WEATHER AND GLACIATION — REEDS 303 



These laminated jrlacial clays, which record the annual retreat 

 and ablation of the last continental ice sheet, have been studied, 

 durinir the last 20 years, in North America, Sweden, Finland, Ice- 

 land, Arjzentina, and the northwestern Himalaya Mountains of 

 India. While the primary object of the investigation has been 

 the establishment of a <ieochronoh)«rical time scale, with the varve 

 or year as the unit of measurement, a study of the variations in 

 thickness preserved in the seasonal and annual layers is of special 

 interest, since it involves weather changes which affectetl the melting 

 of the ice during its retreat. 



The material composing the varved clays was brought directly 

 from the melting ice by subglacial streams and deposited in fresh 

 or sliglitly brackish water lying in front of the receding ice mass. 

 The mud was brought into the lake during the annual melting 

 period — that is. the summer months. The quantity delivered was 

 in all probability proportional to the quantity of ice that under- 

 went melting. It apparently was not directly influenced by the 

 nourishment of the ice as was the retreat of the ice front. Thick 

 varves thus signify a long period of warm summer weather; thin 

 varves imply short summers, with cold and foggy weather. The 

 varve graphs which have been published, showing summer and 

 winter thicknesses, record fairly accurately the yearly variations of 

 the amount of summer temperature. 



The lamination of the clay sediments postulates a strict peri- 

 odicity, not only in the melting of the ice and the deposition of the 

 " summer " layer, but a pause of several months, during which 

 time the supply of sediment is interrupted and the fine clay particles, 

 which remain suspended in the fresh- water lakes following the 

 summer influx, have had sufficient time to settle to the bottom and 

 form the dark "winter" layer, consisting of pure clay. No phe- 

 nomena other than seasonal variation — that is, the alternation of 

 summer and winter — meets this postulate. Varved clay sediments 

 produced by this seasonal variation are being deposited now in Lake 

 Louise, Alberta, Canada (Johnston, 1922). 



In addition to the .seasonal variation, there is an annual or varve 

 variation in which changes in thickness occur from year to year. 

 The thickness of the varves usually varies in different basins from 

 a few millimeters to 3 or 4 cm., sometimes more. For instance, 

 the varves in the Hudson River Basin at Haverstraw, N. Y., average 

 35 nmi. (1.38 inches) in thickness, while those in the Quinnipiac 

 Basin, near New Haven, Conn., average 23.24 mm. (0.02 of an inch). 

 The relative differences as well as other structural featiires, how- 

 ever, remain constant over wide regions as well as over widely 

 scattered occurrences. Thus it is possible to identify and correlate 

 the varves in separate sections. 



