Some of the beds of siltstone and mudstone in the Brushy Basin Member of the Morrison shown in figure 21 contain bentonite, a clay derived from the decomposition of volcanic ash, which indicates the presence of active volcanos in or near the Plateau at the time these beds were deposited. Bentonite swells when wetted, so it is widely used in well-drilling muds, sealing canals, etc. Some bentonitic material has been dug from the Brushy Basin along the Little Park Road just south of the point from which the photograph in figure 21 was taken and was used for sealing irrigation canals in the Grand Valley. The Morrison is not well exposed in the Monument, as the formation is restricted to the higher parts where most of it is hidden by vegetation. The lower part is seen in roadcuts and outcropping ledges along a high stretch of Rim Rock Drive between Artists Point and the head of the west arm of Ute Canyon, where sandstone lenses in the Salt Wash Member are especially thick. The climate during Morrison time was wet enough to support abundant vegetation along the many lakes and streams—at least enough to feed the hungry dinosaurs and other reptiles that roamed the area. Many bones and parts of several skeletons of dinosaurs have been found in the Morrison at several places in The Redlands not far northeast and northwest of the Monument. The most famous dinosaur locality near the Monument is Riggs Hill where, in 1900, the late Elmer S. Riggs of the Field Columbian Museum (now Field Museum of Natural History) dug out part of the first known skeleton of a huge _Brachiosaurus_ (fig. 22). This discovery made quite a splash in the scientific world, for it was the first and only type of dinosaur found whose front legs were longer than its hind legs. The fossilized thigh bone (femur) alone is 6 feet 8 inches long and weighs 549 pounds; the arm bone (humerus), though incomplete, is even longer. The ribs are 9 feet long. A bronze plaque now marks the site of the excavation (fig. 39). In 1901, Riggs removed all but the forepart of a skeleton of _Apatosaurus_ from the southeast side of a large hill of the Morrison Formation just south of the old Fruita bridge. Riggs also found remains of _Diplodocus_, _Camarasaurus_, and _Morosaurus_, and, in 1937, Al Look, prominent writer and amateur paleontologist of Grand Junction, and Edwin L. Holt, an instructor in Mesa College at Grand Junction, found the closely associated remains of _Allosaurus_, _Stegosaurus_, and _Brachiosaurus_ at the western end of Riggs Hill. Dinosaurs generally are thought of as huge creatures—many were huge indeed (fig. 23)—but they came in various sizes and some were quite small.