|A broad shallow sea, the Western Interior Seaway, expanded across the North American continent from the Arctic Ocean to the Gulf of Mexico. Limestone accumulation in the ancient Gulf of Mexico suggests that marine environments supported a prolific warm-water ecosystem. Mountains were uplifted in Idaho, Nevada, and Utah. Sediments eroded from these mountains built broad, coarse-grained alluvial fans along the east side of the mountain front and finer-grained coastal plains further east, near the seaway. Peat accumulated on coastal plains, where rising sea level provided water-logged conditions and allowed for development of freshwater wetlands. Ancient wetland vegetation included varieties of plants typically found today in areas of high rainfall and subtropical or tropical climates. Of particular interest are the flowering plants, which had only recently evolved and began to flourish during this time period. On the east side of the seaway, minor amounts of peat developed indicating that geological and climatological conditions were similar to those in the west.|
*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.).