704 THE ICE AGE IN NORTH AMERICA. 



my knowledge. This delta of bowlders had been covered with 

 a thin deposit of loam, and the city of Pocatello was built 

 upon it. While I was there the city authorities were endeavor- 

 ing to put in a sewer system, but were impeded by the general 

 occurrence of these obstructing bowlders of great size. This 

 bowlder bed is just where the Port Neuf River debouches 

 upon the Snake River Plain. But upon going up the tributary 

 valley a half mile or so the large bowlders ceased, and there 

 was a scoured out channel free from them. 



The explanation of this puzzling phenomenon was soon 

 found in the result of Mr. Gilbert's investigations respecting 

 the history of Lake Bonneville, to which reference has already 

 been made. It was through the Port Neuf River that the 

 great debacle from Lake Bonneville poured when the rising 

 water surmounted the 1000-foot dirt barrier which retained 

 the upper 350 feet of water over an area of 20,000 square 

 miles. Mr. Gilbert estimates that it would require twenty 

 years for a stream as large as Niagara to lower this body 

 of water to the bottom of the dirt dam which was finally 

 broken through, and that a stream as large as that did mean- 

 while pour through the opening leading to the Port Neuf, 

 and so on into the Snake River Valley at Pocatello. Here, 

 therefore, we have not only an adequate cause for the bowlder 

 bed at Pocatello with all its peculiarities, but for the seemingly 

 anomalous facts in the valley at Nampa 350 miles below, and 

 at a level 2000 feet nearer that of the sea. The conditions 

 were such as to favor the rapid accumulation of fine sediment 

 which appears above the mouth of Boise* River, and above the 

 great constriction of the channel which occurs a short distance 

 beyond at Huntington, and continues for a long distance 

 below. 



Light is shed upon the geological date of this debacle by 

 the fossils found at Glens Ferry, about half way between Po- 

 catello and Nampa, and described by Dr. Dall. They are as 

 follows: Goniobasis taylori Gabb (sp.), Lithasia antiqua Gabb, 



