 Foothill regions are often covered in weathered stone or broken slate, or consist of uncovered bare rock (usually igneous, quartzite, sandstone, or shale).The soil of intermountain basins usually consists of clay, gravel, sand, silt, and volcanic ash, much of it laid down by lakes which covered the region during the Oligocene 33 to 23 million years ago.Tablelands are often topped with argillite gravel and weathered quartzite, occasionally underlain by shale.