350 Leverett — Weathering and Erosion as Time Measures. 



ages of the drift sheets under comparison. The rapidity of 

 weathering is especially great in fragmental formations, such 

 as the glacial and fluvial deposits, since a multitude of faces 

 are exposed for attack. It is thus possible to distinguish 

 lesser differences in age than would be detectible in the solid 

 formations. Weathering of glacial material or of fluvial mate- 

 rial will reach depths to be measured in feet and meters, while 

 that in rock formations may reach only a few inches or centi- 

 meters. 



There are certain complications in these studies resulting from 

 differences in climatic conditions and differences in the consti- 

 tution of the drift which have to be properly considered in 

 forming the estimates. Leaching goes on so much more 

 rapidly under humid conditions than under semi-arid that one 

 may easily be misled if he depends entirely upon the depth to 

 which the leaching extends. In the same way erosion studies 

 are liable to be misleading if one does not take into account 

 differences of rainfall, in stream gradient, and in the texture 

 of the deposits. It is necessary also, in glaciated regions, to 

 ascertain the work accomplished by the drainage from the ice 

 sheet, in order to properly compare the work done by the 

 present streams on drift sheets of different age. 



In the glaciated portions of North America there are, fortu- 

 nately, extensive plains of Kansan, Illinoian, and Wisconsin 

 drift and of glacial lake beds, whose conditions of rainfall, 

 stream gradient and texture of deposits, are sufficiently alike to 

 afford excellent subjects for time estimates based on weathering 

 and erosion. It is not claimed that these studies will give time 

 measures definitely expressible in thousands of years, at least 

 not in the present state of our knowledge ; but it is thought 

 that a sufficiently reliable measure of relative age is afforded to 

 form a basis for wide correlations, such as those between the 

 American and European drift sheets. 



The table here presented gives the names applied to the 

 several members of the glacial series in each prominent field 

 of Pleistocene glaciation. Some of the lesser fields, west of 

 the Rocky Mountains, may have a similar complexity, but this 

 has not been so fully established. The Skandinavian field 

 has had a great variety of interpretations and stills awaits 

 a satisfactory nomenclature. It will, perhaps, serve our pur- 

 pose best to make use of the terms employed by the German 

 geologists, though here it must be understood that the earlier 

 literature is not fully in harmony with the later. The order 

 of arrangement in the table is from younger to older in each 

 field. 



