Much of the basic information which the geologist uses to reconstruct the geologic history of a region comes from his examination and interpretation of _bedrock outcrops_. _Bedrock_ is the solid unweathered rock which underlies loose earth material such as soil, sand, and gravel. An _outcrop_, or _exposure_, is a place where bedrock is exposed at the surface. The first chapter of earth history begins with the most ancient rocks known. Because they were formed early in geologic time, these rocks are normally found deeply buried beneath younger rocks which have been deposited on top of them. It is for this reason that earth history is read from the bottom up, for the earliest formed rock layers correspond to the opening chapter in our earthen history book. The later chapters are found in the upper younger rocks which are located nearer the surface. Thus, in “reading” the geologic history of Palo Duro Canyon we start with the oldest “chapter” which is recorded in the Quartermaster Formation (p. 17) of Permian age, for these are the oldest rocks exposed in the canyon. But deciphering earth history is not as simple as it might appear. In many areas the rock layers are not always found in the sequence in which they were originally deposited. In places, great structural disturbances have caused some of the rocky “pages” to become shuffled and out of place; others may be missing completely. Many rocks have been destroyed by weathering and erosion or greatly altered by metamorphism. As a result, the story recorded in these particular rocks is lost forever. These missing “pages” make the ancient story even more difficult to interpret so the geologist must then depend on other evidence that will permit him to “fill in the blanks.” The record revealed in the rocks indicates that our planet is at least 4½ billion years old and that life has been present for more than 3 billion years. During this vast span of time the earth and its inhabitants have undergone many changes. THE GEOLOGIC COLUMN AND GEOLOGIC TIME SCALE The _geologic column_ refers to the total succession of rocks, from the oldest to the most recent, that are found in the entire earth or in a given area. For example, the geologic column of Texas includes all rock divisions known to be present in the State. By the same token, the geologic column of Palo Duro Canyon consists of the geologic formations exposed there. Thus, by referring to the geologic column previously determined for a specific area, the geologist can determine what type of rock he might expect to find in that particular region.