200 
sents several hundred years and the two moraines, together with 
the intervening retreat, about 2000 years. 
How Long Ago Were the Deposits in Brooklyn Botanic 
Garden Formed 
The question, “ How long ago?”, is one of the first to be put 
to a geologist by the layman; but it is one of the most difficult to 
answer. or dating events related to the last glacial epoch a finely 
laminated clay is of special importance. This clay, showing layers 
resembling annual rings in wood, was formed in fresh water lakes 
in iront of the melting ice sheets. It was formed of water-borne 
material coming directly from the ice. Because water is heaviest 
at a temperature of 39.2° F*. the ice-cold (32° F.) melt-water was 
lighter than the bulk of the lake water. It therefore rose to the 
surface, when discharged into the lake. In doing so it brought 
along part of the mud (silt and clay) it had carried from the 
glacier. Being too heavy to remain in suspension the larger 
grains, t.c., the silt and the coarser clay, soon sank to the lake bot- 
tom. The finest clay, on the other hand, being extremely light, 
was unable to settle as long as the water was in motion. First 
during the quietness of the following winter did it slowly sink to 
the bottom. In this way a silty layer was formed in summer and 
a fine clay layer in winter, the two indicating the year. The layer- 
pair, the annual deposit, is called a varve.t| These clay varves have 
been used as a basis for a chronology of the time of waning of the 
last ice sheets. The studies carried out so far suggest that the 
retreat of the ice front from Long Island to central Labrador, that 
is, the total disappearance of the ice, took from 25,000 to 30,000 
years. 
By another laminated clay also indicating years, though formed 
under entirely different conditions on the yeah coast of the 
Gulf of Bothnia, the time since the practical disappearance of the 
*Antevs, Ernst. The recession of the last ice sheet in New England. 
Amer. Geogr. Soc., Research Ser. No. 11. — York, 1922. 
Antevs, Ernst. Retreat of the last ice sheet in Eastern Canada. Geol. 
Survey of Canada, Mem. 146. Ottawa, ne Antevs, Ernst. The last 
glaciation. Amer. Geogr. Soc. Research Ser. No. 17. New York, 1928. 
Antevs, Ernst. Late-glacial pee ane and ice recession in Manitoba. 
Geol. Survey of Canada, Mem. 168. Ottawa, 1931. 
