Dr. A. P. Young—Glaciation of Navis Valley. 249 
vegetation. Except for the presence of some larger rock-fragments, 
the material may be compared with the loose ash which collects round 
the crater of Vesuvius, with which, however, as regards origin, it has 
nothing in common. 
The loose powder on the Reckner slope must be constantly carried 
downward by the melting and falling snows, and to the same extent 
renewed. We have here a hint of the way in which a rock may dis- 
integrate at the level of the snow-limit, and perhaps under the action 
of firn, where alternations of frost and thaw play an important role. 
Orper CHannets oF Uprrr TARrnrtrAt. 
The higher and older outlet channels on the west side, by which 
the Upper Tarntal once drained into the north and south channel 
(¢, d, Fig. 2), are thickly strewed with erratic blocks of Reckner 
serpentine. Judging from the amount of demolition and erosion 
which has been effected since these channels were used, they must 
have been first formed during one of the older glaciations, or at any 
rate before the last general advance. But the presence of erratics in 
these old watercourses implies a movement of ice at a level of 
2550 metres, and a corresponding snow-limit above the ice and below 
the summit ridge; in other words, the snow-line must have made an 
early stand at 2650 metres nearly. The date of this resting stage 
could hardly have been later than the close of the Riss period or the 
first advance of the Wiirm glaciers. Since the serpentine blocks were 
left in these high channels the rear wall of the Griibl has been cut 
back by ice, which could only work efficiently as long as the snow-line 
remained near the level of 2400 metres. 
GuacraL Hisrory. 
The history of the Griibl can only be written on the assumption 
that the snow-line stood on two different occasions at or near 
2400 metres, and, again, twice at about 2650 metres. The sequence 
of events was somewhat as follows:—(1) The older corrie of the 
Gribl, with the higher floor and the rear wall to west of the present 
one, was formed during a sojourn of the snow-line at 2400 metres, 
when the Riss glacier was in retreat. (2) At a later stage of the 
same glaciation, or in the first forward movement of the Wiirm, the 
basin of the Upper Tarntal was excavated down to the level of 
the higher outlets (d), while the snow-line persisted at 2650 metres. 
(3) During the final retreat, that of the Wirm glacier, the snow-line 
again stood at 2400. On this occasion the floor of the Gribl was 
lowered and the demolition of the rear wall broke down the connexions 
at 6 and d, as related above. (4) Lastly, a second stand above 2600 
metres reduced by 30 metres the floor-level of the Upper Tarntal, the 
north-west outlet was pierced and subsequently dammed, since which 
no advance of the ice has taken place. 
Incidentally it may be observed that as far as this evidence goes 
there is no necessity to suppose that the snow-line ever rose above 
2650 metres during the interval between the Riss and the Wiirm period. 
The climatic snow-line to-day is here at least as high as 2750 metres. 
1 See Map, Fig. 1, p. 245. 
