forth within a few miles, probably for centuries, each local advance 
bringing a fresh supply of debris. Masses of dirty ice became iso- 
lated and buried among the rock materials. When they finally melted 
and contributed their load to the moraine, the material above them 
slumped down and irregular hollows were formed on the surface. Such 
pits are known as kettles or kettle-holes. The surface of any mar- 
ginal moraine is generally rough, partly because of the great number 
of such. kettles and partly because of the irregular hummocks or 
knobs between them that result from abrupt variations in the thick- 
ress of the debris. 
The. geologist: gives ene name till to such a heterogeneous ac- 
cumulation of unlayered (unstratified) material. Till usually con- 
sists of fragments of many. sizes, from great blocks of rock to pul- 
verized rock flour. .:This material was either dumped, or pushed a- 
head of, or plastered beneath a glacier. Till was also spread as a 
‘thin veneer over the land behind the outermost, terminal moraine 
either because. it was laid there by the .overriding ice during its 
advance or because it was-let down slowly upon the surface when the 
glacier meltede The name ground moraine is applied to such bodies 
of till. scattered over the area covered by the ice, The thickness of 
a deposit of till may vary a great deal. In some places whole hills 
are made of till, whereas nearby areas are free from any such cover- 
INS. : . : . . ; 2 
~ Blocks and.boulders carried by the ice were deposited at random 
“over the glaciated tract. Those transported far from their source 
are composed of rock material that may be quite different fromthe 
bedrock. underlying their present resting-place. Some rest in the 
midst of till masses or on top of them, and others are now. perched 
atop naked rock ledgese Such ice-carried blocks are called erratics. 
Even while the . ice-sheets were increasing in size, varts of 
then melted during the day time and during the: summer months, esne- 
‘cially those portions that. had reached low altitudes and warmer re- 
gions. The meltwaters were loaded with rock debris, and they depos- 
ited it in layers in front of the ices Such material, ‘known as out- 
wash, consists of. gravel, sand, and silt mixed with small amounts of 
clave As it was transported partly by ice and partly by water, it 
may be called. glacio-fluvial debris. The particles in it became 
rounded and smoother than the more angular fragments generally found 
in glacial +i01) because they were rolled and jarred against one an- 
other during water trarsporte ; 
{ Some melting also occurred during the. construction of the 
inoraine so that masses of stratified outwash sands and gravels are 
occasionally found embedded in the till of the moraine. Small 
streams of meltwater piled canelike masses of outwash in front of the 
ice and resting against ite Sand and gravel also ac -cumulated in 
hollows on the surface of the icee. When the glacier molted away 
from these masses, they stood as isolated hills known as kamesS in 
places a whole series of them were built along the ice front so that 
they form an almost continuous, irregular ridge. Outwash also ac- 
cumulated in broad, low, fan-shaped masses extending outward from 
the ice for several milese Because of their gentle surface slope, 
such features are called outwash plains. 
