Availability of raw materials is not uniform, however. In some areas the glacial sediments contain numerous cobbles. In other areas, these sediments consist primarily of sand, clay or other fine grained sediments containing little usable rock. This is the dry bed of Glacial Lake Agassiz, an area also known as the Red River Valley. Rock of any kind is rare or absent in the lakebottom plain.
Cobbles are, however, abundant in beaches of the extinct lake.
The lake stabilized at different levels during the several thousand years of its existence, and separate beaches were formed at each level. They presently appear as sand and gravel ridges rising above the surrounding clay plain. This distribution is presently hard to explain in terms of geological and glacial history, and needs further examination. SRC is found over a very wide area, from eastern Saskatchewan through southwestern Manitoba, the eastern Dakotas, much of Minnesota and presumably into northern Iowa. This material is extremely variable, especially in color.