In order from oldest to youngest, they are the Pierre Shale, the Fox Hills Formation, the Chamberlain Pass, Chadron Formation, the Brule Formation, and the Sharps Formation. (McDonald 36) A sub category of these formations would be the White River Group, which consists of The Chamberlain Pass, Chadron, and Brule formation. (McDonald 36) Terrestrial sedimentation, which occurred during the Tertiary Era, is responsible for how the White River Group formed. The volcanoes mentioned above started the events that lead to the formation of the Western Interior Seaway, which covered what is now known as the great planes. As time passed, mud located on the sea floor hardened, an once all of the water was completely gone, the sea floor was exposed to air, and the exposed mud became a tropical soil. The Yellow Mounds today showcase this display this process. By around 50 million years ago, smaller remnants of the Western Interior Seaway had completely vanished from the region, replaced with a low plain. Then, he rolling gray hills of the Chadron formation were formed within a warm and humid environment. Large rivers carried a vast amount of sentiment from the Black Hills down to the Badlands, which was once a geological basin, or a low point. From 34-40 million years ago, climate went from a sub-tropical to cooler, drier air. During this time, river channels and windblown sentiments deposited the Bruele formation. Rivers also began to flood, leaving the sand, mud, and other elements too pile up, which led to a vast amount of fossils. The highest peaks in Badlands are what is known as the Sharpes Formation. This was formed from the rocky ford ash that came from a volcanic eruption to the west of Badlands. Sentiments that formed this were from ancient sea, rivers, flood plains, and windblown volcanic ash that was deposited 75-25 million years ago, which then formed the sedimentary rock layers seen today.
In order from oldest to youngest, they are the Pierre Shale, the Fox Hills Formation, the Chamberlain Pass, Chadron Formation, the Brule Formation, and the Sharps Formation. (McDonald 36) A sub category of these formations would be the White River Group, which consists of The Chamberlain Pass, Chadron, and Brule formation. (McDonald 36) Terrestrial sedimentation, which occurred during the Tertiary Era, is responsible for how the White River Group formed. The volcanoes mentioned above started the events that lead to the formation of the Western Interior Seaway, which covered what is now known as the great planes. As time passed, mud located on the sea floor hardened, an once all of the water was completely gone, the sea floor was exposed to air, and the exposed mud became a tropical soil. The Yellow Mounds today showcase this display this process. By around 50 million years ago, smaller remnants of the Western Interior Seaway had completely vanished from the region, replaced with a low plain. Then, he rolling gray hills of the Chadron formation were formed within a warm and humid environment. Large rivers carried a vast amount of sentiment from the Black Hills down to the Badlands, which was once a geological basin, or a low point. From 34-40 million years ago, climate went from a sub-tropical to cooler, drier air. During this time, river channels and windblown sentiments deposited the Bruele formation. Rivers also began to flood, leaving the sand, mud, and other elements too pile up, which led to a vast amount of fossils. The highest peaks in Badlands are what is known as the Sharpes Formation. This was formed from the rocky ford ash that came from a volcanic eruption to the west of Badlands. Sentiments that formed this were from ancient sea, rivers, flood plains, and windblown volcanic ash that was deposited 75-25 million years ago, which then formed the sedimentary rock layers seen today.