A nearly homogeneous (uniform composition) rock layer may be identified as a FORMATION. Formations are usually spread over a wide area like a large blanket and are stacked on top of each other with the oldest at the bottom and the youngest at the top. When viewed in the walls of the canyon, they resemble a huge layer cake and may be traced along the canyon walls. [Illustration: Figure 1. GEOLOGIC MAP OF PALO DURO CANYON] [Illustration: Figure 2. Stratigraphic section at Palo Duro Canyon with Geologic Time-scale for reference.] GEOLOGIC TIME-SCALE AGE M.Y. CENOZOIC QUAT. HOLOCENE PLEISTOCENE 2 TERTIARY PLIOCENE 13 MIOCENE 25 OLIGOCENE 36 EOCENE 58 PALEOCENE 63 MESOZOIC CRETACEOUS 135 JURASSIC 181 TRIASSIC 230 PALEOZOIC PERMIAN 280 PENNSYLVANIAN 310 MISSISSIPPIAN 345 DEVONIAN 405 SILURIAN 425 ORDOVICIAN 500 CAMBRIAN 600 PRECAMBRIAN The lowest and oldest exposed formation in Palo Duro Canyon is the Quartermaster. It was deposited near the edge of a shallow sea that occupied a wide area in Texas some 280 to 230 million years ago (Figure 3). This was during the Permian Period, the last period of the Paleozoic Era. The sediments in the park area were carried from the east and deposited in a nearshore environment. Sedimentary structures, such as crossbeds and ripplemarks, are present throughout the Quartermaster Formation. Halite casts suggest that there was a high rate of evaporation as sedimentation occurred. Gypsum (altered anhydrite) is also interpreted to be an evaporite deposit. The gypsum is now seen as horizontal white layers of alabaster and satin-spar varieties within the Quartermaster Formation. The Quartermaster Formation is mostly siltstone and shale, and is commonly a distinctive red color. This red color is the result of combining oxygen from the air with the iron in the sediments (oxidation) much as a nail rusts after it has been exposed for a long period. The bedded gray zones represent times when there was enough fresh water from the land to offset temporarily the oxidation process. Smaller circular gray areas have organic nuclei that produced local areas of chemically altered iron by a process called reduction.