|As sea levels fell and sediments continued to be delivered to the coast, the seaway began a slow retreat. Carbonate production decreased and marine sedimentation was dominated by mud. These conditions would continue to the end of the Cretaceous Period. Mountain building steadily progressed eastward as older rocks were thrust faulted over younger deposits. Volcanism intensified in western Montana. Evidence of volcanic activity has even been found in the seaway itself in the vicinity of Austin, Texas. As the shoreline built seaward, an area of well-drained alluvial plains developed between the mountains and the coastal plain in Arizona and western New Mexico. Environments favorable for wetland development and peat accumulation expanded south to Texas and north to Alberta. Early Campanian coals of significant economic value are present in Wyoming, Utah, Colorado, and New Mexico. A high quality coal bed 100 feet thick, for example, is currently being mined from the Adaville Formation in southwestern Wyoming.|
*See Roberts, L.N.R., and Kirschbaum, M.A., 1995, Paleogeography of the Late
Cretaceous of the Western Interior of middle North America-- Coal
distribution and sediment accumulation: U.S. Geological Survey Professional
Paper 1561, 115 p., (1 pl.).