Dr. A. P. Young—Glaciation of Navis Valley. 259 
(2) On slopes at lower levels, where local precipitation is reinforced 
by the flow from above, the activity of water must be greater than on 
the high ridges which receive only the snow or rain falling directly 
on them. 
RELATION OF THE CORRIE-FLOOR TO THE SNOW-LIMIT. 
In general, reliable evidence for any one stand of the snow-line will 
be found on suitable slopes under ridges which rise only a few hundred 
metres above the snow-limit. ‘his is the condition for the formation 
of corries. In such cases the ice-tongues will be short, the firn-line 
will coincide with the snow-line. 
Under higher ridges the ice-tongues of the same period will be 
longer, the beds will be in the form of troughs rather than basins. In 
still higher ground the beds of glaciers belonging to successively 
higher snow-lines will join to form a continuous groove, which gives 
no indication of a stationary period. 
Further, low ridges too near the centres of glaciation will be 
whelmed in ice from the high ground. Corries will thus be formed 
only near the border of the tract for the time being under glacial 
conditions, the different stands of the snow-line will leave their traces 
on successively higher ridges nearer the centre, the lowest corries 
corresponding to maxima of cold being on the extreme border of the 
area occupied by ice. Such is, in fact, the distribution of corries 
actually observed. 
All pronounced corries are held to bear precise and unequivocal 
testimony to some one stand of the snow-line, during which the ice 
just filled the hollow. None of this evidence can be neglected. 
Corries with moraine dams must have been filled with ice during the 
latest retreat, the loose material being unfitted to withstand the 
thrust of moving ice of any later advance. 
The basin-like hollow of the corrie is the counterpart of the trough 
occupied by the long glacier tongue. Amphitheatres with ill-defined 
floors are either the sources of longer ice-streams or are the walls of 
ancient corries, the floors of which have been broken down by 
a subsequent advance of the ice. 
This explanation of the origin of corries differs somewhat from those 
hitherto in use. The site of the corrie was determined by the height 
of the snow-line, the height of the ridge, the catchment area, the 
conditions of precipitation and insolation on which depended the 
length of the ice-tongue. A watercourse or groove led the firn in 
the required direction. There was no other marked hollow. Where 
several corries are found at nearly the same level it cannot be supposed 
that each was determined by a pre-existing pit or funnel.! The 
formation of corries is not limited to extremes of glaciation. At the 
time the corrie was formed the snow-line was not on the level of 
the floor,” but considerably higher. The level of the corrie-floor is in 
no way related to the level of the ice-surface in the main valley to 
which the corrie-watercourses were tributary.® 
- | See James Geikie, The Great Ice Age, 1894, p. 237. 
2 See Bohm, Die alten Gletscher der Mur und Miinz, 1900, p. 19; also Richter, 
Geomorphologie, pp. 15, 75. 
3 See Penck, Die Alpen im Hiszeitalter, p. 285. 
