183 
Modern glaciers normally move at the rate of 100 to 300 feet a 
year. We do not know the rate at which the continental ice 
—" 
sheet advanced but, if we assume a yearly rate of say 100 feet, 
the time required to bring the boulder of Medina Sandstone (No. 
18 of our labeled boulders) from its probable point of origin near 
Albany, about 150 miles, would have been approximately 8000 
years.! This figure is in close agreement with that arrived at 
by another method described by Dr. Antevs in the following pages. 
— 
Jasper County, Georgia), showing 
Fic. 13. Cut in a non-glaciated region 
surface soil (Iredell loam) derived by the disintegration of the underlying 
— 
bedrock (Diabase). (Photo by D. D. Long.) f. Fig 
The geological history of Long Island is, of course, only an 
episode or chapter in the glacial geology of North America, which 
records the events of the last or Quaternary Ice Age on that conti- 
nent. The Botanic Garden has been fortunate in securing the 
15280 (ft. per mile) * 150 (miles) = 792,000 (ft. from Albany, N. Y., to 
New York City). 792,000 (feet) + 100 (ft. per year) = 7920 (years). 
