56 REPORT UNITED STATES GEOLOGICAL SURVEY. 



that tke flow did not occur in a lake, but in a deeply eroded surface of 

 dry land. 



The present Yellowstone Lake is so well known as to need no extended 

 description here. Its shores are generally flat and timber-covered, mead- 

 ows occurring at rare intervals. The only mountains that approach it 

 so closely as to give it their drainage are the Yellowstone Mountains to 

 to the east; even these do not rise directly from its shores, but stand 

 back from 5 to 10 miles. 



A good idea of the character of the surrounding country may be had 

 by observing that if the waters of the lake were raised 200 feet the wa- 

 ter surface would be more than doubled. The history of the lake bas 

 not yet received the attention its importance demands. The great ex- 

 tent of the deposits of the ancient lake are dwelt upon at length, by Dr. 

 Hayden in the reports for 1871 and 1872. 



My own observations tend to show that his estimates of the thickness 

 of the deposits and of the elevation of the old shore-line are too great 

 and contrary to what he surmised, and to what any observer might be 

 led to suppose, the ancient lake expanded to the north and east consid- 

 erably more than to the south and west, where the country seems open 

 through to the broad plateaus of the Pacific Slope. My own observa- 

 tions in the southwest are, however, by no means satisfactory. 



In Plate (XXXI) 4 I present a sketch of the Yellowstone Lakes, an- 

 cient and modern, the shore-line of the ancient lake being defined as 

 far as possible by the actual occurrences of its deposits. Where these 

 deposits have been removed by subsequent erosion, the outline is con- 

 tinued along the contour which represents the ancient lake level. 



The level of the present lake, as determined by the most recent ob- 

 servations, is 7,738 feet. Among the many barometric observations 

 made on the highest levels of the lake deposits there is no single one 

 which reaches 300 feet above the present lake, the highest record be- 

 ing made at a point near the Howard road, some 8 or 9 miles west of the 

 Mud Geyser. This, by aneroid, is 8,028 feet. 



On Pelican Creek the highest terrace has a general level of 7,950 feet. 

 On the eastern shore of the lake, and so far as I have observed on the 

 south, the highest terraces are not over 100 feet above the preseni lake. 

 It is possible that higher levels exist. The heavy growth of forest 

 makes it very diflicult to trace beach-lines that might otherwise be fol- 

 lowed with the greatest ease. 



The most extensive deposits occur in the valley of Pelican Creek and 

 in Hayden's Valley. The region occupied by the Pelican Creek deposits 

 was formerly a wide bay of the ancient lake. The deposits consist of 

 coarse sandstones and conglomerates, both of which seem to have been 

 greatly altered by solfataric action. The series of fine terraces bordering 

 Pelican Creek are probably in some cases river terraces. 



The western shore of the ancient lake has not differed greatly from 

 that of the present lake, but on the south its waters have probably cov- 

 ered a very extended area. 



Promontory Point existed as an island, but most of the flat tongues 

 of land separating the arms were covered by the waters of the old lake. 



The low divide that seperates the waters of the Yellowstone from the 

 Snake Eiver drainage is from 2 to 10 miles from the lake shore and must 

 have been almost on a level with the ancient lake. 



There are no indications, however, that the lake ever had an outlet 

 in this direction, although one is at a loss to see why such a large part 

 of the drainage of the rhyolite plateau should cut its way through the 

 great metamorphic range to the north, when the divide to the south and 

 west presents such inconsiderable barriers. 



