If you look down into any of the large canyons in the Monument, you will
notice a brick-red formation, the Chinle, which forms steep slopes at
the foot of the high cliffs and lies upon the dark Proterozoic rocks
along nearly straight lines of contact. Such a straight-line contact is
particularly well shown about midway up the high bluffs along the
northeastern boundary of the Monument (fig. 37). If the red layer and
all overlying rocks were stripped away, these straight lines would be
the exposed edges of a remarkably smooth, nearly flat erosion surface on
the top of the dark Proterozoic rocks, as shown in the last diagram of
figure 9. A vast amount of time passed between the carving of this
surface and the deposition of the red Chinle, and no record of the
events during this time is preserved in the Monument.
During the latter part of the Proterozoic Eon and parts of the long
Paleozoic Era that followed, the dark rocks were submerged beneath the
sea several times and received sediments now found in areas to the
northeast and southwest. Beginning in the Pennsylvanian Period some 330
million years ago (fig. 61), a large upfold of the rocks, or anticline
(fig. 27), known to geologists as the Uncompahgre Highland, rose high
above sea level, probably reaching its highest level in Late
Pennsylvanian or Permian time. This old highland formed an imposing
chain of mountains in about the position of the present Uncompahgre
Plateau.
After the old rocks were pushed up into these high mountains what became
of them? From the moment the mountains began to rise, their rocks were
buffeted by wind, pounded by rain, pried open by frost, scoured by
debris-laden streams and, perhaps by glaciers, and the loosened rock
particles were dissolved or carried to the sea. Most rocks are brittle
enough to crack when bent by Earth forces. Such cracks, called joints,
are easy targets for erosion. The freezing of water in joints tends to
pry the rocks apart. The breakup of the rocks was hastened by the
chemical attack on rock minerals by water charged with oxygen and carbon
dioxide. When land plants became established in later geologic eras,
soil acids formed from decaying vegetation also assisted materially in
breaking up the rocks.
These same erosion processes are going on today, but their effects are
scarcely noticeable from year to year except in soft earth after storms
or floods. During eons of time, however, the mountains were again worn
down to a nearly level plain. Missing between the red Chinle and the
dark rocks are many thousands of feet of rocks, some of which once
covered this surface and still occur in other regions less affected by
erosion. This gap in the rock record, which represents more than a
billion years, is known to geologists as a great unconformity. Missing
are part of the lower Proterozoic rocks, all the upper Proterozoic
rocks, all those of the Paleozoic Era, and part of those of the Triassic
Period of the Mesozoic Era. (See figs. 7 and 61.)
Traces of primitive life have been found in some Proterozoic rocks in
the form of lime-secreting algae and casts of worms, but no fossils of
more advanced types have been found because at that time the primitive
animals seemingly had not yet developed shells or skeletons. The ensuing
Paleozoic Era saw the appearance and great development of shellfish,
fish, amphibians, reptiles, and primitive plants. Some of the rock
layers of ages missing at the Monument may be seen as near as Glenwood
Springs to the northeast and Gateway to the southwest.