Silt and Siltstone

Silt is a common sedimentary rock composed of tiny particles smaller that sand size yet larger than clay size (1/16 - 1/256 mm). It is found in stream deposits, lake beds and most commonly, in Kansas, as loess. Loess is wind-blown silt which covers much of the High Plains of western Kansas and along bluffs of the Missouri River in northeast Kansas. It occurs to some extent in many other areas in the state. Loess is typically a yellowish-buff porous silt that outcrops with steep faces along hillsides and valley walls. Loess contains white or cream-colored concretions (1-2" in diameter) which are composed of calcium carbonate and have been called Loess Kinchen (little children of the loess).

Some of the finest and thickest soils in the world are formed in the upper part of thick deposits of loess. As wind moves the small particles only, a soil that develops is free from boulder and pebbles. Loess deposits are built up by successive dust storms (not unlike the dust storms of the 1930's).

More than 90% of the soil in Thomas, Sherman, Cheyenne, Greeley, Hamilton, Wichita, Scott, Lane and other western counties consist of the upper part of these loess deposits.

In northeastern Kansas along the Missouri River, loess can range in thickness from 60 to 100 feet. Farther from the river, it may be no more than 5 feet thick. These deposits are the result of fine material ground by the advancing ice sheet and deposited on the flood plains by glacial meltwater. The material was later reworked by winds. It is thought that most of the loess in northeastern Kansas was laid down more than 50,000 years ago.

Siltstone is consolidated or compacted silt. This rock may be found as thin slabby beds in many of the Pennsylvanian formations of eastern Kansas. Many siltstones contain layers rich in tiny flakes of mica which glitter in the sun. The mica is concentrated along the bedding planes where the rocks break easily.

Table 2 - Particle Size
Sedimentary Rocks

Size

Fragment

Rounded, Subrounded, Subangular
Aggregate

Angular
Aggregate
> 256mm Boulder Boulder gravel, Boulder conglomerate

Rubble
256 - 64mm Cobble Cobble gravel, cobble conglomerate

  Breccia
64 - 4mm Pebble Pebble gravel, pebble conglomerate
4 - 2 mm Granule Granule gravel  
2 - 1/16 mm Sand Sand, Sandstone Grit (1/2 - 1 mm)
1/16 - 1/256mm Silt Silt, Siltstone  
< 1/256 mm Clay Clay, Shale