THE CONTINENTAL CRUST
By the beginning of the Proterozoic, upward of 80 percent or more of the pre-
sent continental crust was already in existence (Fig. 41). Most of the material
presently locked in sedimentary rocks was at or near the surface. Therefore,
ample sources of Archean rocks were available for erosion and redeposition. Sed-
iments deriv ed directly from primary sources are called graywackes, often
described as dirty sandstone.They commonly occur in folded sedimentary rocks
such as those in the Alps and Alaska. Most Proterozoic graywackes composed of
sandstones and siltstones originated from Archean greenstones. Another com-
mon rock type was a fine-grained quartzite. It was a metamorphic rock derived
from the erosion of siliceous grainy rocks such as granite and course-grained
sandstone called arkose.
Conglomerates,consolidated equivalents of gravels, were also abun-
dant in the Proterozoic. Nearly 20,000 feet of Proterozoic sediments form
the Uinta Range of Utah (Fig. 42), and the Montana Proterozoic belt
system contains sediments more than 11 miles thick.The Proterozoic is also
Figure 40The dashed
lines indicate the extent of
the late Precambrian ice
age in Australia.PROTEROZOIC METAZOANSAdelaideNorth PoleUluru
(Ayers Rock)