THE PLEISTOCENE OR GLACIAL PERIOD. 415 



(2) Of the efforts that have been made to measure in years the 

 post-glacial interval, those based upon the recession of Niagara and 

 St. Anthony Falls are the most important, and are all that can be 

 considered here. 1 It is important, however, to note precisely what is 

 being measured. In both these instances, the measurement attempted 

 is the time occupied in the recession of the falls from the point of their 

 initiation to their present positions. It is as important to know when 

 they began their gorge cutting, as to know how long they have been 

 occupied in it. The gorge-cutting of the Niagara Falls could not have 

 begun until the Mohawk outlet of the ice-ponded lakes, previously 

 sketched, was abandoned, because the escarpment through which the 

 cutting subsequently took place was still submerged while the lake 

 discharged through the Mohawk valley. The time measured by the 

 Niagara cutting was only that which has elapsed since the ice-border 

 retired from the northeast flank of the Adirondacks sufficiently far 

 to permit the waters of the ancestral Lake Ontario to find an outlet 

 lower than the Niagara escarpment, and no very effective cutting 

 could take place until the waters were withdrawn to something near 

 their present level. 



If the border of the ice-sheet at this stage (Fig. 522) is compared 

 with the border of the ice at the maximum Late Wisconsin stage 

 (Fig. 470), it will be seen that a retreat of the ice-border, measured 

 along the axes of the more protrusive lobes, of some 600 miles had 

 taken place. In the course of this retreat, about a score of morainic 

 ridges had been formed. Some of these appear to have represented 



1 References on Niagara: Pohlman, Am. Assoc. Adv. Sci., Vol. XXXII, 1883, 

 and Vol. XXXV, 1887; Science, Vol. II, 1S83, and Vol. VIII, 1886; Trans. Am. Inst. 

 fein. Eng., Vol. XVII, 1889, and Eng. and Min. Jour., Vol. XLVI, 1888. Wright, 

 Am. Jour. Sci., 3d ser., Vol. XXVIII, 1884; Sci. Vol. V, 1885; Bibliotheca Sacra, 

 1884 Proc. Am. Assoc. Adv. Sci., Vol, XLVII, Science, new ser., Vol. VIII; Am. 

 Geol., Vol. XXII, 1898; Pop. Sci. Mo., Vol. LV, 1899, and Am. Jour. Sci., 3d ser., 

 Vol. XXVIII. Gilbert, Am. Jour. Sci., 3d ser., Vol. XXXII, 1886; Science, Vol. 

 VIII, 1886; Proc. Am. Assoc. Adv. Sci., Vol. XXXV, 1887; Rept. N. Y. Com. State 

 Res. at Niagara, 6th Rept. 1890, and Chapter in Physiography of the United States. 

 Upham, Am. Jour. Sci., 3d ser., Vol. XLV; Jour. Geol., Vol. I, 1893; Am. Geol., Vol. 

 XI, 1893, and XVIII, 1896, and Pop. Sci. Mo., Vol. XLIX, 1896. Spencer, Am. 

 Jour. Sci., 3d ser., Vol. XL VIII, 1894, and Am. Geol., Vol. XIV, 1894; and Taylor, 

 Bull. Geol. Soc. Am., VoL IX, p. 84. 



St. Anthony Falls: Winchell, N. H. Fifth Ann. Rept, Natl. Hist, and Geol. 

 Surv. of Minn., 1876; Geol. of Minn., Vol. II, 1888, Twenty-third Ann. Rept., 1894; 

 Southall, The Epoch of the Mammoth, p. 373. 



