546 F. G. CLAPP GLACIAL PERIOD IN NEW ENGLAND 



lying formations. Where the Wisconsin ice moved over the great sand 

 and gravel plains of Long island its deposits consisted chiefly of gravel. 

 In New York, Pennsylvania, and New England the boundary of a sand- 

 stone outcrop can often be located very closely by the abrupt appearance 

 of a large proportion of sandstone boulders in the drift. On mount 

 Katahdin, Maine, a mountain of solid granite, the most careful search 

 fails to find a trace of foreign drift; nevertheless the topography of the 

 mountain shows that the ice has passed nearly, if not entirely, over it. 

 The movement of the ice was everjTvhere so slow that it seems to have 

 incorporated boulders of tlie underlying rock and often deposited tliem 

 within a few hundred feet of their parent ledges. The greater proportion 

 of gravel and of rounded material in the Wisconsin ice is thus explained 

 by the abundance of gravel in the pre- Wisconsin retreatal deposits.- This, 

 like the boulders, was scooped up and sometimes deposited within a short 

 distance of its original resting place. In the same way, ice in passing 

 over a clay plain can take up nothing but clay, and when it retreats the 

 clay, with its incorporated gravel and boulders, is deposited largely within 

 the boundaries of the original deposit. In a vertical cut the reworked 

 portion of pre-Wisconsin clay is generally distinctly visible in the form 

 of a non-stratified and non-fossiliferous upper portion, 5 to 8 feet in 

 thickness, which contains more abundant boulders and is somewhat more 

 sandy than the stratified clay below. 



As the Montauk till was very thick in places, its retreatal deposits must 

 have been of great bulk. We should naturally expect to find the Wiscon- 

 sin til] composed largely of apparently unglaciated and little glaciated 

 material, as is actually the case. In such localities as on the south side 

 of Sebago lake, from which the ice must have scooped up large quantities 

 of gravel, we should expect to find an extremely gravelly type of till, such 

 as is actually found there. 



3. Occurrence of underlying stratified deposits. — The classic paper on 

 till underlying stratified deposits in this region is the report by Marbut 

 and Woodworth on the cla3^s about Boston.* A few additional cases, in 

 Revere, Chelsea, and Everett, were described in 1902 by Brown. f In 

 many places throughout northeastern New England similar exposures 

 have been seen which show stratified deposits underlying till of Wisconsin 

 type. Examples of clays having these relations are given on pages 536- 

 539. A few instances of sands and gravels underlying the Wisconsin till 

 are as follows : 



In a small roadside excavation west of lake Auburn, Maine, a deposit 

 of very stony till was seen resting on stratified sand. 



♦ Seventeenth Ann. Kept. U. S. Geological Survey, part 1, pp. 989-1004. 

 t R. M. Brown : Am. Jour. Sci., fourth series, vol. xiv, pp. 445-450. 



