The Badlands are a landscape whose apparent stillness masks constant change, as hard infrequent rain attacks loosely cemented clays and sandstones, gouging new gullies and carrying off as much as two to four inches of surface a year from steep, unprotected slopes. At night the buttes and outcroppings take on eerie shapes, and dawn brings shifting colors. Over time this jagged land will become smooth and rounded through one vast shaping motion in which the river cutting through soft rock, burning lignite, and sudden downpours all play a part. Lignite coal seams, sparked by lightning or grass fires, can smolder underground, and the intense heat bakes surrounding rock into red clinkers visible on nearby hilltops. Huge house-sized masses of earth also shear from bluffs and creep downhill as intact slumpblocks, moving so slowly that most shear scars weather away before the blocks finally come to rest.