Even the most casual observer will soon note that not all of the
canyon’s rocks have been equally affected by erosion. Indeed, it is the
nature of this _differential erosion_ that gives Palo Duro Canyon the
rugged sculptured appearance that accounts for much of its beauty.
Visitors to Palo Duro Canyon commonly ask why the rock formations are so
diversely shaped. The answer to this question lies in the rocks
themselves. Because the various rock strata are of unequal hardness,
they erode at different rates of speed. Hence, the harder, more
resistant rocks, such as the sandstones and conglomerates of the
Trujillo Formation, form the shelves, ledges, and “caps” of the rock
sculptures. The Lighthouse (fig. 31) and other pedestal rocks (fig. 16)
are good examples of land-forms produced by differential erosion. The
“hoodoos” mentioned earlier are also the products of this type of
erosion (figs. 16 and 20).
[Illustration: Fig. 20. Talus slopes (arrow) are well developed on
the east side of Capitol Peak and in places obscure the
Quartermaster red beds. Note the “hoodoo” at the south (left) end of
the structure.]
Softer rocks like shales and clay are more readily eroded and they
normally form slopes rather than cliffs or ledges (fig. 12). Grooves,
recesses, and caves have also developed in some of the less resistant
rocks such as the shales and gypsum beds of the Quartermaster Formation.
Catarina Cave (fig. 27) which has formed in the red and white shales of
the Spanish Skirts (fig. 26) is a good example of this type of feature.
Caves of this type afforded protection to both man and wild animals
since the dawn of history, for their remains have been found in a number
of similar caves.
Thus, within a relatively short time—geologically speaking—the familiar
land-shaping processes described above have joined forces to provide
Texas with one of its most remarkable natural attractions. But
interestingly enough, the same geologic processes that created these
unusual formations are busily at work destroying them. As time passes
and erosion progresses, the caps of the pedestals are worn away and the
underlying shales crumble and are washed into the valley below. Yet even
as the old land-forms are being destroyed, wind, water, ice, and man are
attacking the canyon walls to produce still more of these interesting
erosional remnants.