Ancient Rocks and Events
The dark rocks that floor all the large canyons of the Monument ([fig. 6]) and form the high bluffs along the northeastern boundary (figs. [37], [38], [40], [41]) are of early Proterozoic[18] age—among the oldest known rocks of the Earth. Most were once sand and mud that spread out on the bottom of the sea and later hardened into sedimentary rocks ([fig. 9]-1). After thousands of feet of such rocks had accumulated, they were squeezed, bent, and lifted up by slow but mighty movements of the Earth’s crust to form high mountains perhaps like the Rockies. Heat and pressure that developed at great depth in the roots of these mountains changed the sediments into metamorphic rocks known as schist (finely banded) and gneiss (coarsely banded) ([fig. 9]-2). The rocks are about 1½ billion years old ([fig. 7]).
Later in Proterozoic time, about a billion years ago, molten material from below was forced upward along cracks or faults and cooled slowly to form thin seams or dikes and irregular bodies of granite ([fig. 9]-3). Dikes are called pegmatite when they contain large crystals of pink feldspar, white or clear quartz, black tourmaline, and large flakes of white mica. Small pegmatite dikes that pass through the older schist and gneiss may be seen along roadcuts in Fruita and No Thoroughfare Canyons.
BLOCK DIAGRAMS OF EARLY PROTEROZOIC EVENTS (after Edwin D. McKee). (Fig. 9)
① Layers of sand, mud, and other sediment accumulated in the sea and later were hardened into sedimentary rocks.
② The strata were compressed, bent, and uplifted into high mountains. Heat and pressure at great depth changed the sediments into banded schist and gneiss.
③ Molten rock flowed upward along cracks or faults. Upon cooling it formed lava at the surface and granite or pegmatite beneath.
④ During eons of time the forces of erosion wore down the mountains to a nearly level plain.