Kansas Rocks and Minerals - Gravel, Conglomerate, Till


Gravel deposits vary greatly in mineral composition, size, shape and color. The term "gravel" usually means a rock composed of particles ranging from sand to pebble or boulder sizes (2mm +)(Table 2). Glacial outwash gravels or Tills occur in northeastern Kansas. Till (sometimes called boulder clay) forms low, rounded rolling hills covered by loess, soil and vegetation. Consequently very few good outcrops occur. Materials that make up the till include: limestone, sandstone, quartzite, granite, basalt and many others. The quartzite boulders are hard red rocks and their nearest source is southeastern South Dakota. Most of the these rocks have been weathered by the melting of the ice sheets. Often they have been weathered so much that some granites can be crumbled with your bare hands. Some of the quartzite boulders are so hard and well preserved that it cannot be broken with a hammer. Some of the glacial till have been scratched or polished by the glacial action of rubbing and grinding of the material against other rocks in the ice.
Stream deposited gravels underlie the High Plains in the western part of the state. The material that makes up these gravels include: feldspar, agate, quartz, native copper, granite, basalt, quartzite, etc.

In the Flint Hills and in southeastern Kansas, gravels consist mainly of just one mineral - chert or flint, which are weathered from Paleozoic limestones. These brown, hard, resistant gravels commonly cap the uplands in Anderson, Cowley, Elk, Greenwood, Lyon, Morris, Wabaunsee, Geary, Riley, Pottawatomie and Marshall counties.

Conglomerate is a hardened, generally cemented gravel and like sand, silt and clay, has been formed by the breaking down of older rocks and by later redeposition. Commonly it is found interbedded with layers of sandstone. It also occurs at the base of Pennsylvanian formations, for example near the town of Baldwin in Douglas County. Small areas of hard conglomerate are found in many gravel pits in Tertiary and Pleistocene deposits. Conglomerate and gravel are used in making concrete, in surfacing roads and as railroad ballast.

Size

Rounded, Subrounded, Subangular

Angular

Fragment

Aggregate

Aggregate
> 256mm Boulder Boulder gravel, Boulder conglomerate Rubble
256 - 64mm Cobble Cobble gravel, cobble conglomerate  
64 - 4mm Pebble Pebble gravel, pebble conglomerate Breccia
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  

Table 2 - Particle Size - Sedimentary Rocks