By the close of the Precambrian, the region was intermittently submerged under shallow seas, and for 500 million years various types of sedimentary rocks were formed. During the Paleozoic (542 to 251 million years ago) sandstone, shale, limestone and dolomite were deposited. Though most of these sedimentary rocks have since eroded away from the central Teton Range, they are still evident on the northern, southern and western flanks of the range. One notable exception is the sandstone Flathead Formation which continues to cap Mount Moran. Sedimentary layering of rocks in Alaska Basin, which is on the western border of Grand Teton National Park, chronicles a 120-million-year period of sedimentary deposition. Fossils found in the sedimentary rocks in the park include algae, brachiopods and trilobites. Sedimentary deposition continued during the Mesozoic (250-66 million years ago) and the coal seams found in the sedimentary rock strata indicate the region was densely forested during that era. Numerous coal seams of in thickness are interspersed with siltstone, claystone and other sedimentary rocks. During the late Cretaceous, a volcanic arc west of the region deposited fine grained ash that later formed into bentonite, an important mineral resource.