98 Dr. Bit Riche PrcUer—Thc Merjelen Lake. 



length and about 50 metres (164 ft.) 1 in depth, formed by the 

 Aletsch glacier. The cross valley or depression in the gneiss 

 formation between the Aletsch and Viesch glaciers was. in times of 

 more extensive glaciation, evidently filled b}' an offshoot of the 

 former glacier into the Viesch valley, although the fine mud which 

 covers the bottom of the lake is not, as has been averred, old 

 moraine, 3 but simply deposit of the particles held in suspension 

 by the lake. Under the glacial conditions of the present day, the 

 Aletsch glacier is unable to invade and fill the lake basin, the more 

 so as the lake is, in addition to glacier water, fed by spring water, 

 and notably from a small tarn on the Merjelen Alp about 760 ft. 

 above the lake-level, so that the water in the lake is kept in motion, 

 and permanent congelation is thus prevented. Indeed, the melting 

 power of the lake water is evidenced by the fact that, although the 

 surface of the Aletsch glacier slopes gently into the lake, the slope 

 is, below the water's edge, undermined and melted down to a vertical 

 wall or cliff. The glacier thus acts as a retaining wall to the lake, 

 while the latter retains, that is, prevents the intrusion of, the glacier. 



The natural outlet of the lake is to the west, through the Aletsch 

 glacier ; but when the lake exceeds a certain level, it has at its 

 eastern or narrow end an additional outflow to the Viesch glacier, 

 over a low divide or saddle which, at ordinary level, separates it 

 from the drainage area of that glacier. The lake thus possesses the 

 peculiar feature of having two outlets in opposite directions. 



In addition to its contiguity to the Aletsch glacier, and to the 

 intensely, almost unique, greenish-blue colour of its water, 3 the 

 Merjelen lake possesses, as is well known, the further peculiarity, 

 not only of sudden fluctuations of level, but of emptying itself at 

 irregular intervals, and sometimes within the space of eight or 

 twelve hours, through a temporary passage in the glacier wall, the 

 phenomenon being on certain occasions accompanied by a great noise, 

 which is sometimes heai'd at a distance of no less than six miles, and 

 is produced partly by the oveidianging roof of the glacier falling in 

 and partly by the rush of water through the glacier. 



This remarkable Alpine phenomenon recurred during the night 

 from Monday to Tuesday, 23rd to 24th September last. Already, in 

 the autumn of 1894, a partial emptying had taken place, the lake 

 being then reduced to about one-third of its volume at high water. 

 As is usual in the case of partial emptying, there remained two 

 lakelets or tarns — an upper, smaller and shallow one, and a lower, 

 larger one, deepening rapidly towards the glacier end, the two basins 

 being divided by a path which leads to Concordiahiitte and is sub- 

 merged when the lake is at high level. During the hot and dry 

 summer of 1895 the lower basin rose again ; at the end of August 



1 The exact depth deduced from the Swiss Contour Map is 47 metres (154 ft.), 

 while that measured by Prof. Ramsay in 1858 was 157 ft. 



2 Ph. Gosset, Jahrb'uch S.A.C. 1887-8. 



3 This colour appears to correspond approximately to shade v of Prof. F. A. 

 Forel's scale of eleven shades of lake waters. — F. A. Forel, "Le Lenian," vol. ii, 

 p. 464. Lausamie, 1895. 



