BEARING OF THESE STUDIES 101 



many questions connected with these researches remain to be 

 studied. Some of these problems may be pointed out. 



On account of the great depth of the varve sediments the bot- 

 tom layer was reached practically only in the northern part of 

 the Connecticut Valley. It is, therefore, to be hoped that any 

 opportunity to measure the lowest strata be used. Series with 

 bottom varves are necessary for the determination of the exact 

 rate of recession. If a sufficient number of sections with bottom 

 can be secured, the position of the ice front for every winter can 

 be mapped. Such a study has been carried out by De Geer 

 (1912, PI. 2; 1912a, p. 464) of the Stockholm district. Detailed 

 determination of the rate of recession probably yields the best 

 material known for studying the long climatic periods, i. e. 

 periods from a few decades to hundreds or thousands of years. 



The zones of readvance of the ice should be studied in detail. 

 The extent and number of oscillations, time involved, and the 

 rate of retreat before and after the readvances are questions of 

 great importance for the understanding of the causes. Stadial 

 moraines should be sought and mapped particularly in these 

 zones. 



Determination of the length of time of the periods recorded by 

 the stadial moraines which Taylor (1903) mapped in the Berk- 

 shires should be of great climatological interest (cf. p. 92). 



Clay measurements in other lakes and valleys are very valua- 

 ble for elimination from the normal curve of local, non-climatic 

 features. 



The history of the lakes presents interesting problems. A 

 detailed study of their water levels is being made by Goldthwait. 

 This will throw light upon the extension of the lakes (cf. p. 9), 

 the sedimentation, the tilting of the land, the changes of level 

 of the lakes due to lowering or raising of their outlets (cf. pp. 

 8, 51), and so forth. Determination of the minimum duration 

 of the lakes by counting the varves will give an idea of the rate 

 of the warping of the land. 



The measured profiles represent a vast amount of material for 

 the study of sedimentation. Part of this is given in the descrip- 



