6o4 WILLIAM J. MILLER 



ACTION OF ICE 



Intraformational contorted clays have, by various writers, been 

 attributed to the action of ice but, as a rule, there has been little or 

 no attempt to really analyze the structures involved. Salisbury 

 and Atwood/ in their discussion of an extinct glacial lake near 

 Devil's Lake, Wisconsin, figure and very briefly describe intraforma- 

 tional contorted clays. They say: "The grounding of an iceberg 

 on the surface before the overlying layers were deposited, or the 

 action of lake ice, may have been responsible for the singular phe- 

 nomenon." In accordance with criteria set forth in the foregoing 

 discussion of the Connecticut Valley clays, the writer believes that 

 this corrugated zone must have resulted from differential movement 

 after deposition of the overlying beds. The remarkable uniformity 

 of thickness of the contorted zone; its relatively regular (nearly 

 straight) upper surface; and the gently bent immediately overlying 

 beds all strongly indicate that the corrugations developed under 

 weight of the overlying beds. If the corrugations were caused by 

 thrusting action of ice upon surface layers would not the con- 

 torted zone show notable variations in thickness and irregularity 

 of its upper surface, and would not the overlying beds fail to show 

 appreciable evidence of having been deformed? It is, however, 

 conceivable that the corrugated zone may have resulted from 

 differential movement brought about by the crowding action of 

 ice against the upper portion of the whole body of clay, thus setting 

 up a differential motion within its mass. Either this, or differential 

 movement brought about under the action of gravity (in case the 

 clays are at least moderately tilted), appears to have produced the 

 corrugations. 



J. Geikie,^ in his description of the early postglacial deposits of 

 the basin of the Forth in Scotland, says : 



Here and there also the beds (sands and clays) are much crumpled and 

 confused, great sheets of clay being rolled over and over, and involving the 



associated sands for considerable distances These are exceedingly 



irregular, and are just of such a character as we should expect would result 

 from the grounding of ice rafts. 



' Salisbury and Atwood, Jour. Geol., Vol. V (1897), p. 143. 

 * J. Geikie, The Great Ice Age (1894), pp. 271-72. 



