GEOLOGr AND GEOGEAPHY OF OLAT DEPOSITS 



117 



From the facts as observed, quoted above, the following may be 

 deduced. That during the retreat of the ice sheet from the 

 Hudson valley the glacial streams deposited as kames a great 

 amount of ground up material, principally shale ; the material 

 found underlying the clays along the upper portions of the 

 valley. 



That subsequent to the retreat of the glacier there was a 

 depression of the land, which, according to Dr. Merrill,* 

 amounted to 80 feet at 'New York city and near Schenectady to 

 about 360 feet. 



During this period a great amount of plastic clay was depos- 

 ited produced by glacial attrition of the shales and limestones, 

 the latter no doubt giving to it the marly character and influenc- 

 ing its color. 



The upper portion of the clay is more siliceous and overlying 

 it is an extensive deposit of sand, indicating a change in the 

 nature of the material washed into the estuary. During the 

 period of submergence much of the siliceous matter washed into 

 the estuary was deposited at the mouths of the tributary streams 

 to form deltas. 



Terrace and sand pit, Dutchess Junction. 



It has been suggested by Dr. Merrill f that the change in the 

 estuary deposits from clay to sand might be due to the exposure 



* Amer. Jour. Sci., June, 1891. 

 + Amer. Jjur. Sci., June, 1891. 



