Ok-A-Beh Marina Sign 1 Reading the Rocks at Ok-A-Beh
Rocks tell two kinds of stories. First, the character of rocks tells a story of processes, climate, and environment when the rocks formed. Second, the geometry of the beds and the relationship to other rock layers tell the story of mountain building and erosion. The canyon walls here at Ok-a-beh (bend of river in the Crow language) are excellent exposures of the Mission Canyon Limestone, Amsden Formation, and Tensleep Sandstone. The sequence records the conditions about 300 to 350 million years ago. During the time of Mission Canyon Limestone deposition, the area was a shallow, widespread sea much like the Caribbean; reef buildups also were common. Later, the Amsden tells the story of the sea receding and exposure of the land surface to the atmosphere, resulting in deposition of red (oxidized) sediment. At this time, fresh water circulating in the Mission Canyon Limestone dissolved solution features, such as cavities and caverns, that often collapsed into sink holes like those forming periodically in Florida. These cavities make this rock an excellent ground-water aquifer and oil and gas reservoir. The inset photo shows some of the solution features with springs issuing from them. As the sea slowly returned, the light-colored sandstones of the Tensleep were deposited as beaches and sand dunes. The mountains you see around you are about 70 million years old. Imagine the area the way it was about 10 million years ago-a plain, sloping gently eastward, that had been formed by erosion of large volumes of sediment from the mountains and deposition in the valleys. Early in its history, the Bighorn River established a meandering course across this plain. When the region was slowly uplifted, the river began cutting down, superimposing itself through the young sediment and continuing down into the underlying bedrock of the buried Bighorn Mountains, forming Bighorn Canyon. The story in the rocks continues. What changes could be happening? Do you think it is possible for the canyon to be cut any deeper?