Colorado National Monument - Grand Valley

This view is looking north down Fruita Canyon toward the Grand Valley. Rocks in the foreground and along the creek are Proterozoic metamorphic rocks (mostly gneiss, schist, and migmatitic pegmatite) about 1.7 billion years old. These rocks were once deeply buried. However, by Late Triassic time (about 201 to 235 million years ago), two or more episodes of uplift and mountain building had resulted in erosion of all overlying strata. That was when the red mud of the Chinle Formation was deposited on the eroded surface of the much older rocks. The contact between the Proterozoic rocks and the Triassic Chinle is an angular unconformity (Scott and others, 2001; Baars, 1998). The Chinle and younger Mesozoic rocks blanketed the Proterozoic rocks. But once again, uplift and mountain building has resulted in erosion and the exposure of the Proterozoic rocks at the earth's surface.