HHUSHKV 



The Hirer Terraces of the Orleans Basin. 



469 



tion of pretending to approximate closely to the age of the ter- 

 races, but simply to fix minima which everyone can accept. 



Basing the estimates solely on the depth of the erosion, we 

 will get the following result: The rock canons have depths 

 respectively of 20, 35, 45, 315, 200 and 120 feet. The corre- 

 sponding periods of time will be 2,000, 3,500, 4,500, 31,500, 

 20,000 and 12,000 years, or a total for the terrace-forming period 

 of 73,500 years. 



But the above is obviously not a fair estimate, as it fails to 

 take into account the very important factor of different widths 

 of the several canons. I will allow the Modern canon an average 

 width of 750 feet, which is a liberal estimate. With higher 

 canons I will be more strict in my estimates. The upper two 

 may be considered approximately equal in width, because the 

 lower occupied as much new territory as it left in the form of 

 the 675-foot terrace. 



The average widths of the canons are respectively 750, 1,500, 

 2,000, 3,000, 5,000 and 5,000 feet, and the corresponding cross- 

 sections, 15,000, 52,500, 90,000, 945,000, 1,000,000 and 600,- 

 000 square feet. By this method we derive as the length of 

 time occupied in the formation of the canons, 2,000, 7,000, 

 12,000, 126,000, 133,000+ and 80,000 years, or a total for the 

 terrace-forming period of 360,000 years. The age of the 120- 

 foot terrace, which I correlate with the Intermediate glacial 

 deposits, will thus be 21,000 years. This agrees fairly well with 

 estimates of the probable age of the Iowan drift of the Missis- 

 sippi Basin, and constitutes another point in favor of consid- 

 ering the Intermediate drift as pre-Wisconsin in age. 



The last estimate of the age of the respective terraces seems 

 large, but is probably fairly conservative. It is based on the 

 supposition that the abnormally rapid rate of erosion of the 

 hi<;h-grade Modern canon obtained throughout the terrace 

 period. However, it is a well-known fact that the rate of erosion 

 greatly decreases as the valley widens and the grade lowers. 

 Taking this into consideration, it is probable that the age of 

 the Modern canon could be cut to 1,000, or even 500 years, 

 without necessarily reducing the age of the 850-foot terrace 

 below the 360,000 years which I have assigned to it. However, 



