Eocene Late Eocene Time: 38 to 36.6 mya Duration: 2.6 million years    Early Eocene Time: 57.8 to 52 million years ago Duration: 5.8 million years   There were two very different periods of coal formation during the Eocene. The most important occurred in the western part of the North America during the Early Eocene. A less important coal-forming event occurred in the Late Eocene.   Early Eocene coal developed in intermontane valleys associated with the still-rising Rocky Mountains. These coals are very thick (up to 280 feet) and low in ash and sulfur. The coal is subituminous and very similar to that of the Paleocene intermontane valleys. Enormous quantities of this coal are found in Colorado, Wyoming and Montana. Large amounts of lignite of the same age are found in Montana and North Dakota.    The younger coals of the Late Eocene occurs as hundreds of small, thin coal deposits scattered throughout western North America. Their widespread occurrence suggests that intermittent swampy conditions existed but the accumulation of peat was not as extensive as earlier in the period. Currently, these younger coals are not commercial.

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