THE PIRACY OF THE YELLOWSTONE 27 I 



With the lowering of these two barriers, other barriers were 

 uncovered in the path of the stream above. The most important 

 of these is a ridge of firm rhyolite in the bottom of the Thistle 

 Creek narrows. This became a large factor in the history of the 

 Yellowstone Lake, when in the cutting of the canyon at this point, 

 this firm rhyolite was reached, at a level about sixty feet above 

 the present lake. The lake level since then has waited on the 

 lowering of this one barrier. It is the only barrier which now 

 determines the lake level, altho it seems plausible that in 

 earlier stages, a barrier at Mud Geyser, and perhaps even the 

 Upper Fall barrier, were agents also in maintaining the lake at 

 the sixty-foot terrace, the action on each barrier being much 

 deferred by the lack of gradient due to the former higher eleva- 

 tion of these lower barriers. 



This is the postglacial history of Yellowstone Lake and 

 Canyon as it may be read from the data in hand. The whole 

 great lake, with its drainage basin of about fifteen hundred 

 square miles, was captured by the little Sulfur Creek canyon, 

 taken bodily from the Snake River and the Pacific slope, and 

 added to the Lamar River and the Atlantic slope. And the 

 volume of water in the captive stream was so great as to 

 dominate the lower valley of the Lamar, and reduce that older 

 stream to the rank of a minor tributary. 



John Paul Goode. 

 University of Chicago. 



