THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XXV III. — Weathering and Erosion as Time Measures /* 

 by Frank Leverett. 



The writer presented a paper, under the above title, at the 

 Baltimore meeting of the Geological Society of America, 

 which set forth the advantages of estimates of weathering and 

 erosion in making correlations between the drift sheets of 

 Europe and America. Since the correlation of European 

 with American drifts is to be presented as a special paper in 

 the international glacial magazine, Zeitschrift fiir Grletscher- 

 kunde, the present paper will aim only to set forth the value 

 of weathering and erosion in making correlations, and will be 

 restricted mainly to our American deposits. The topographic 

 sheets now published embrace each of the drift formations to 

 a sufficient extent to enable the student to use them in com- 

 paring glacial formations of different age. They will thus 

 serve a valuable purpose in the class room. The present 

 paper will fulfill its mission if it helps to a proper understand- 

 ing of topographic sheets illustrating each of the glacial drifts. 



There are certain situations in which the student is able to 

 determine, by erosion alone, the relative ages of the different 

 drift sheets. This can be done where the surface of the drift 

 was left in a flat or featureless condition so that the present 

 reliefs are almost wholly the result of stream action, and it is 

 especially valuable where the streams have been working 

 under sufficiently high gradient to admit of the cutting of well- 

 defined channels. There are places where stream gradients 

 are so low that little or no deepening of the drainage lines has 

 occurred. In such situations and in situations where the drift 

 topography is of a ridged or complicated character, weathering 

 naturally is made a leading criterion for judging of relative 



* Published by permission of Director U. S. Geological Survey. 



Am. Jour. Sci. — Fourth Series, Vol. XXVII, No. 161. 

 24 



•May, 1909. 



