BEARING OF THESE STUDIES 103 



tion of the sections (Ch. III). A closer treatment requires better 

 knowledge of the surfaces of the lakes than we have now. Prob- 

 lems are: the spreading of the sediments in the lakes, i. e. the 

 decreasing in thickness of the varves and in coarseness of the 

 material outward from the mouth of the glacial river; sedimen- 

 tation and depth of water (cf. Fig. 19 and pp. 51, 69, 82); sedi- 

 mentation and size of the drainage area; fluctuations in the sedi- 

 mentation during the summer, as recorded in thick silty varves; 

 the mechanical and chemical character of the sediments — 

 studied in Sweden by Oden. 



Drainages of lakes ponded by ice or by glacial deposits consti- 

 tute a subject of interest for detailed study (cf. p. 69). So also 

 do changes in the size of the drainage areas of the lakes, as the 

 ice receded, and their relation to the sedimentation. 



Driven southward by the ice, plants and animals re-immigrated 

 to the newly exposed land. Hardy species followed closely the 

 receding ice edge, and others came as fast as conditions be- 

 came favorable for them. Remains of the earliest flora are pre- 

 served in the varve clays, and a stratigraphical study of them 

 should be of both botanical and climatological interest. By meas- 

 uring the varve series in which the plant remains occur and 

 identifying these varves with the normal curve, the position of 

 the ice edge, when the plants were embedded, can be fixed. 

 Sandegren's (1915) study of the post-glacial flora and climate 

 at Ragunda in northern Sweden, as recorded by the plant remains 

 in the varve silts, is a standard work because of its accuracy. Of 

 considerable interest are quantitative and stratigraphic studies 

 of plant pollen in the clays, like those carried out in Sweden 

 by L. von Post and others. 



The physiographic development and the geologic factors, such 

 as erosion by rivers and by the sea, weathering, etc., can be 

 studied under new conditions when the actual length of time 

 since the release from the ice shall have become known. Knowing 

 that southern New England was uncovered about 5,000 years 

 earlier than the northern part, comparative studies can be made. 



The question of local glaciers in the Adirondacks, Catskills, 

 and White Mountains, after the departure of the ice sheet, 

 might be settled by studies of the clays. 



