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.