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.
[Illustration: BLOCK DIAGRAMS OF EARLY PROTEROZOIC EVENTS (after
Edwin D. McKee). (Fig. 9)]
[Illustration: ① Layers of sand, mud, and other sediment accumulated
in the sea and later were hardened into sedimentary rocks.]
[Illustration: 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.]
[Illustration: ③ Molten rock flowed upward along cracks or faults.
Upon cooling it formed lava at the surface and granite or pegmatite
beneath.]
[Illustration: ④ During eons of time the forces of erosion wore down
the mountains to a nearly level plain.]