THE PLEISTOCENE OR GLACIAL PERIOD. 



361 



(1) that they were accumulated beneath the ice under special con- 

 ditions, and (2) that they were developed by the erosion of earlier 

 aggregations of drift, much as roches moutonnees are developed. 

 Under the first of these general views, it has been suggested (1) that 

 the bars of rivers give the clue to the origin; (2) that protuberances 

 of rock gave occasion for the lodgment; ^3) that the balance between 

 load and strength of movement furnishes the key to their explanation, 



Fig. 495. — Drumlins shown in contour near Sun Prairie, Wis. (U. S. Geol. Surv.) 



a slight but not excessive overload being the condition necessary for 

 their development; and (4) that they may be, in some way, connected 

 with longitudinal crevasses. 1 



1 Papers on Drumlins. — Hall, Geol. Fourth District of New York, 1873, pp. 414-5; 

 Lapham, Smiths. Contr. for 1855; Shaler, Proc. Bos. Soc. Nat. Hist. 1870, pp. 196-204; 

 C. H. Hitchcock, ibid, Vol. XIX (1876), pp. 63-67; Matthew, Geol. Surv. of Can., 

 Rept. 1877-79, pp. 12-14, EE; Upham, Proc. Bos. Soc. Xat. Hist. 1879, pp. 220-234, 

 ibid., Vol. XXIV (1889), pp. 228-242, Geol. of X. H., Vol. Ill (1878), Am. Geol. 

 Vol X (Dec. 1892), pp. 339-360, and Bull. Geol. Soc. Am. Vol. Ill (1892), p. 142; 

 Stone, Proc. Bos. Soc. Nat. Hist., Vol. XX (1880), p. 434,; Johnson, Trans. N. Y. 

 Acad. Sci., Vol. I (1882), pp. 78-89, and N. Y. Acad. Sci., Vol. II, pp. 249-266; 

 Chamberlin, Geol. of Wis., Vol. I (1883), p. 283, Proc. Am. Assoc. Adv. Sci. 1886, 

 p. 195, Third Ann. Rept. U. S. Geol. Surv., 1883, p. 306, and Jour, of Geol., Vol. I, 

 p. 255-267; Dana, Am. Jour. Sci., Vol. XXII (1883), pp. 357-361; Davis, ibid,, 

 Vol. XXVIII (1884), pp. 407-416; Chalmers, Geol. of Can. Rept. 1881-9, Vol. IV, 

 p. 23; Salisbury, Geol. Sun-, of New Jersey, Rept. 1891, p. 74, and Glacial Geology of 



