646 THE PLEISTOCENE PERIOD 



Duration of the Glacial Period 



The desire to measure the great events of geological history in 

 terms of years increases as our own time is approached. The un- 

 certainties attending such measurements are, however, so great 

 that the results have an uncertain value, and do little more than 

 indicate the order of magnitude of the time involved. Attempts 

 have been made (i) to estimate the relative duration of the several 

 glacial and interglacial epochs, and (2) to estimate in years the time 

 since the close of the glacial period. 



1. The best data for estimating the relative duration of the 

 several glacial stages are found in the central basin of the Mississippi, 

 for here only are all members of the drift series present. The 

 criteria that have been used in estimating relative duration embrace 

 (i) the amount of erosion of the several drift sheets, (2) the depth of 

 leaching, weathering, and decomposition of its materials, (3) the 

 amount of vegetable growth in interglacial intervals, (4) the climatic 

 changes indicated by interglacial and glacial floras and faunas, (5) the 

 time needful for the migration of faunas and floras, particularly 

 certain plants whose means of migration are very limited, (6) the 

 time required for advances and retreats of the ice, and some others. 

 A few of these, as the first, are subject to direct measurement; but 

 most of them are matters of judgment. By the use of these data, 

 it has been" estimated that the time since the Kansan drift was 

 deposited is some 15 to 20 times as long as the time since the last 

 glacial epoch. 



2. Of the efforts that have been made to measure in years post- 

 glacial time, those based on the recession of Niagara and St. Anthony 

 Falls are the most .significant. 1 In both these instances, the meas- 

 urement attempted is the time occupied in the recession of the falls 

 from their starting point to their present positions. 



If the length of the Niagara gorge be divided by the average an- 

 nual retreat since the falls were first located by accurate surveys, 



1 References on Niagara: Gilbert, Am. Jour. Sci., 3d ser., Vol. XXXII, 1886; 

 Science, Vol. VIII, 1886; Chapter in Physiography of the United States, and Bull. 

 U. S. Geol. Surv. Upham, Am. Jour. Sci., 3 d ser., Vol. XLV; Jour. Geol, Vol. I, 

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

 1896. Spencer, Evolution of the Falls of Niagara; Taylor, Bull. Geol. Soc. Am., Vol. 

 IX, p. 84, and Vol. XXIV. 



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

 Surv. of Minn., 1876; Geol. of Minn., Vol. II, 1888, 2 3 d Ann. Rept., 1894; Southall, 

 The Epoch of the Mammoth, p. 373. 



