Toronto and Scarhoro' Drift Scries. — Uphaiii. 315 
on the windward low region of the Laurentian lakes whence 
the ice had recently retreated. 
The thick stratified clay and sand beds fomiing the lower 
half of the Scarboro' section were evidently amassed as a 
delta or broad alluvial slope, perhaps wholly above the water 
level in the Ontario basin. So large a supply of sediments 
seems referable to their derivation partly from englacial drift 
exposed by ablation on the margin of ice-fields within the 
drainage area of the delta-forming streams. In this tract of 
confluence between the great eastern and central lobes of the 
Laurentide ice-sheet, represented by the angle of the drift 
boundary at Salamanca in southwestern New York, there un- 
doubtedly was brought an exceptional volume of the en- 
glacial drift by the confluent glacial currents. Much of these 
clays, and sands may therefore have come from englacial and 
finally superglacial drift of the neighboring ice-sheet on the 
northeast, being brought by streams from' its melting; while 
the driftwood and leaves of trees, and mosses of peat bogs, 
growing within a few miles westward, were contributed to the 
same deltas by streams flowing from a wooded land area bor- 
dering the ice, such as Russell found adjoining the Malas- 
pina ice-sheet in Alaska and even extending its forest growth 
several miles upon the drift-covered marg-in of the departing 
ice. 
When the delta, with its fanlike lakeward slope, had at- 
tained the maximum depth of nearly 200 feet in the Scarboro' 
section, lying partly beneath the present lake level, it was 
deeply channeled by the principal streams, which no longer 
carried so abundant silt for deposition on that outer part of 
the delta area. 
Later a moderate readvance of the ide-sheet seems to have 
brought m.uch englacial till, boulder clay, over this district, 
to be deposited when that thin ice margin melted away ; and 
uijon this till the streams flowing from the ice front spread a 
thickness of twenty-nine feet of cross bedded sand. With the 
oscillations of small extent and short duration which the ice 
border often made during its accumulation of marginal mor- 
aines, the same alternations of boulder clay and sand were 
repeated until four distinct deposits of till and three of in- 
tervening sand beds, marking four ice advances and reces- 
