Geology
The Tongue River basin is part of the larger geologic structure known as the Powder River basin. The term Powder River basin can refer to the topographic drainage basin lying to the east of the Tongue River drainage basin, but the term is used in this part to denote the larger geological structure which stretches from the Black Hills to the Big Horn Mountains and which includes the Tongue River drainage area.
The Powder River basin is shaped like a large shallow bowl, with its westernmost rock formations lying against the Big Horn Mountains. As these mountains uplifted over eons of geologic time they lifted and tilted the sedimentary rocks from the Powder River basin, which were then eroded away, creating the plains that stretch eastward from the mountains into the basin. Generally there are older sedimentary layers closer to the mountains and younger layers farther away.
As the Tongue flows out from the Big Horn Mountains it passes over the uplifted layers of increasingly younger sedimentary rocks. In the Big Horn Mountains the Tongue flows in its mountain canyon of Madison Limestone, which was deposited during Early to Middle Mississippian time, about 359 to 326 million years ago. As the Tongue leaves the mountains it flows through younger formations, including the distinctive thick red Chugwater Formation, deposited during the Triassic time, 250 to 199 million years ago. Shortly after leaving the mountains, the Tongue River enters an area dominated by a thick layer of buff colored sandstones and silty clay. This sedmintary layer is named the Tongue River Sandstone, because its outcrops are so predominant in the Tongue River basin. The Tongue River Sandstone is the youngest of three "members" which form the Fort Union Formation, the other two members being the Lebo Shale Member and the Tullock Member.
The buff colored sandstones and shales of the Tongue River sandstone are visible all along the greater part of the Tongue River from Dayton, Wyoming to a point north of Ashland, Montana. In this stetch, the sandstone layers of the Tongue River member hold ground water so that the highlands on each side of the Tongue River valley are often covered with pines. As the river approaches Miles City the valley changes appearance to grassy rolling hills, as the river leaves the Tongue River formation and flows through the Lebo shale and the Tullock sedimentary formations.
The Tongue River sandstone member outcrops widely over portions of southeast Montana and northeast Wyoming and it is best known for its coal. The Tongue River member has approximately 32 coal seams with a combined thickness in excess of 300 ft. The thickness of the separate coal seams varies radically from place to place—in some places the beds are thick but over a relatively short distance the seam can pinch out to nothing. Where these coal beds are thick and also close to the surface in the Powder River Basin in northern Wyoming and southern Montana they are mined in large open pit mines, like the mines along the Tongue in the vicinity of Decker, Montana. Tongue River coal is low in sulfur content and coal-fired electric generating places throughout the United States demand Tongue River coal so they can meet federal emission standards. Because of this demand, about 40% of the coal now used to generate electricity in the United States is mined from the Tongue River sandstone coal seams in the Powder River Basin, producing 14% of carbon dioxide emissions in the United States.
Where the Tongue River now flows in Montana and Wyoming, the sedimentary rock formation that is today known as the Tongue River sandstone began to form about 60 million years ago, when mountain uplifts began rising from a shallow sea. The Black Hills uplift on the east, the Hartville uplift on the southeast, and the Big Horn Block on the west created a flat, swampy low lying plain, with slow moving rivers flowing northwest to deltas along a shallow sea. At this time the climate in the area was subtropical, averaging about 120 inches (3,000 mm) of rainfall a year. For some 25 million years, the floor of this plain was made up of thick deposits of sandy silt from the surrounding mountains, with many rivers, deltas, backwaters and swamps, all covered by forests and vegetation. At that time, from 35 to 60 million years ago, the area where the Tongue River now flows would have appeared as a dense swampy jungle. Over long periods of time, the heavy plant growth died and accumulated as peaty layers in the large backwaters and swamps all across the basin. Periodically more sandy silt deposits would wash in from the mountains, completely burying the layers of organic peaty materials. Eventually the climate became drier and cooler. The area passed through more long periods of geologic time, during which new sedimentary layers buried this entire sandy silty layer along with its deposits of peat, under thousands of feet of newer sediments, compressing the sandy silty deposits into the Tongue River sandstone of today, and also compressing and changing the layers of peaty organic material into thinner layers of lignite coal. Over the last several million years, much of the overlying sediment has eroded away, bringing the sandstone layer with its seams of coal to the surface again in the Tongue River area.
The Tongue River sandstone forms cliffs, hills, buttes and bluffs along the river and throughout the basin. From the Decker area downstream to about Birney the river flows through the prairie in a canyon carved from the Tongue River sandstone. The upper part of this canyon is dammed to form the Tongue River Reservoir.
The sandstone hillsides and bluffs along the Tongue and its tributaries often have reddish bands running through them or they are capped with resistant reddish layer. These red layers were formed millions of years ago. Coal seams outcropping in the Tongue River sandstone caught fire, probably from prairie fires that started by lightening. The fires burned from the outcropo back into the coal seams, and the fire finally went out when they burned so deeply into the coal seam that the fire was smothered. These fires burned for a long time and they were extremely hot, and they baked and changed the structure of the sedimentary rocks that lay just over the coal seam until it became a hard "clinker" substance and turned a reddish brick color. These red "clinker" beds are often more resistant to erosion than the silty sandstone, so they appear on the higher parts of bluffs, and buttes on either side of the valleys of the Tongue River basin are often capped by beds of this baked and fused rock that are five to twenty feet thick. Besides the beds of reddish "clinker" larger concretions can be found that appear at first glance to be similar to melted glass or even pieces of volcanic rock. Although of a different appearance than the clinker these odd looking concretions are also formed by the burning coal beds, with the difference in appearance being due to the difference in content of the material in the overlying bed that was heated to very high temperatures. The reddish "clinker" is crushed and used to surface roads roads throughout the Tongue River basin.
North of the Yellowstone, dinosaur fossils have been found in Cretaceous era rock formations, but dinosaur fossils have not been found in any members of the Paleozoic Fort Union Formation, including the Tongue River sandstone. However plant fossils are common in the Tongue River sandstone, and many imprints of leaves and fronds have been found and collected by scientists and fossil hunters.
Read more about this topic: Tongue River (Montana)