220 LIFE OF THE PLEISTOCENE 



preserved material. It is obvious that but a small percentage of the remains 

 of the biota could by any possibility be preserved during the invasion and 

 occupancy of a gigantic ice field, such as is known to have covered the engla- 

 ciated region. It is likewise evident that the greatest care is necessary in 

 using this data, in order that errors may be eliminated; and only that informa- 

 tion can be used that is known beyond reasonable doubt to be referable to the 

 particular interval discussed. There are many records that cannot be admitted 

 because of lack of precise geologic data. In many cases it is difficult to deter- 

 mine whether certain deposits are to be correlated with the till sheet (Kansan, 

 Iowan, etc.) upon which they rest, or are the product of much later geologic 

 influences. Likewise, it is practically impossible to correlate the deposits 

 beyond the englaciated territory with those within this area. It is evident, 

 therefore, that only that datum is available which lies in or between the drift 

 material deposited by the great continental ice sheets. 



b. Imbrication of Drift Sheets 



Fortunately, the different drift sheets are imbricated and we are thus able 

 to study their physical characteristics, and to trace one drift sheet until it 

 disappears beneath that of a later period. Beneath the later drift sheets, the 



Figure 4. Diagram to illustrate superposition and imbrication of drift sheets (After 

 Chamberlin and Salisbury). 



earlier deposits are largely discontinuous and in many places, especially be- 

 neath the later Wisconsin drift, are practically absent. The five drift sheets 

 are superimposed as shown in the annexed diagram 5 (figure 4). "Theoretically 

 and perhaps really, the several sheets of drift are imbricated as shown in the 

 figure, but each sheet of drift is discontinuous beneath the overlying one, and 

 this discontinuity goes so far that beneath the Wisconsin drift, for example, the 

 several sheets are more commonly wanting than present. " It is to be noted 

 that the earlier, or Nebraskan drift sheet, does not extend beyond the limits 

 of the next younger, or Kansan, and the limit of glaciation of this period is 

 believed to have been considerably less than that which followed. In New 

 Jersey, an old drift, the Jerseyan, extends beyond the limits of the later drift 

 and is believed to be the equivalent of the Nebraskan. 6 

 • 



6 Chamberlin and Salisbury, Geology, III, page 394. 

 • Chamberlin and Salisbury, Geology, III, p. 384. 



