The unusual features of the Needles district are due in some part to the character and thickness of the underlying rocks but in large part to erosion along joints and faults. Joints are fractures along which no displacement has taken place, and faults are fractures along which there has been displacement of the two sides relative to one another (fig. 76). The Cedar Mesa Sandstone comprises 500 to 600 feet or more of hard well-cemented buff, white, and pink beds of massive sandstone. On the basis of the type and amount of deformation and erosion of the Cedar Mesa Sandstone and underlying rocks, the Needles district can be divided into three differing areas: (1) an eastern area where the rocks are relatively undeformed but are carved into an intricate series of canyons, including Salt Canyon and the upper reaches of Davis and Lavender Canyons—the section of the district that contains most of the arches and Indian ruins; (2) The Needles proper, where tensional forces have cracked the brittle Cedar Mesa Sandstone into a crazy-quilt pattern of square to rectangular blocks separated by joints widened by erosion, creating a myriad of spires and pinnacles; and (3) The Grabens, where the previously jointed rocks were later subjected to additional tensional forces that produced a series of nearly parallel faults that trend northeastward and separate downdropped blocks of rock, called grabens, from intervening stationary or upthrown blocks of rock, called horsts. Let us examine each of these areas in the order named. For traveling to most features a four-wheel-drive vehicle is strongly recommended. Some visitors negotiate the jeep trails with dune buggies or motorcycles, but four-wheel-drive vehicles are considered safer and generally more reliable. A few trails can be traveled only on foot. Squaw Flat, in the western part of the relatively undeformed area, is a nearly flat area of lower Cedar Mesa Sandstone covered here and there by a thin layer of sparsely vegetated soil and surrounded by generally low hilly erosional forms in the upper part of the sandstone. Short canyons and alcoves in the sandstone hills along the west side afford excellent semi-private campsites, each of which has its own paved access road, picnic table, and trash can (fig. 39). Moreover, ground water at shallow depth in the underlying sandstone has encouraged the growth of exceptionally large piñon and juniper trees that provide welcome shade. [Illustration: SQUAW FLAT CAMPGROUND, in the Needles district, in Cedar Mesa Sandstone. Large piñon and juniper trees draw ground water from this sandstone. (Fig. 39)] SALT, DAVIS, AND LAVENDER CANYONS