warm, muddy, shallow sea with abundant marine mollusks called the Sundance Sea started to spread from Alaska south to Wyoming. More than of soft gray fossil-rich shale and thin beds of limestone and sandstones were deposited. After the sea withdrew, the Jurassic and Lower Cretaceous-aged Morrison and Cloverly Formations were laid down on low-lying tropically humid flood plains. These formations erode into colorful badlands of red, pink, purple, and green claystones and mudstones, and yellow to buff sandstones. Large and small dinosaurs roamed the abundant vegetation and swamps. Western Interior Seaway expands and retracts Brightly colored rocks continued to be deposited as the final period of the Mesozoic, the Cretaceous dawned. Another warm, shallow sea, the Western Interior Seaway, then partly and sometimes completely covered the Teton region along with most of Wyoming, About of drab-colored sand, silt, and clay with some coal beds, volcanic ash layers, and minor amounts of gravel were deposited. The Western Interior Seaway retreated eastward from the Teton region around 85 million years ago, marked by deposition of the Bacon Ridge Sandstone. Extensive coal swamps formed along and followed the retreating seashore, leaving coal beds 5 to thick in the Upper Cretaceous strata. Examples of these coal beds are visible in abandoned mines found in the eastern margin of the park. A modern analog of this depositional environment is the hot and humid climate of the Florida Everglades. About of compacted plant material is needed to form of coal. Fine-grained volcanic ash from volcanoes west and northwest of the Teton area was periodically deposited in the quiet shallow water of the Western Interior Seaway throughout Cretaceous time. Ash deposited in this manner was later altered to bentonite; a type of clay used in the foundry industry and as a component of oil well drilling mud. Elk and deer in Jackson Hole use exposures of bentonite as a (bitter) salt lick. Bentonite swells when wet, which causes landslides that sometimes block access roads into Jackson Hole. Cretaceous-aged rocks in the Teton region form part of a huge east-thinning wedge of crust that is locally almost thick. Most of these rocks are from debris eroded from slowly rising mountains in the west. Bentonite, crude oil and natural gas are commonly produced from the various Cretaceous formations. Enormous coal reserves, with some beds reaching 50 to thick, are a potentially vast resource. By the end of the Cretaceous,