DENVER & RIO GRANDE WESTERN ROUTE. 



229 



traveler may get his first general view of the Great Salt 

 Lake basin. Originally this plain was only a desert, but now it is 



would still be conspicuous by reason 

 of its position. As it is, no geologic 

 insight is necessary to discover it, for 

 it is one of the pronounced features 

 of the country. It confronts all be- 

 holders and insists on recognition. 

 The tourist who visits Ogden and Salt 

 Lake City by rail sees it on the 

 Wasatch [Mountains] and on the 

 islands of Great Salt Lake and makes 

 note of it as he rides. The farmer 

 who tills the valley below is familiar 

 with it and knows that it was made 

 by water ; and even the cowboy, find- 

 ing an easy trail along its terrace as 

 he ' rides the range,' relieves the mo- 

 notony of existence by hazarding a 

 guess as to its origin." 



Gilbert followed this shore line, 

 studied it in detail, and mapped it 

 throughout most of its sinuous course. 

 The map copied from his report (see 

 fig. 60) shows the greatest extent of 

 Lake Bonneville as compared with the 

 present Great Salt Lake. 



The history of Lake Bonneville goes 

 back to a time before man was known 

 on the globe, or possibly about to the 

 time of his first appearance, but in 

 any event the conditions that led to 

 the formation of that great body of 

 water could not have been due to 

 man's activities and hence must have 

 been the result of climatic change. 

 Gilbert (U. S. Geol. Survey Bull. 612, 

 pp. 96-97, 191.5) gives the history of 

 Lake Bonneville as follows : 



"The latest of the periods into 

 which geologists divide past time wit- 

 nessed a series of climatic changes 

 which affected the whole earth, and 

 * * * tlie element which recorded 

 its changes most clearly was tempera- 

 ture. Tliere were several epochs of 

 cold, and they were separated by 

 epochs of warmth. During the cold 

 epochs the high parts of the Wasatch 

 Range held a system of glaciers, and 

 in one of them several ice tongues pro- 

 truded so far beyond the mouths of 

 80697°— 22 16 



the mountain canyons that they 

 heaped their moraines on the floor of 

 Jordan Valley, only a few miles from 

 the place where Salt Lake City now 

 stands. In that epoch of cold the rate 

 of evaporation was far slower than 

 now, and evaporation was at so great 

 a disadvantage in its contest with 

 precipitation that there was immense 

 expansion of the water surface. When 

 the lake was largest it v^-as comparable 

 in area and depth with Lake Michi- 

 gan ; it had eleven times its present 

 extent. In attaining this great ex- 

 panse the water surface rose to a po- 

 sition more than 1,000 feet above its 

 present level. 



" To this great body of water geolo- 

 gists apply a distinctive name — Lake 

 Bonneville — and they have given much 

 attention to its history, which is writ- 

 ten in shore lines, deltas, channels, de- 

 posits, and fossils. The shore lines [Pis. 

 LXXXIX, .4, and XCVI, B] appeal 

 most to the traveler and may be seen 

 from car windows at several points. 



"As a matter of definition a shore 

 is merely the meeting place of land 

 and sea, or of land and lake, but as 

 a matter of land form it is much more. 

 At the shore the lashing of storm 

 waves works changes in the land, giv- 

 ing it new shapes. At some places the 

 land is carved away ; at others it is 

 made to encroach on the water. 

 Where it is eroded the limit of erosion 

 is marked by a cliff, and below the 

 water is a shelf of gentle slope. 

 Where additions are made they take 

 the form of beaches or bars, which rise 

 little above the water level and are 

 composed of sand or gravel. At some 

 places a bar spans a bay from side to 

 side; elsewhere it is incomplete, pro- 

 jecting from a headland as a spit. 



" The waves of Lake Bonneville were 

 as powerful as those of Lake Michigan 

 and fashioned the shore into an elabo- 

 rate system of cliffs, beaches, and spits, 

 and when the waters finally fell to the 



