Dr, A. P. Young—Glaciation of Navis Valley. 247 
The annexed diagram (Fig. 2) shows in plan the main hydro- 
graphical features. A pronounced north and south groove ¢, d, is seen 
to end at d at a level of about 2500 metres, just in front of the lower 
end of the Lower Tarntal. This groove was at one time the bed of 
the brook from Upper Tarntal, which was then tributary to the 
Lower Tarntal. By the retreat of the wall the connexion has been 
broken in two places, namely, along the dotted line d¢ and near the 
junction at d. 
At this earlier stage the wall at the back of the Griibl stood further 
to the westward as shown by the broken line e, f. The Griibl was then 
a simple corrie and received the drainage of both the higher valleys 
below the junction at d. Two other important changes have since 
been brought about. The floor of the Upper Tarntal has been further 
excavated so that the level of the latest outlet is lower by some 
20 metres than the old watercourse at 6. The floor of the Griibl has 
also been further eroded. A former level of this floor is no doubt 
indicated by the small plain which now makes a conspicuous feature 
in the landscape as seen in Pl. XIX, Fig. 2. This plain is now some 
20 or 30 metres above the corrie floor. 
In the case of the Griibl the process of ‘gnawing back’ must have 
gone on at an unusually rapid pace, the vertical joints in the 
dolomitic rocks facilitating the work of demolition, most of which was 
effected when the snow-line stood at levels between 2200 and 2500. 
As long as the floor of the corrie was above the snow-line the 
blocks may have been detached in great numbers, but being under 
firn could not have travelled very far. When the snow-line was 
above 2500 the ice-tongues were much reduced in mass and length 
and could contribute little to the transport of the boulders. We 
come back later to the detailed history of the erosion of the Griibl. 
BiocK-waLL AND BuiocKk-wastr. 
The east and west ridge carrying the Reckner and Little Reckner 
summits is almost girdled by a great wall of serpentine blocks. The 
Staffelsee on the south side is somewhat below the foot of the wall. 
The ridge-line of the wall is at 2600 metres, in parts higher. Fig. 3 
of Plate XX is a photograph taken from a knoll about 2600 metres 
high in the Upper Tarntal to the north-west of Reckner. The crest 
of the block-wall is seen projected against the snow-field in the middle 
of the picture and can be traced eastward as far as the contour can be 
followed to the left. 
At first sight the wall might be taken for a moraine, from which, 
however, it differs in the absence of any binding material. Most 
probably it is the result of the demolition which went on when the 
snow-line stood for a time at 2650 metres; the building of the wall 
was thus contemporaneous with the work of the glacier-tongue 
which filled the Upper Tarntal during or since the last retreat. The 
blocks were loosened by alternate frost and thaw, and remained near 
their original site or rolled down the slope by their own weight till 
checked. The pile has no doubt received later accessions and is 
probably increasing still; but its growth was far more rapid when the 
snow-line was lower. 
