonates. Peoria Loess thickness ranges from 1-8 m in the mapping area. Locally, some areas may be less than one meter in thickness near unit boundaries and on steep slopes.

Ql Loveland Loess

Mid-Pleistocene

Silty clay to clayey silt

Loveland Loess is older than the overlying Peoria Loess. Loveland Loess can be distinguished from Peoria Loess by its characteristic red or pink color and finer clay-rich texture. Some areas mapped as Loveland Loess along the eastern side of the Big Blue River Valley contain areas of reworked Loveland Loess mixed with older alluvial sands and gravels.

Qt Glacial till

Mid-Pleistocene

Clay, with sand and gravel

Tills in Nebraska were deposited by glaciers between ~ 2,500,000 and 600,000 years ago (Pre-Illinoian aged). Tills in the mapping area are generally hard, reddish brown, or bluish gray clay to silty clay with varying amounts of sands and gravels.

F Fill

Built up areas where natural sediments are obscured, such as cities, gravel pits and dams.

W Water