CHROlTOLOGY — DOltGLASS AlTD ANTfiVS 



315 



at a temperature of 38° to 39.2° ; and the water stratification may 

 have been constantly inverse with the coldest water at the top. Tlie 

 icy water coming from the glacier was consequently lighter than 

 the lake water, and, even if discharged at the bottom of the lake, 

 rose to near its surface. In doing so it brought along part of the 

 suspended mud which was quickly distributed by waves and cur- 

 rents, the finer the mud, the farther it was spread (fig. 1). 



Having arrived in the upper layers of the lake water, the mud 

 began to separate according to coarseness and to sink, the coarse 

 grains fairly fast, the fine particles at extremely slow rates. Silt 

 grains would settle in a few days or weeks. Coarse clay particles, 

 as well as part of the fine clay, also sank to the bottom during the 

 summer. On the other hand, the bulk, or a great part, of the finest 

 particles still remained in suspension when melting ended with the 



Outlet 



V/ind- 



Geological Surve;,', Canada. 



-4=^ 



FiGUKB 1. — I'rubable water circulation iu a glacial lake, aud probable water tempera- 

 tures, F°., (luring summer. (From Antevs, Geol. Surv. Ciiuada, Mem. 146, fig. 21.) 



arrival of winter. Because of very slow sinking, these minute 

 particles could not reach bottom individually in the course of a year. 

 However, owing to the perfect calm under the ice cover and to the 

 development of a slight salinity of the water as a result of partial 

 dissolution of the silicic acid of the mud, the particles flocculated or 

 aggregated into small lumps, which settled before spring. It is this 

 separation of the grains and particles by their different rate of fall 

 through the water that produced the distinct lamination of the clay, 

 causing the deposition of a silty layer in the summer and a clayey 

 layer in the winter. This pair of layers, representing the annual 

 deposit, is the varve. 



The chief conditions of formation of the varved glacial clay 

 were, therefore, that at least part of the fine-grained mud came 

 from a melting glacier, that the water in which deposition took place 

 was fresh, or practically so, and was heavier than the river water, 

 so that this water and the contained mud could rise to the upper 

 strata of the lake and the mud separate and settle according to size 

 of grains and particles. 



