Much of the coal in the eastern U.S. comes from swamps that existed during the Carboniferous Period, 355 to 295 million years ago. However, in the western U.S. coal swamps formed between 100 and 55 million years ago, in the Middle to Late Cretaceous Period and the Paleocene Epoch of the Tertiary Period. During this time, Colorado was situated along the shoreline of a large, shallow seaway that extended from Canada to Mexico throughout the central U.S. This shoreline moved back and forth during the course of time. Fresh-water swamps formed along the coastal plains adjacent to the shoreline of this seaway. The climate was very warm and humid, with abundant vegetation on the coastal plain. During that period Colorado’s environment looked similar to modernday South Carolina’s coastal plains and swamps, but with dinosaurs. As the vegetation died and sank to the bottom of the fresh-water swamps, it built up large deposits of decomposed, spongy organic matter called “peat.” This saturated peat built up to form bogs that were a few feet to over hundreds of feet thick. Over geologic time, sand and clay sediments covered this peat. More and more sediment was deposited on top of the peat weighing it down and squeezing the water out of the peat. Burial compacted the peat and eventually turned the sediments into rock. High temperatures and pressures over millions of years converted the peat into different types of coal. Generally, the greater the pressure, the harder the type of coal that is formed. This entire process is called “coalification.” Mountain-building processes also affect coalification. Thermal processes within the coal beds can be initiated by igneous intrusive activity and deep-seated uplift. Tertiary-age uplifts and intrusions into the coal-bearing rocks affected the coal beds. Generally, these coal beds were thermally cooked, which upgrades the “rank” of the coal. Bituminous coals can be upgraded to anthracite rank when thermally cooked by igneous intrusions. Anthracite altered by igneous intrusions is located in Crested Butte, Somerset, and Walsenburg.