PART 1.] McMahon : Notea of a tour Ihronyh liangrany and Spiti. 
C7 
Mr. J. B. Lyall, in his settlement report of tlio Kangra district (1874), states 
(page 164) that “light showers of rain occur” in Spiti, “in July and August; 
and in winter, snow falls only to the depth of 2^ feet.” As “ 10 inches of snow 
rouglily represents 1 inch of rain ” (Huxley’s Physiography, p. 6J), 21 feet of 
snow is ecp\iv!ilont to a rainfall of 3 inches. Bven on the assumption that the fall 
of snow on the high peaks and passes is greater than that mentioned by Mr Lyall, 
the average for the whole of the Spiti valley cannot he in excess of 10 inches in 
the year, and the bulk of this falls as snow. 
No hot winds penetrate to these elevated and mountain-locked regions to 
cause sudden thaws ; the clear frosty air is usually without a cloud, and the snow 
“melts gently” (Lyall, p. 162). 
That denudation proceeds slowly in the region under consideration will, 1 
think, be evident when we consider that though the Spiti (a tributary of the 
Sutlej) flows over comparatively soft limestones and friable slates, whilst the Sutlej 
at Wangtu has to cut its way for miles through intensely hard gi’anitic gneiss, yet 
the bed of the Spiti opposite Shalkar is still 4,724 feet above the Sutlej at 
Wangtu,’^ the distance between the two points being only 45 miles as the crow 
flies. 
The deepening of the bed of the Satloj at Wangtu must be a very slow 
process, but clearly the excavation of the Spiti valley is a still slower operation. 
The fall of the Sutlej at Wangtu is about 50 feet per mile. 
1 think, with reference to the only data we have to enable us to form a rough 
idea on the subject, that the rate of the deepening of the bed of the Spiti at the 
point under consideration is probably not more rapid than 1 foot in 400 years. 
At this rate it would require 960,000, or in round numbers say about one million 
of years,^ for the excavation of the Spiti valley between Shalkar and the Ga 
station. 
At the rate of 1 foot in 400 years, the total depth excavated in 80,000 years 
would be 200 feet only; so that if the last glacial epoch terminated as recently as 
80,000 year’s ago, the date fixed by Dr. Croll on astronomical data (pp. 325-327, 
Climate and Time), the Upper Sutlej and Spiti rivers, and the agents of denud¬ 
ation operating on the slopes of their watersheds, cannot have cfl^accd the broader 
marks of the work done by glaciers during that pieriod. If then any great increase 
of glaciation during the last glacial epoch took place in the region under consider¬ 
ation, we ought to find decided traces of ice sculpture in our Himalayan rrpland 
slopes and valleys. 
A similar conclusion appears to have been arrived at by experienced geo¬ 
logists regarding England. “ Post-glacial denudation generally,” states Mr. Good- 
1 The Sutlej at Wangtu, 116 miles beyond Simla, is 5,200 feet above the sea, 
2 This is less than half the rate adopted by Geihie for his average valley, “lor we hnd l)y a 
simple piece of arithmetic, that at the rate of denudation which we have just postulated as probably 
a fair average, a valley 1,000 feet deep may be excavated in 1,200,000 years, a pei’iod which, in the 
eyes of most geologists, will seem short enough.” Juke’s and Geikie s Manual, p. 431. 
