320 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



next purpose has been to correlate geological events with this time 

 scale. The third aim has been to connect, as far as possible, the 

 late-glacial chronology with our Christian era. The fourth aim has 

 been to determine, by means of the varve chronology, the length of 

 time employed for the performance of certain geophysical, chemical, 

 and biological changes, so as to have meters that may help us to 

 estimate the time factor for similar changes and processes when 

 no direct means of determining time is available. 



Varve records consequently have been obtained for considerable 

 parts of the age of waning of the North American ice sheet. The 

 length of time that the last ice sheet kept its greatest extent in Long 

 Island is not directly determined, but a comparison of the bulk of 

 the two terminal moraines with that of moraines whose time factor 

 is known suggests that this entire niarginal zone represents roughly 

 2,000 years. The ice sheet began to withdraw almost immediately 

 after reaching its southernmost line. From varve measurements, 

 moraines, and other features the time of release of the belt extending 

 from the terminal moraines to Newburgh, N. Y., and Hartford, 

 Conn,, may be estimated at about 5,500 years. The rate of retreat of 

 the ice front has been determined from Hartford northward to St. 

 Johnsbury, Vt., with exception of one narrow zone at Claremont, 

 N. H. Besides the main line of clay measurements in the Connecti- 

 cut Valley, long controlling lines have been obtained in the Hudson 

 and Merrimac Valleys. The time occupied by the ice recession from 

 Hartford to St. Johnsbury is about 4,100 years. Since the distance 

 is 185 miles, the rate averaged 240 feet a year, or 22 years to a mile. 

 The actual rate of melting back varied considerably. At North- 

 ampton-Amherst, Mass., and probably again at Claremont, N. H., 

 the ice border halted and readvanced. At Woodsville, N. H., the 

 recession was as much as 1,100 feet a year, the highest amount ob- 

 served in North America outside of Manitoba. Shortly after, the 

 recession grew slower, and at St. Johnsbury it came to a stop, fol- 

 lowed by a short advance. 



The ice border of St. Johnsbury may have trended westward to 

 the Adirondacks and then southwestward to south of Lake Ontario. 

 In the belt extending from this line to North Bay and Mattawa the 

 rate of decay of the ice has not been determined by varve measure- 

 ments because of scarcity of clays, but the uncovering coincided 

 almost precisely with the life of Lake Algonquin, a very prominent 

 lake occupying the basins of the three upper Great Lakes. The sev- 

 eral successive stages constituting Lake Algonquin, the changes of 

 level of the Ottawa region, varve series, and other things, all indicate 

 that the time occupied by the ice release of central Ontario was long, 

 some 10,000 years, in round numbers. 



