262 /. P. GOODE 



that both these valleys were old and well developed before the 

 rhyolites were poured out to form the Park plateau in Pliocene 

 time. The lower courses of both these valleys are masked 

 by the rhyolite flows, and the lake depression itself may be 

 suspected to be a great mountain valley obstructed by lava 

 flows. 



The divide west of the lake lies on the flat-topped rhyolite 

 plateau, and at various places there are cols of significant shape 

 and altitude. Plainly some of them have been lines of drain- 

 age, showing that at some time water has flowed across the 

 divide, making well-defined valleys. The stage road from the 

 Upper Geyser Basin to the "Thumb," as the west arm of the 

 lake is locally called, passes through one of these notches at the 

 continental divide east of de Lacy Creek. It is rather a narrow 

 valley, with walls perhaps a hundred feet high, cut right across 

 the crest of the divide, yet flat-bottomed and at present marshy 

 and undrained. 



It is believed that this whole region has been covered with 

 ice moving west from the Absarokas and north from the Tetons, 

 and it may easily be supposed that in the unequal recession 

 of the ice margin, obstructed drainage would give rise to over- 

 flow to the west, establishing channels that would be aban- 

 doned on a further recession of the ice. But there is one such 

 channel which gives evidence of very long use even after the ice 

 had left the plateau. This is a "windgap" between Overlook 

 and Channel mountains at D in the map, page 263. Here a 

 canyon with walls several hundred feet high cuts across the pres- 

 ent divide, down almost to the contour of 7900 feet. Yet this 

 surprising notch is poorly drained, puny streams starting from 

 the marshy col and flowing to opposite oceans. The eastern one 

 is an unnamed branch of Grouse Creek, the one to the west, called 

 Outlet Creek, leads into the Heart Lake basin and so south to 

 the Snake River. This notch" has been recognized as a former 

 outlet of the lake, and the fact is well known that the lake was 

 once at this altitude, about one hundred and sixty feet above its 

 present level. Lacustrine deposits are recorded on the United 



