22 DYNAMICAL GEOLOGY. 



Time. Several attempts have been made to estimate 

 the time occupied in this process. Mr. Lyell estimated it 

 at 35,000 years.* A large part, if not the whole of this 

 time, belongs to the present geological epoch, and was 

 probably witnessed by early man. 



Other Falls. Many other perpendicular falls have 

 receded in a similar way and given rise to similar gorges. 

 The most remarkable of these are the Falls of St. Anthony. 

 The Mississippi River, at Fort Snelling (mouth of the 

 Minnesota River), is traversed by an escarpment which 

 separates a higher from a lower plateau. The river runs 

 on the upper plateau as far as Minneapolis, then drops, 

 by a nearly perpendicular fall, into a gorge one hundred 

 feet deep, runs in this gorge eight miles, and then emerges 

 on the lower plateau at Fort Snelling. Here, again, we 

 have the upper plateau capped by a hard limestone, under- 

 laid by a soft sandstone. Here, also, the wearing away of 

 the underlying sandstone causes the limestone to project 

 in overhanging tables which fall from time to time into 

 the chasm below, and so the fall works backward. There 

 is no doubt that the Mississippi at one time fell over the 

 escarpment at Fort Snelling, and has worked its way back 

 to its present position, and that this all took place during 

 the present geological epoch, and while man inhabited the 

 continent. Professor Winchell has estimated that, at its 

 present-rate recession, it would take not more than 8,000 

 years to accomplish the work. 



Minnehaha River is a tributary running into the Mis- 

 sissippi about six miles below the falls. It therefore, at 

 one time, fell into the gorge. It has now worked itself 

 back about two miles, and forms the beautiful " Minne- 

 haha Falls," made celebrated by their description in Long- 

 fellow's " Hiawatha." 



The Columbia River, where it breaks through the 

 Cascade Range, has cut a gorge fifty miles long and 1,000 

 * Later estimates make it about 11,000 years. 



