At the very northeastern corner of Fossil Butte National Monument can be
seen an outcrop of the Thaynes Limestone of Triassic age, a marine unit
deposited some 200 million years ago. This formation is noted for its
rich and varied assemblage of marine fossils, mostly forms related to
oysters and clams. The Thaynes Limestone consists predominantly of
sandy, gray limestone and green-gray limey siltstones. The formation
weathers to a dark brown in the lower part and a grayish-yellow in the
upper. The fact that the Thaynes Limestone is exposed in the monument at
all is because of the faulting that took place just prior to the
deposition of the Evanston Formation of latest Cretaceous and earliest
Tertiary age (refer to section on structure) and the erosion of the
Tertiary rocks that once covered it.