BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 1, pp. 195-202, pl. 3 March 21, 1890 



STRUCTURE AND ORIGIN OF GLACIAL SAND PLAINS. 



BY WILLIAM MORRIS DAVIS OF HARVARD COLLEGE. 

 [Read by title befm-c the Society December 27, 1889.) 



CONTENTS. 



Page 



External Form and Internal Structure 195 



Hypothesis of Origin 106 



Deductive Extension of the Hypothesis " 196 



Verification of the Hypothesis 107 



Evidence from Cross-bedding 1- 198 



Katio of Sand-plain Growth to Ice Melting 199 



Origin of Depressions in Sand Plains 199 



Local and Temporary Growth of Sand Plains 200 



Sand Plains generally formed in local Bodies of Fresh Water 201 



Relation of Sand Plains to other Glacial Deposits 201 



Points needing; further observation . 202 



External Form and Internal Structure. — Plains of stratified gravelly sand, 

 half a mile or more in diameter, standing mesa-like above the adjacent 

 valley ground, are common in many parts of New England. They lie on 

 striated ledges and till, and hence are at most not older than the closing 

 stages of the latest glacial epoch. Their distinct marginal slopes give no 

 indication of more than a small measure of erosion, and hence their present 

 form may be taken as essentially equivalent to their initial constructional 

 form. They are well stratified throughout, and this, along with their defi- 

 nite marginal slope, indicates them to be deposits made in bodies of standing 

 water. 



The general surface of these sand plains is very even, but fails of being 

 level by reason of a gentle slope, generally to the south, of ten, twenty, or 

 thirty feet to the mile. Their margins are in most cases well defined, having 

 slopes of from 10° to 30°. They present two very distinct forms of outline, 

 illustrated in plate 3. The northern quarter of the perimeter possesses a 

 number of strongly concave curves, descending by steep slopes to kettle- 

 hollows, often holding swamps or ponds; and the cusp-like points between 

 these curves extend northward into a group of gravelly ridges and sandy 



XXVI— Butt. Geol. Soc. Am., Vot,. 1, 1889. (195) 



