The Fort Union Formation is a geologic unit containing sandstones, shales, and coal beds in Wyoming, Montana, and parts of adjacent states.

In the Powder River Basin, it contains important economic deposits of coal, uranium, and coalbed methane.

[1] The Fort Union is mostly of Paleocene age and represents a time of extensive swamps as well as fluvial and lacustrine conditions.

The rocks are more sandy in southwestern Wyoming and more coal-bearing in northeast Wyoming and southeast Montana, reflecting a general change from rivers and lakes in the west to swamps in the east, but all three environments were present at various times in most locations.

[1] One such bed, the Wyodak Coal near Gillette, Wyoming, is as much as 110 feet (34 m) thick.