212 matured flfcfracles. 



and Iowa is probably to a great extent a water 

 deposit. It is the kind we find in the bottom 

 of a pond that has stood for many years, and 

 it would seem that at some period all this 

 prairie country with the black soil was the 

 bottom of a great lake. 



The facts of a glacial period are beyond 

 question, but when it occurred, and how it 

 occurred are questions that many have tried to 

 answer. So far, all that we can say of them 

 is that some of them are shrewd guesses. The 

 evidences adduced for determining the time, 

 are the erosion caused by rivers and streams 

 since the ice subsided. Some of the rivers 

 and outlets of lakes had their courses changed 

 by the action of the ice, so that when it sub- 

 sided new water courses were formed, and the 

 erosion that they have produced from that 

 time to the -present furnishes the data for de- 

 termining the time since the subsidence of the 

 ice at any particular point. For instance, 

 Niagara Falls was undoubtedly at one time 

 situated at Queenstown, a number of miles be- 

 low its present position. And the time that it 

 has taken to grind out the great gorge that 

 exists between that point and the present falls 

 is approximately a measure of the time that 

 has elapsed since the subsidence of the ice at 

 that point. Various estimates have been 

 made to determinate the rate of erosion. The 

 earlier ones put the time at about 35,000 years. 



