The lithology of the lighter and darker red shales at the Cheltenham Badlands site is categorized as smectite-poor, due to the clay content, ranging from 58% to 68% clay, and the dominant clay minerals, illite and chlorite. The non-swelling illite in the shales can cause high pressures with repeated wetting and drying, failing the shale structures. The difference in the water absorption between illite and chlorite clay minerals can cause differences in swelling which results in larger shards of the shales breaking into smaller shards. Once the shards are reduced to tiny and flaky shards, they become compacted and smooth; however, surface cracks during drying periods can lead to further erosion.
