THE PIRACY OF THE YELLOWSTONE 



269 



The trend of physiographic history in the region was sud- 

 denly changed when the col was cut thru by the advancing 

 canyon. The water of the lake began to flow out to the north, 

 the increased volume very greatly hastening the deepening and 

 widening of the trench. The lake level was rapidly lowered, 

 the Overlook Mountain outlet was suddenly abandoned, and 

 with this change the continental divide was transferred to its 

 present position south of the lake. The lowering of the lake 

 level was extremely rapid for a hundred feet, while the outlet 

 was cutting in the decomposed rhyolite merely. In the hundred 

 feet of rapid lowering but slight traces of shore action on the 

 lake could be expected. But this rapid lowering was checked 

 when the river reached the 7800-foot contour, for it came upon 

 a wall of firm, undecomposed rhyolite standing squarely across 

 its path — the site of the present Great Falls — and the river 

 settled down to the task of sawing this barrier in two. It is still 



at the task, with nearly a quarter of its 

 work yet to do. This barrier is only 

 about a hundred feet thick, and is very 

 plainly marked in the brow of the canyon 

 wall, forming a narrow gateway thru which 

 the water rushes. The inner walls of this 

 gateway are very precipitous, as may be 

 seen in the familiar view of the Great Falls. 

 Immediately above and below this gate- 

 way the canyon walls fall away to a wide 

 V-shape in section. The plan, Fig. 4, shows 

 the relation of the barrier to this fall, and 

 how the canyon is narrowed to the precipitous gateway in the 

 barrier. As seen from the down-stream side, this barrier is evi- 

 dently cut down a little over half its height, and one may easily 

 conjecture that this fall, which is now 312 feet high, must have 

 earlier been much higher, perhaps even 700 feet. The present 

 brow of the fall is near the up-stream face of the barrier, and 

 standing at the brow one may see that the firm rock of the bar- 

 rier projects at the bottom on the east side of the stream, as a 



Fig. 4. — Diagram of the 

 barrier at the great fall. 



