The Book Cliff's, a prominent feature in east central Utah, extend from near Castlegate, Utah, eastward into Colorado. They were formed under semi-arid conditions by the more rapid erosion of the soft illancos shales underlying n thick sequence of late Cretaceous sandstones. In general the cliff scarps face south with the north dip slope forming the south flank of the Uinta Basin. The strata exposed in the Book Cliffs range in age from Upper Cretaceous to Lower Tertiary.