TERTIARY LAKE BASIN OF FLORISSANT 147 



saws Still remain in the stump. A photograph of one of these stumps 

 is reproduced in Fig. 3. 



It was long ago pointed out that the drainage and destruction of 

 the lake were probably brought about by the tilting of the region 

 from the southeast, reversing the drainage and causing the lake to 

 overflow its northwestern rim and cut down its barrier at that end. 

 It is beheved that the valley formerly drained southward into the 

 Arkansas River, but now, with the exception of the extreme southern 

 portion, the drainage is westward into the South Platte. The south- 

 eastern end of the basin, instead of being lower than the northwestern 

 extremity, is now about 800 feet higher.^ If the lake was in fact thus 

 drained, it is interesting to note that at present the basin formerly 

 occupied by it still drains partly to the south, as a glance at the accom- 

 panying map will show. This means either: (i) that the drainage 

 was not wholly reversed, but that the axis of the deformation crossed 

 the lake near the southern end, thus dividing it into two portions, 

 one draining into the Arkansas and the other into the South Platte; 

 or (2) that the drainage of the southern portion has been captured 

 in more recent times by the headward progression of the channel from 

 the south. 



In the literature of the subject there appears to be no hint as to 

 the origin of the lake, and in the field we have found nothing to suggest 

 any of the known causes of mountain lakes except the warping of 

 the earth's crust. As there is evidence of a general tilting of the region 

 either during or subsequent to the deposition of these beds, it seems 

 quite possible that it was only a continuation of the process which 

 originally created the lake. The great majority of mountain lakes 

 of Colorado are glacial, in this respect differing from the lakes of the 

 plains. Lake Florissant was certainly not glacial. The geological 

 map shows a volcanic accumulation south of the basin in such posi- 

 tion as to act as a possible barrier. This we did not visit. It should 

 be examined with reference to this problem before any final conclusion 

 is reached. Barriers of gravel and boulders produced by the out- 

 wash from high-grade lateral gulches and in other ways would not 

 seem sufficient to account for this lake and its deposits. 



» Twelfth Ann. Rept., Hayden Survey, Part i, p. 274. 



