Drumlins containinfj 3fodified Drift. — Upham. 885 
was only slackened while these hills were being accumulated. 
But glacial readvance probably attended the formation of 
drumlins in central New York, where Davis has observed sec- 
tions of these till accumulations lying on beds of gravel and 
sand which were somewhat eroded before the deposition of 
the till. 
Wherever free drainage from the ice border permitted the 
waters of the glacial melting and rains to flow away, bearing 
the fine rock flour and clay )n suspension, the nucleal and 
basal modified drift accompanying the drumlin till consist of 
gravel and sand, which could be deposited from running- 
streams. In New Hampshire, during my observations of many 
sections of drumlins and of allied lenticular slopes of till rest- 
ing on rock hills, a few instances were noted having such 
coarse modified drift, from a few inches to five feet or more in 
thickness, lying between lower and upper deposits of till 
which were interpreted as respectively subglaeial and engla- 
cial drift when the ice-sheet was finally melted away.* 
On the other hand, where drainage could not take place 
freely during the departure of the ice-sheet, the water being- 
ponded against the ice border, as in the extensive basins of 
lakes Winnipesaukee and Squam in central New Hampshire, 
very remarkable beds of horizontally stratified clay, some- 
times free from stones and gravel, but occasionally containing 
man}!- small rock fragments dropped from the overlying ice, 
were formed between a lower subglaeial till and an upper en- 
glacial and finally superglacial till. The upper till varies in 
thickness up to 10 feet; and the stratified clay next below is 
usually only a few feet thick, but in some places, well ex- 
posed by excavations for brick-making, it is 20 to 30 feet 
thick. It occurs, here and there, at all altitudes uja to 300 
feet above these lakes on the adjoining drift-covered high rock 
hills; but no deposits of stratified clay are found in this dis- 
trict without the thin envelope of the upper till.f 
Comparing these observations with the sections of strati- 
fied brick clays noted by Marbut and Woodworth in Somer- 
ville, Medford and Saugus, Mass., we may well ask, in the few 
places where those stratified clays are seen to be overlain by 
*GeoIogy of N. H., vol. iii, 1878. pp. 289-291. 
tGeology of N. H., vol. in, pp. l.'Jl-137. 
