THE PLEISTOCENE OR GLACIAL PERIOD. 413 



The Duration of the Glacial Period. 



The desire to measure the great events of geological history in 

 terms of years increases as the events approach our own period and 

 more intimately affect human affairs. The difficulties attending 

 such attempts are, however, formidable, and the results have an uncer- 

 tain value. At best they do little more than indicate the order of 

 magnitude of the periods involved. Geological processes are very 

 complex, and each of the cooperating factors is subject to variations, 

 and such a combination of uncertain variables introduces a wide range 

 of uncertainty into the results. 



Efforts to determine the date and duration of the glacial period 

 fall mainly into two categories: (1) efforts to estimate the relative 

 duration of the several glacial and interglacial epochs, and (2) efforts 

 to measure in years the interval since the close of the glacial period. 



(1) The best data for estimating the relative duration of the sev- 

 eral glacial stages are found in the region bordering the Mississippi 

 river, for it is there only that all members of the series are present. 

 There only also do they come into such relations with one another 

 as to furnish fair facilities for comparison. The criteria that have 

 been used in estimating relative duration embrace (1) the surface 

 erosion and the cutting of special gorges, (2) the depths of leaching 

 and weathering, (3) the internal changes, (4) the decomposition of 

 the pebbles and bowlders, (5) the amount of vegetable growth in 

 interglacial intervals, (6) the climatic changes indicated by floras 

 and faunas, (7) the times needful for the migration of faunas and 

 floras, particularly certain plants whose means of migration are very 

 limited, (8) the times necessarily required for advances and retreats 

 of the ice. and similar means. A few of these are subject to direct 

 measurement, as the relative amounts of erosion; but for the greater 

 part they are matters of judgment, in which the value of the result 

 is much affected by the personal equation. 



A collation of the judgment of five of the glacial geologists who 

 have most studied the data in their most favorable expressions is the 

 basis for the estimates embodied in the following table. In this case, 

 the time-datum for each sheet of till is the stage at which it began 

 to suffer erosion, which, of course, would be slightly after the beginning 



