[Illustration: COMMON KINDS OF GEOLOGIC STRUCTURES produced by deformation of the earth’s crust. An original horizontal rock layer may be upfolded into anticlines, downfolded into synclines, and broken by either reverse or normal faults. A fault is a fracture or a zone of fractures within the earth’s crust along which movement has taken place. A reverse fault is one generally produced by compression (squeezing together), and the hanging-wall block has moved up with respect to the footwall block. A normal fault is one generally produced by tension (pulling apart), and the hanging-wall block has moved down with respect to the footwall block. All these kinds of structures are present in Yellowstone National Park. (Fig. 13)] HORIZONTAL (undeformed) REVERSE FAULT (compressional) Hanging wall Footwall Forces ANTICLINE (upfold) Crest Limb NORMAL FAULT (tensional) Footwall Hanging wall Forces SYNCLINE (downfold) Limb Trough During the Laramide orogeny, many folds and faults formed in the northwestern part of the Park, in the area now occupied by the Gallatin Range (fig. 14, section A-A′). In south-central Yellowstone, the Paleozoic and Mesozoic sedimentary rocks were tightly folded into three anticlines separated from one another by synclines and faults (fig. 14, section C-C′). Movement along one reverse fault in this area was locally more than 10,000 feet. As the lands were uplifted and contorted, they came under vigorous attack by the ever-present agents of erosion. Tremendous quantities of rock were stripped from the highlands, and the debris was carried by streams into the adjacent lowland basins and deposited mostly as sand and gravel. As the highlands continued to rise, the basins continued to sink, and in a short period of time great thicknesses of basin-fill sediments accumulated locally. One such deposit, the Harebell Formation of latest Cretaceous age in south-central Yellowstone (fig. 5), is more than 8,000 feet thick. Other similar anticlines, synclines, and reverse faults no doubt extend far into the interior of Yellowstone National Park, and perhaps entirely across it in places, but they lie buried beneath a thick capping of volcanic rocks. Nevertheless, it seems safe to conclude that none of the Park area escaped the effects of the great forces of the Laramide orogeny. These forces, regardless of how they originated deep within the earth, seem to have been compressional (fig. 13), pushing the upper layers of the earth’s crust from the east and northeast toward the west and southwest. This interpretation is based on the style of the structural features just described, which shows that the steep limbs of folds, as well as the direction of movements along reverse faults, point toward the west or southwest (fig. 14).