nearly featureless plain, perhaps resembling the vast flat areas in which similar Precambrian rocks are now exposed in central and eastern Canada. At the close of Precambrian time, about 600 million years ago, the plain slowly subsided and the site of the future Teton Range disappeared beneath shallow seas that were to wash across it intermittently for the next 500 million years. Paleozoic deposition Deposition resumed in the Cambrian period and continued through the Paleozoic era, creating nine major formations which together are 4,000 feet (1,200 m) thick (the only geologic period in the Paleozoic not represented is the Silurian). These formations were deposited in a shallow sea and later became a discontinuous mix of dolomites, limestones, sandstones, and shales. These formations are relatively undeformed for their age even though periodic upwarp exposed them to erosion, creating uncomformities. Fossilized brachiopods, bryozoans, corals, and trilobites are found in the carbonate rocks with the best examples found outside of the park in the Alaska Basin. The most complete examples of these formations are found to the west, north, and south of park borders. On the edge of a shallow seaway Early in Cambrian time a shallow seaway, called the Cordilleran trough, extended from southern California northeastward across Nevada into Utah and Idaho. The vast gently rolling plain on Precambrian rocks to the east was drained by sluggish westward-flowing rivers that carried sand and mud into the sea. The site of the Teton Range was part of this plain. Slow subsidence of the land caused the sea to spread gradually eastward during Middle Cambrian time flooding the Precambrian plain. Sand accumulated along the beaches just as it does today. As the sea moved still farther east, mud was deposited on the now-submerged beach sand. In the Teton area, the oldest sand deposit is the 175 to 200 feet (53 to 60 m) thick Flathead Sandstone. The partly marine Flathead Sandstone is reddish-brown, very hard, brittle and exposures can be found on the north and west flanks of the Teton Range and Gros Ventre Range. Mud was laid down on top of the Flathead Sandstone as the shoreline advanced eastward across the Teton area. The resulting soft greenish-gray shale with beds of purple and green sandstone near its base, became the 100 feet (30 m) thick Wolsey Shale Member of the Gros Ventre Formation. Some shale shows patterns of cracks that formed when the