THE OVEELAND EOUTE — COUNCIL BLUFFS TO OGDEX. 



97 



From 



o 



more 



advantage in its contest with precipita- 

 tion that there was immense expansion 

 of the water surface. When the hike was 

 largest it was comparable in area and 

 depth with Lake Michigan; it had eleven 

 times its present extent. In attaining 

 this great expanse the water surface rose 

 to a position more than 1 ,000 feet above 

 its present level. 



To this great body of water geologists 

 apply a distinctive name. Lake Bonne- 

 ville, and they have given much atten- 

 tion to its history, which is written in 

 shorelines, deltas, channels, deposits, and 

 fossils. The shore lines 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, giving 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, w^iich 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 

 I>owerful as those of Lake Michigan and 

 fashioned the shore into an elaborate sys- 

 tem of cliffs, beaches, and spits; and 

 when the waters finally fell to lower levels 

 they left behind the shapes theii* waves 



had made. The base of each aurvivaig 

 shore cliff is a horizontal line, and so is the 

 crest of each beach, bar, and spit, and 

 these features in combination trace the 

 outliae of the old lake as a level contour 

 about the sides of the basin and the faces 

 of mountains that were once islands in the 

 lake. 



In rising and falling the waters lin- 

 gered at many levels, and so there are 



many ancient shore lines, but two of 

 them are more conspicuous than the rest 

 and have been named. The highest of 

 all is the Bonneville shore lino^ and 375 

 feet lower lies the Prove shore line. The 

 Bonneville line represents a relatively 

 short stand of the water and is conspicu- 

 ous chiefly because it marks the bouad- 

 aryof wave action. All the slopes below 



it have been more or less modified by the 

 waves, but the slopes above it retain the 

 shapes which had been given them by 

 other agencies. The Provo line repre- 

 sents a long stand of the water and is con- 

 spicuous because it is strongly sculptured. 

 In all the early historj^ of the great lake 

 its basin was closed, like that of the mod- 

 ern lake. The water surface rose and fell 

 in response to climatic changes, like that 

 of its modern remnant. The last great 

 rising was the highest and terminated the 

 series of oscillations by creating an outlet. 

 The lowest point of the basin's rim was at 

 Red Rock Pass (90 miles by rail north of 

 Ogden), and when the water rose above 



cross 



pass 



tributary to Snake River, the chief branch 

 of the Columbia. Through the creation 

 of IhLs outlet the Bonneville Basin, which 

 had previously contained an independent 

 interior drainage system, became part of 

 the drainage system of the Pacific Ocean. 

 Red Rock Pass was not. a mountain 

 pass, a notch in a rocky crest; it was 

 merely the highest point on the axis of 

 a valley between two mountain ranges. 

 Valley and ranges ran north and south 

 and the valley wa^ floored by alluvium 

 washed from the ranges. From the Red 

 Rock summit the valley sloped gently 

 northward toward the Portneuf and south- 

 ward toward Bear River. The formation 

 at the summit consisted of soft earth, and 

 as soon as overflow began a channel 

 formed. The deepening of the channel 

 increased the volume of the stream by 

 lowering the outlet of the lake, the greater 

 stream was more efficient in deepening 

 thp> r-TinnnpL and these two causes inter- 



92213°— Bull. G12— 15 



■7 



