CHARACTER OF THE PLATEAUS OF MODIFIED DRIFT. 187 



t 



sea, that part of our eastern coast having stood during the Late Glacial 

 or Champlain epoch from 100 to 200 feet lower than now, with gradual 

 increase in the amount of the depression from south to north * In 

 Newton, Hingbam, and other towns five to fifteen miles west, south, and 

 southeast of Boston, such plateaus, well studied by Davis,f Gulliver, % 

 and Crosby and Grabau, § appear to represent a series of glacial lakes 

 hemmed in between the land on the south and the retiring crenate ice- 

 margin on the north, with outlets southeast and east to the sea, the rela- 

 tive heights of sea and land there in the Champlain epoch having been 

 probably almost the same as now. More recently, Woodworth || has 

 described somewhat similar modified drift deposits in the district of 

 Narragansett bay, where they seem referable to deposition in fanlike 

 slopes by the flooded and overloaded streams from the summer melting 

 and rains on the contiguous receding ice-fields. 



In Saint Paul the very nearly level expanses of these high tracts of 

 gravel and sand imply a lacustrine origin ; and the contour of the 

 district, in connection with the courses of its moraine belts, implies that 

 this was the site of a glacial or ice-dammed lake. The surface of the 

 glacial lake during the early part of its existence was about 250 feet 

 above the present river, or 930 to 940 feet above the present sealevel, 

 these being the heights of the Hamline and Como plateaus. A little later, 

 when the plateau crossed by Phalen avenue a mile east of lake Como 

 was formed, the lake level had fallen apparently five or ten feet. At the 

 time of formation of the Summit Avenue plateau, it had been further 

 lowered several feet, having then apparently an elevation of about 915 

 feet. Another depression of some ten feet is recorded b) r the small 

 plateau on which Western avenue intersects Lawson street. Belonging 

 to a time intermediate between the last two, an esker and plateau, to- 

 gether nearly a mile long, were formed close north of East University 

 avenue ; but three very small plateaus which occur in a series extending 

 thence a mile east and northeast, between Mississippi and Edgerton 

 streets, were amassed b} r the continued action of the same glacial stream 

 while the surface of the lake was falling more notably. At the time rep- 

 resented by the most eastern of these little plateaus, crossed by Whitall 

 street, the lake had been reduced to an altitude of 870 or 875 feet. 



These modified drift plateaus will be described in a numerical order 

 corresponding with figures by which they are designated on the accom- 

 panying map (plate 15). The first three are connected with each other, 



* Geology of New Hampshire, vol. iii, 1878, pp. 155-164, 170, 171. 



j W. M. Davis : Bulletin Geol. Soc. Am., vol. i, 1890, pp. 195-202. 



tF. P. Gulliver : Journal of Geology, vol. i, 1893, pp. 803-812. 



gW. O. Crosby and A. W. Grabau : Am. Geologist, vol. xvii, pp. 128-130, Feb., 1896. 



||J. B. Woodworth : Am. Geologist, vol. xviii, pp. 150-168, Sept., and p. 391, Dec, 1896. 



