Gneiss, a coarsely banded rock (fig. 7), and schist, a finely banded rock, are the most common kinds of metamorphic rocks in Yellowstone. Originally, the gneiss probably was granite, and the schist was a shale or sandstone. Outcrops of the gneisses and schists occur only in the northern part of the Park (pl. 1), where they form the central cores of some mountain ranges such as the Gallatin Range (fig. 3). They also lie buried beneath younger rocks in many other areas of the Park.
From the time of the metamorphic event, when the gneisses and schists were formed, until the deposition of sediments of the Cambrian Period (figs. 5 and 6), there is virtually no record. It is reasonably certain, however, that several times during this 2.1-billion-year interval the region was intensely squeezed and uplifted into high mountains and then deeply eroded. By the end of Precambrian time, approximately 570 million years ago, the ancient Yellowstone landscape had been reduced by erosion to a flat, stark, almost featureless plain, which was soon to be flooded by a shallow sea encroaching from the west. This very old surface is now partly exposed in some places across the Buffalo Plateau, at the north edge of the Park (fig. 1).
[Illustration: LAMAR RIVER. View downstream (west) along the Lamar River in Lamar Canyon. The rocks along the river banks are coarsely banded Precambrian gneisses more than 2.5 billion years old, some of the oldest rocks in Yellowstone National Park. (Fig. 7)]
[Illustration: Closeup views show coarse banding and texture of the gneiss; minerals include quartz, feldspar, and biotite (black mica).]
From the appearance of the rugged, mountainous terrain of Yellowstone National Park, it is difficult to visualize a time when this region lay close to sea level, at times even below sea level. Yet the evidence is clear that from the Cambrian Period to the latter part of the Cretaceous Period, a span of about 500 million years, vast stretches of western lands were flooded repeatedly by broad shallow seas that often reached from Canada to Mexico (fig. 8). During these great floodings, widespread horizontal beds of sand, silt, clay, limy mud, and other sediments were deposited on the ocean floors, along the adjoining beaches and wide tidal flats, and across the broad flood plains of large rivers that emptied into the seas. All of these ancient sediments have now hardened into compact well-layered sandstones, shales, and limestones (figs. 9 and 10). These _sedimentary rocks_ have been divided into 25 or more distinct formations in the Yellowstone region (fig. 5), where they locally attain a combined thickness of more than 10,000 feet.