316 



ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1931 



If these conditions had not been filled, i3ractically all the fine mud 

 would have flocculated and settled during the summer, together with 

 the coarse fractions forming a massive clay. Homogeneous clays 

 were formed where glacial brooks discharged into strongly saline 

 waters, and they are now deposited where ordinary brooks and rivers 

 empty into regular lakes of our latitudes (pi. la). 



If the debris was uniformly distributed in the ice, the quantity 

 of mud brought into a glacial lake during a summer was propor- 

 tional to the amount of ice melting. The thicknesses of the varves, 

 therefore, record the relative amount of ice melting during the dif- 

 ferent years. Since ice melting is found, by observations in the Alps 

 and elsewhere, to be determined chiefly by summer temperature, the 



Figure 2. — Mode of formation of glaci-fluvial deposits (esker gravel, sand, silt, and 

 varved clay) in fresh water off tbe receding ice front during three successive years. 

 The bottom varve to the right in the figure is varve number three to the left in the 

 figure, roints to the left were uncovered two years earlier than points to the right. 

 Section through center of glacier orifice. At a distance in lateral direction from 

 the mouth of the glacial river the varve is thin and consists of fine sand, silt, and 

 clay at the very ice edge 



relative thickness of the varves was defined, in the last analysis, by 

 the total summer heat. Thick varves, therefore, mean warm and 

 long summers ; thin varves, cold and short summers. 



While several geologists interpreted the layer-pair in the glacial 

 clay as the annual deposit, Gerard de Geer, of Sweden, went further 

 and, in 1885, propounded a method to use it for a geochronology of 

 the waning stage of the last Pleistocene ice sheets. The method is 

 based on the fact that, when the ice sheet terminated in water, its 

 edge formed the proximal limit of the clay varves. As the ice edge 

 retreated the varves extended farther and farther in centripetal 

 direction. The varves accordingly cover one another as shingles on 

 a roof (fig. 2). ' 



The field operations consist in measuring continuous series of 

 varves in exposures, if possible from the bottom of the clay. The 

 limits of the varves are marked on strips of strong paper giving the 



