The lake remained at the Bonneville level for a long time. Then, continuing its upward movement, the water reached Red Rock Pass in northern Cache Valley, and, through this lowest point in the rim of the basin, overflowed into the Snake and Columbia river systems. The Red Rock outlet carried the water through a loose gravel formation which was cut away so rapidly that within a comparatively few years the lake dropped 375 feet. When the outflowing water encountered a resistant limestone at the base of the gravel, the lake level again became constant and remained so until the Provo, largest of all Lake Bonneville terraces, was formed. Near Salt Lake City the Provo terrace appears at the foot of Ensign Peak and at Fort Douglas. Recognizable also in many other parts of the Great Basin, it stands in bold relief on the west side of the Wasatch and Oquirrh Mountains.
At the end of the Provo epoch the lake again resumed its downward course. After forming several intermediate terraces, the water dropped to a point approximately 300 feet above the present level of Great Salt Lake. Here for perhaps more than 1,000 years the waves ate into the jagged shoreline until they had produced the Stansbury Terrace, third in size and importance. In many places the record of the lake at the Stansbury level has been destroyed by erosion, but the terrace may still be seen from the Salt Lake-Ogden highway between the Municipal Baths and Becks Hot Springs (seeTour 1c). Between the Stansbury level and that of Great Salt Lake the waters of Bonneville left a great many small terraces, many of which appear today on the mountains at the north end of the present lake. More than fifty terraces, each recording a stage in the history of Lake Bonneville, have been recognized at various places within the Great Basin.
During the rise and fall of the ancient lake, streams flowing out of the mountains built deltas at the mouths of canyons. Many such deltas were formed while the lake stood at the Bonneville level, but nearly all of them were washed away and their sands and gravels reconsolidated into deltas on the Provo and other lower levels. Perhaps the most important remaining delta of the Bonneville stage is in Ogden Valley, where the village of Huntsville stands. At the Provo stage six major deltas--the Ogden, the Weber, the Logan, the Bear, the Provo, and the Sevier were built. In addition, many smaller deltas were scattered along the Wasatch from Santaquin to Brigham City. Much of the finest agricultural land in the State is of deltaic construction.
Sand and gravel bars formed by the action of longshore currents are further evidence of constructive forces at work in the old lake. Notable among these are the Stockton Bar at the north end of Rush Valley in Tooele County and the bar at Jordan Narrows near the Salt Lake-Utah County line. These embankments effectively separated Rush Lake and Utah Lake from the parent body of water. Rush, Utah, and Great Salt Lakes are thought to be present-day remnants of Lake Bonneville. Near Wendover, where Bonneville waters once covered the land, great flats extend westward into Nevada.
Although it is probable that Lake Bonneville teemed with fish and invertebrate forms, the only evidence of their existence is in the discovery of gastropods in lake deposits near the northwestern limits of Salt Lake City. The story of vertebrate life in the Bonneville area, however, is more nearly complete, for commercial removal of gravel along the base of the Wasatch Mountains has resulted in many enlightening discoveries. Along the shores of the old lake thousands of years ago roamed the hulking mammoth, the ponderous musk ox, and the predecessors of the modern camel and horse. Some of these creatures, attracted by water and abundant vegetation, wandered out on the deltas and sank in treacherous swamplands bordering the lake. Others were overcome by stronger beasts, and spring floods scattered their bones through gravel deposits at the water's edge. Although animals peculiar to glacial climates were common during the Provo stage of Bonneville history, they did not disappear entirely with the receding lake. In 1928 a post-Bonneville lava cave near Fillmore yielded the skull of a camel, apparently taken into the cave by some ancient carnivore.