78 THE GLACIER EPOCH OF AUSTRALASIA. 
of great mass and weight are floating masses of we. Glaciers 
which descend into the sea break off in huge masses, as ice- 
bergs or ice sheets, and carry their burden of moraine stuff, 
which often include masses of rock of enormous weight, until. 
the mass of ice melts or is overturned, when the rocks are 
discharged and fall to the quiet sea bottom as erratics. 
Shore ice of Arctic regions when breaking up may, in a 
similar way, transport and discharge erratic boulders over the. 
sea bovtom. 
PRINCIPAL EVIDENCE OF INTENSE GLACIAL ACTION. 
In regions, or at periods when the climate was refrigerated 
sufficiently to cause permanent snowfields on the tops of the 
higher plateaux of mountains,and merely caused glaciers to 
form in the sub-alpine valleys which melted before reaching 
the lowlands, the evidences of ice action are confined to those 
indicative of milder glacial effects, viz., lateral and terminal 
moraines in the higher valleys, perched blocks, roches- 
moutonneés, scooped lake basins, striated,polished, and grooved 
rocks and boulders, lines of striation of fixed rocks running 
in harmony with the direction of the particular valleys in 
which such marks occur. But when refrigeration of climate 
becomes as intense as in the pleistocene glacial epoch of 
Northern Europe and America, which covered all the surface 
of the land, save the highest peaks of mountains in these 
regions; when glacial ice sheets—no longer mere ice rivers 
confined within mountain valleys—spread, as in polar lands, in 
devastating “ seas of ze” across the whole land and interven- 
ing lakes and seas, the action of the travelling sea of ice 
produces effects of quite a distinctive character from the 
limited drums and longitudinal heaps of moraine stuff of 
mere upland zce rivers. 
The detrital matter of wide-spreading ice sheets, in moving 
slowly over the irregular surface of the lowlands of a country, 
is best described by reference to the character of what is now 
known to be partly the produce and partly the form of redis- 
tribution of superficial materials, as seen in the so-called tilk 
or boulder clay of Scotland. 
The character and derivation of the “ till” formed by the 
intense glacial action of the great land ice sheet in Scotland is 
graphically described by Wallace in his “Island Life” 
(pp. 109-112), whose views are in accordance with those so 
ably advocated by Professor Jas. Geikie in his well-known 
work, ‘“‘ The Great Ice Age.” 
The Glacial “ Till” of Scotland.—Dy. Wallace writes :— 
“Over almost all the lowlands, and in most of the Highland 
valleys of Scotland, there are immense superficial deposits of 
