a 
PIR Py sa 
W. M. Davis—Gorges and Waterfalls. 127 
chell* to result from a displacement of the river from its older 
and wider channel that ran west of the present course from a 
point just above Minneapolis, down to the wide channel of the 
Minnesota River, three miles above Fort Snelling. The Missis- — 
sippi, forced by an ice or a drift blockade to depart from its old 
channel, rose till it reached the level of the limestone bluffs of 
its eastern bank; a lake was thus formed from which abundant 
deposits of fine brick-clay remain. The overflow ran across 
country about eight miles to Fort Snelling, where the ol 
channel was again taken ; in plunging over the limestone bluffs, 
the waters gave birth to the Falls of St. Anthony. Since that 
time, the falls have receded to their present position, leaving a 
narrow valley below them; and from the records of earl 
exploration Professor Winchell has computed that this recession 
of eight miles has occupied somewhat less than nine thousand 
years. The Falls of Minnehaha are in a side ravine branching 
from this new channel of the Mississippi a little over a mile 
above the original site of the Falls of St. Anthony at Fort 
Snelling. Their ravine was therefore begun about a thousand 
years later than the Mississippi channel, and being the work of 
a much smaller stream, their recession is only about one mile > 
from the front of the new Mississippi bluffs. Several still smaller 
falls along the bluffs have the same origin. h 
St. Croix and the rapids of St. Louis rivers, also in Minnesota, 
may probably be referred to postglacial erosion, but explicit 
statements of their origin are not to be found. The southern 
peninsula of Michigan must also furnish many examples of 
newly made falls and gorges, for the streams there are de- 
scribed as generally flowing on the drift, but sometimes cutting 
down into the rock. 
Professor I. C. White, of the Geological Survey of Pennsyl- 
vania, has given several excellent descriptions of gorges. Wal- 
lenpaupack Creek, dividing Wayne = ike counties in the 
northeastern part of the State, is one of the most striking 
examples of its class. In preglacial times, it had carved out a 
wide and deep channel in Catskill strata from the Pocono 
plateau northeastward to its junction with the Lackawaxen, 
having a descent of four hundred feet in eleven miles without 
_ any notable falls so far as can be discovered. During the 
Glacial period, the lower end of the channel was filled with drift 
to a depth of three hundred feet, so that, where the ice melted 
_ away, the re-established drainage found a shorter cut and lower 
passage to the Lackawaxen about three miles above its former 
mouth. For ten miles above the obstruction, the ’Paupack 
bE meanders through a broad, flat, swampy valley, the bottom of 
_ arecently extinct lake, with a fall of only half a foot ina mile: 
* Geol. Minn., 4th Ann. Report, 1876, 178. 
