Palo Duro visitors—regardless of age—seem to have an innate curiosity
about the canyon’s rocks. This is not surprising, for most of the
features of the park landscape are composed of or have been sculptured
from solid rock. In short, much of the natural beauty of Palo Duro
Canyon has been derived from the character of its exposed rock
formations and the effect of geologic agents upon them.
Because rocks are the raw materials of geology and the stuff from which
landscapes are formed, it will be helpful for the visitor to know
something about the general characteristics of rocks and their role in
the development of the landscape. Rock is everywhere around us and is
one of the most common objects in the world, yet few people can actually
define a rock. So, at the outset it should be stated that _a rock is a
naturally formed aggregate of minerals_, and _a mineral is a naturally
occurring substance which has a fairly definite chemical composition,
distinctive physical properties, characteristic internal structure, and
which commonly occurs in definite shapes called crystals_. Although not
an exact scientific or legal definition of a mineral, the above
explanation is satisfactory for the purposes of this publication.
Although most visitors show considerable interest in the canyon’s rocks
and minerals, few of them know the story behind the rocks. They do not
know how the rocks were formed, of what they are composed, how they
change, and how they differ. More important, they fail to realize the
historical significance of the rocks and how they can be used to
interpret events that occurred in the canyon many millions of years ago.
Thus, before one studies the geologic story of Palo Duro Canyon, it is
helpful to know something about the various kinds of rocks. There are
three major classes of rocks in the earth’s crust: _igneous_,
_sedimentary_, and _metamorphic rocks_.
_Igneous rocks_ solidified from an original molten state. Common
examples of igneous rocks include granite, basalt, and volcanic ash.
Although no igneous rocks are found in Palo Duro Canyon, they are widely
exposed in parts of West and Central Texas.
_Metamorphic rocks_ were originally igneous or sedimentary in origin.
However, these rocks have undergone such great physical and chemical
change that they have been transformed into a different kind of rock.
Thus, metamorphic changes alter limestone to marble or sandstone to
quartzite. No metamorphic rocks crop out in the canyon, but, like the
igneous rocks, they are common in some parts of the State.
All of the geologic formations exposed in Palo Duro Canyon are composed
of _sedimentary rocks_. These are rocks that have been formed by the
compaction and cementation of rock and mineral fragments called
_sediments_, or by the precipitation of material from solution.
Sandstone, conglomerate, shale, and caliche (_see_ p. 26) are examples
of sedimentary rocks that are exposed in the canyon.
Sedimentary rocks are typically _stratified_, that is, they occur in
layers or beds called _strata_. In addition, sedimentary
rocks—especially those of marine origin—commonly contain _fossils_.
These fossils are traces or evidence of prehistoric plants and animals
that have been preserved in the rocks, and they may provide clues as to
the age of rocks and the manner in which they were formed. Fossil
remains have been found at a number of places in the park and these are
discussed later.