Prof. James Geikie’s Address. 469 
are only three possible modes in which those materials could have 
_ been introduced into the ground-moraine : either they consist of super- 
ficial morainic débris which has found its way down to the bottom 
of the old glaciers by crevasses; or they may be made up of the 
rock-rubbish, shingle, gravel, etc., which doubtless strewed the — 
valleys before these were occupied by ice; or, lastly, they may have 
been derived in chief measure from the underlying rocks themselves 
by the action of the ice that overflowed them. The investigations 
of Penck, Blaas, Bohm, and Briickner appear to me to have demon- 
strated that the ground-moraines are composed mostly of materials 
which have been detached from the underlying rocks by the erosive 
action of the glaciers themselves. Their observations show that the 
regions studied by them in great detail were almost completely 
buried under ice, so that the accumulation of superficial moraines 
was for the most part impossible; and they advance a number of 
facts which prove positively that the ground-moraines were formed 
and accumulated under ice. I cannot here recapitulate the evidence, 
but must content myself by a reference to the papers in which this 
is fully discussed. These geologists do not deny that some of the 
material may occasionally have come from above, nor do they doubt 
that pre-existing masses of rock-rubbish and alluvial accumulations 
may have been incorporated with the ground-moraines; but the 
enormous extent of the latter, and the direction of transport and 
distribution of the erratics which they contain cannot be thus ac- 
counted for, while all the facts are readily explained by the action 
of the ice itself, which used its sub-Glacial débris as tools with which 
to carry on the work of erosion. 
Professor Heim and others have frequently asserted that glaciers 
have little or no eroding power, since at the lower ends of existing 
glaciers we find no evidence of such erosion being in operation. But 
the chief work of a glacier cannot be carried on at its lower end, 
where motion is reduced to a minimum, and where the ice is per- 
forated by sub-Glacial tunnels and arches, underneath which no 
glacial erosion can possibly take place; and yet it is upon observa- 
tions made in just such places that the principal arguments against 
the erosive action of glaciers have been based. If all that we could 
ever know of glacial action were confined to what we can learn 
from peering into the grottoes at the terminal fronts of existing 
glaciers we should indeed come to the conclusion that glaciers do 
not erode their rocky beds to any appreciable extent. But as we do 
not look for the strongest evidence of fluviatile erosion at the mouth 
of a river, but in its valley- and mountain-tracks, so if we wish to 
learn what glacier-ice can accomplish, we must study in detail some 
wide region from which the ice has completely disappeared. When 
this plan has been followed, it has happened that some of the 
strongest opponents of glacial erosion have been compelled by the 
force of the evidence to go over to the other camp. Dr. Blaas, for 
1 Penck: Die Vergletscherung der deutschen Alpen. Blaas: Zeitsch. d. Fer- 
dinandeums, 1885. Bohm: Jahrb. d. k. k. geol. Reichsanstalt, 1885. bd. xxxv. 
Heft 3. Briickner: Die Vergletscherung d. Salzachgebietes, etc., 1886. 
