Geological History
There are many old rocks in Yellowstone National Park. Some of the oldest rocks in this park are Gneisses, which is a metamorphic rock, and Schist, which is also a metamorphic rock. These rocks supposedly came around the Precambrian Era. Then around 570 to 75 million years ago when Yellowstone was under a sea, deposition created many new sedimentary rocks like sandstone, limestone, shale, dolomite, and many other rocks. When the Absaroka Eruption happened, breccia, another type of sedimentary rock came along. The national park got it's name from the yellow sandstone on the banks of yellowstone river.