ANCIENT WATER LEVELS OP CHAMPLAIN-HUDSON VALLEYS 179 



The large delta of the Mohawk extending from Schenectady 

 toward Albany is a witness of the fine sands and clays which 

 poured into Lake Albany from the west^ in which direction lay 

 the great glacial lakes whose development coincided with the 

 retreat of the ice front across the Mohawk valley. The stage of 

 the great glacial lakes with which the delta appears to be equiva- 

 lent is that of Lake Iroquois with its outlet at Rome and thence 

 draining into the Hudson valley. 



Conditions under which the Albany clays were deposited. The 

 conditions under which gravel and sand are deposited both above 

 and below the level of standing water are much better understood 

 than is the case with the sedimentation of clays, particularly 

 those deposits with which we are here concerned, the rock-flours 

 of glaciated districts. At the present time, there is an abundant 

 literature concerning the clays of existing and vanished glaciers, 

 in which, however, there is scant discussion concerning the factors 

 which control the deposition of clays. 



There is a variation in the delivery of clay from a glacier 

 dependent on diurnal and seasonal changes of temperature in the 

 atmosphere, subject to modification by the passage over the 

 glacier of those whirls of the atmosphere known as cyclonic move- 

 ments or storms with their accompanying precipitation in the 

 form of rain or snow. 



Diurnal changes of temperature and their effect on glacial clays. 

 With each rotation of the earth on its axis in middle latitudes, 

 a glacier is alternately exposed to the sun's heat and shielded 

 from this cause of melting. During the day, the eftect of insola- 

 tion is to swell the glacial drainage with water carrying detritus 

 set free by the melting of the ice. Other things being equal a 

 larger quantity of clay will be carried out of a glacier at day than 

 at night when the streams are checked. The greater volume and 

 velocity of the streams discharging directly from a glacier into 

 a water basin during the day will tend to carry the suspended 

 clay particles farther out and allow of their wider distribution 

 by stream-made currents than at night when not only is there 

 less clay delivered but the transporting agencies are less effective. 

 But the clay deposited under these day conditions will contain 

 more coarse mineral particles than the night clays when only 

 the finest rock-flour escapes to the area of clay deposition. There 



