The Fort Union Formation and the Ice Age

The Fort Union formation was deposited not long after the dinosaur extinction, about 65 million years ago. Rivers originating in mountain uplifts to the west and south carried abundant sediment to the nearby shallow inland sea to the east, crossing the subtropical, swampy coastal plain of eastern Montana on their way. As the rivers shifted, the swamp vegetation and peat were covered with thick deposits of sand, silt, and clay. In time, it compacted to form coal. The sand, silt, and clay cemented and compacted into sandstone, siltstone, and mudstone. The coal near Plentywood is lignite, which has a lower energy content than higher ranked coal. Most of the coal is buried under layers of the soft rock, but is locally exposed in hills and road cuts. The scattered red rock of the Fort Union Formation is clinker. Clinker forms when lightning strikes, wildfires, or spontaneous combustion causes the coal to ignite and burn. The fires might burn for centuries. The intense heat of the burning coal bakes the adjacent sedimentary layers creating the dense, hard red rock. For thousands of years, northeastern Montana was covered under a massive ice sheet. Glaciologists aren’t sure why the ice ages began, but the process of glaciation is known because of the mark it leaves on the landscape. About 190,000 years ago, glaciers pushed their way onto the northern Great Plains of Montana from the north and northeast. As the glacier moved south, it blocked the Missouri River, forcing it to seek an ice-free channel farther to the south. The ice sheet lasted until about 130,000 years ago when it melted away. The ice left behind a radically changed landscape, dotted with small ponds and glacial till. Large boulders, called erratics, now litter the plains in this area along with extensive gravel beds deposited by streams carrying debris off the glaciers. Some of the erratics came from as far away as northern Manitoba and consist of granite rocks from the Canadian Shield and limestone and dolomite torn from rock outcrops west of Lake Winnipeg.