After the materials of the sedimentary rocks were deposited and covered by younger layers, they generally became saturated or partly saturated with ground water containing small amounts of dissolved minerals. Some of these minerals precipitated from solution and cemented the loose particles into rocks of varying hardness. Thus, most of the sandstones are partly cemented with the mineral calcite, composed of calcium carbonate (CaCO₃), but some are cemented also with silica (SiO₂) or hematite (Fe₂O₃). Look almost anywhere in the Monument and you will see that the rocks are piled up in layers of different color, thickness, and hardness—much like a vast layer cake. In most of the Monument, these layers are flat or slope gently down to the northeast, but along the northeastern boundary they are sharply bent or broken as though the cake had been carelessly placed over the edge of a table and had sagged. Let us consider these layers one by one, beginning with the oldest at the bottom, for each is a partial record of events long past. Layers of rock that can be easily recognized and distinguished from other layers are called formations and are named after a place where they are well exposed. For the name to be accepted for general usage it must be the first published description in a technical report of a particular sequence of rock layers. The places after which the formations of the Monument were named are given in the rock column (fig. 7), and the outcrops of the formations are shown on the geologic map (fig. 8). In the pages that follow, the geologic events that shaped the Monument we see today are discussed in chronological order, beginning with the oldest rocks that floor the deep canyons. [Illustration: Petroglyph] [Illustration: GEOLOGIC MAP of Colorado National Monument and vicinity, simplified and greatly reduced from part of maps at scale 1:31,680 by Lohman (1963, 1965a). For additional surficial deposits in the Grand Valley and Orchard Mesa see Cashion (1973). (Fig. 8)]