252 Dr. A. P. Young—Glaciation of Navis Valley. 
On the analogy of the present nival gradient the ancient snow-limit 
in the Navistal should have been lower than in the Gschnitztal. 
The latest post-Wiirm stand recognized in the Tarntal at about 
2650, certainly not lower than 2600 metres, represents a depression 
below the present snow-line of about 150 metres. It may turn out to 
be the equivalent of the Daun stage, when the snow-line is supposed 
to have been lowered to the extent of 300 metres." 
The two pre-Wiirm stages of the Tarntal with snow-lines at 2400 
and 2650 metres respectively cannot be identified with any of the 
stationary periods hitherto recognized. 
Dertinition: ‘Nrvat Pranse’ anp ‘ Nrvat GRADIENT’. 
In the discussion which follows account must be taken of the 
contemporary variation in the height of the snow-line in different 
parts of the Alps. 
The small map compiled by Hess* from a great number of 
observations shows the present snow-line near the central ridge some 
800 metres higher than that on the border of the region now covered 
by snow. 
The imaginary surface containing all the different snow-lines of the 
same period may be called the ‘nival surface’, or more simply 
‘nival plane’. The latter term is sufficiently appropriate to present 
conditions ; the rise of the snow-line over considerable spaces being, 
as Hess’s map shows, nearly proportional to the horizontal distance. 
The angle made with the horizon by the nival plane is the ‘nival 
gradient’. 
User or Mars 1n TracInc ANCIENT SNOW-LINES. 
In order with the help of maps to trace over a wider area the 
extension of the snow-line during any one stationary period, it is 
necessary to know for each stage the orientation of the nival plane; 
only in this way will it be possible to sort correctly into their 
respective groups all the traces of the several nival surfaces. | 
The snow-line in the Alps of to-day is, as we have seen, found to 
rise as much as 600 or 800 metres as the axial ridge is approached 
from the border of the snow-covered district. The change in level 
is no doubt mainly due to variation in the amount of annual 
precipitation, the belt of maximum rainfall being in places as low 
down as 1800 metres,*® seldom much above 2000 metres.* 
It cannot on @ prior? grounds be assumed that the zone of maximum 
precipitation stood at the level at which it is found to-day, or that it 
varied according to any definite rule with reference to the position of 
the nival plane. It has, however, been inferred from the heights of 
corrie-floors and other evidence that the snow-lines of previous cold 
periods ran on slopes approximately parallel with that of to-day.® 
' Penck & Briickner, Die Alpen im Hiszeitalter, pp. 348, 639. 
> H. Hess, Die Gletscher, 1904, p. 74. 
3 Tbid., p. 44. 
4 J. Hann, Handbuch der Klimatologie, 1908-10, p. 258. 
5 A. Penck & E. Briickner, loc. cit., p.1144. E. Richter, Gletscher der Ostalpen, 
p- 277 (quoted by Penck). 
