WCP2222

[1]

73. Harley Street

24^th March 1869 —

Dear Wallace

It would be of great use to measure carefully such deltas as those of the Rhone at the head of the lake of Geneva for it would be possible to make a rough approximation to the time which it has required for its deposition. This however would not give us the date of the melting of the glacier which we all suppose to have once filled that lake-basin of Geneva, for we cannot be sure that there were no upper lakes which had first to be filled both in the valley up the Rhone and [2] several of its tributaries. If I mistake not there are geological proofs of such ^higher basins having existed & after being filled up they have had their barriers cut through. I examined several lateral ^{valleys or} feeders of this upper Rhone particularly that of Useigne [Euseigne] which has been filled up to the height of about 500 f^t with unstratified till or moraine-like matter through which the small river has afterwards excavated its deep channels. I confess I cannot understand how that same lateral glacier accomplished [3] the filling up of a valley with such moraine matter nor how the valley near Botzen [Bolzano] in the Tyrol 700 f^t wide & 500 deep (see ‘Principles’ 10^th Ed. vol. 1 — p.p.336-337)¹ after having been eroded in the hard porphyry were in like manner choked up, instead of being deepened by the glacier which filled them with till & glaciated stones. Yet I grant of course that this same till being the result of glacial abrasion shows that ice can as we see it do, wear away ^{some parts of} its stony bed & convert rock into mud. I know [4] of no calculation which we can make from ascertained data as to the relative duration of the time which has elapsed since the Glacial Period was over & that ^{the time} which was comprised between the beginning & end of the same epoch. You seem to conjecture that the post-glacial ages were equal to three times the Glacial Period, but when I heard D^r Otto Torell² give a lecture on his map of the extinct glaciers of the north of Europe & of the advance and retreat of the Scandinavian land-ice, I confess that the succession of events, like the [5] [p.2] oscillations of level alluded to in the "Antiquity of Man" p. 285 —³ seem to me far greater when measured by geographical change than any post-glacial revolutions to which the geologist can refer. I have taken 224.000 years for the grand oscillations but it is true (see p. 285) that this is very vague as I do not know when the post-glacial period began or when the extinct glaciers disappeared. At the rate of subaërial denudation assumed by Geikie⁴ & Croll⁵we shall bring down a great intensity [6] of glacial cold to a very modern period assuming that we care to explain the Glen-Roy roads by the theory of glacial lakes.

This argument I believe I mentioned to you.⁶ Take two deposits of the glacial period of which the marine shells are well known, first, those of Erol [Errol] in Perthshire & Elie in Fife which Torell declares to be an area more glacial than the area of Spitzbergen [Spitsbergen] of which he knows the fauna so intimately, and compare these with the more southern though still an arctic fauna [7] of Mőel Tryfane [Moel Tryfan] which repose ^{at the height of 1400 ft} on a glaciated sea bottom. These two deposits are more different from each other in their climatal aspect and the change in the distribution of species, than is the Mőel Tryfane fauna when compared with that now living in the contiguous British Seas. But I think that Bridlington is a much older fauna than Erol & Elie & we have no reason to suppose that Mőel Tryfane belongs to the end of the glacial period.

The general absence of lakes in all latitudes in spite of the known activity of those forces which [8] elevate & depress the earth’s crust, is a proof that running water which is active in eroding river channels & is always striving to drain completely the land, is as a rule capable of counteracting the power which in a rainless area would soon form land-locked depressions, some of them below the level of the sea, as we see in parts of the Sahara & Asia Minor. Still it may be asked why are there not in the Great river basins of the Great temperate & tropical countries more swamps or incipient lakes especially where the vegetation is so rank as to preserve such shallow depressions. I am [9] [p.3] not so great a traveller as you are & am altogether inexperienced in regard to tropical countries liable to earthquakes & modern changes of level, the only great hydrographical basin affected by a recent earthquake which I have seen is that of the valley of the Mississippi where the shocks of 1811-12. caused what is called the sunk country of New Madrid. You perhaps know my description of this at page 235 vol II.⁷ Second visit to [the] United States — if not I will lend it to you— The depressed country which I examined only in part is said to be about [10] 80 miles from north to south by 30 in breadth. The erect trees though dead are still in great part standing erect in the water. The submergence is said in the deeper parts to be about 30 feet, but this I could not verify. The filling up of the lakes & swamps in this sunk ground is going on generally with extreme slowness by the falling of decayed timber and the growths of a luxuriant swamp vegetation. As to the sediment of rivers their waters are so filtered by the dead grasses & brushwood on the margins of the lakes & swamps that the water is kept perfectly [11] clear all the year round & the coal or lignite resulting hereafter from these lacustrine deposits will be pure & unmixed with earthy matter. But in the course of centuries the rivers after raising their banks will now & then burst through them & deposit sand & mud upon the peat & around the stumps of erect trees the remains of the submerged forests. Perhaps other earthquakes may re-visit parts of this tract & extend the sunk country by subsidence or narrow its bounds by upheaval. At present the prospect to the geologist is a very close imitation [12] of the old coal measures with numerous fossil forests of erect trees such as we see in Nova Scotia[.] But if this same change had happened in the lowlands of some arctic region or in any northern latitude during the glacial epoch, the principal & minor depressions of the sunk country would have been preserved by ice for centuries until some other subterranean movement depressing one region & elevating another like the earthquake of New Zealand in the year 1855 (p[.] 82 "Principles" vol II)⁸ might augment the depth & width [13] [p.4] of some portions of the area & isolate others, for every lake so caused there would be a great many inorganic lakes partly old river & or glacier channels blocked up by ice transported matter. If the same country be submerged, icebergs & the transported matter which they sometimes carry might still further interfere with the drainage regular system of drainage should it again emerge into land. We have no right to expect that two or three thousand years hence there will be lakes in the basin of the Mississippi because we [14] know not whether the subterranean movements will recur frequently & whether before a return of them all the present swamps will be filled up, even if the direction (whether upward or downward) & geographical extension of such movements shall be favourable to the formation of such lakes. We are not perhaps entitled in tropical countries to look for more than incipient lakes, although Humboldt⁹ tells us that in the earthquake of Carraccas ^Caracas 1790 the granite soil sank down 300 f^t & over a space 800 yards in diameter in the forests of Aripao the trees remained [15] green for several months under water (9th Ed. Principles p. 10^th Edition. Vol II p.112)[.]¹⁰

I foresee that until we obtain a more certain knowledge of the manner in which ice can erode rocks & the rate at which it can cut through them & above all, its power to throw out the broken rock or sand, or mud, to which it gives rise by erosion we shall be in the dangerous possession of an ideal force which like the old fashioned "convulsion of nature" or catastrophe or diluvial wave of our predecessors is able to effect whatever it is required to do & to supercede [16] almost every other agency. If Agassiz¹¹ had adopted Ramsay’s¹² theory no lake basin at the equator in Africa or elsewhere would to him present the least difficulty. It is as easy to scoop out the basin of Lake Superior as any small & shallow lake in Sweden. I must confess that I have no reason to complain because I have somewhere granted in the "Elements" that mountain tarns may be due to ice[.]¹³ If ice when so near the water shed can nevertheless act like an artesian auger & cut vertically [17] [p.5] through mica schist (I have one with which I am familiar in my eye) & if then like the auger it can draw up the pounded material & cast it out of the boring[,] I scarcely see any reason why the glaciers of the Lago Maggiore should not scoop out a depth of 2600 ft having only to push up the abraded matter along an inclined plane which though it mounts up 2600 f^t only slopes according to Ramsay at an angle of 2½ degrees, whereas the tarn I speak of in the parish of Clova, Forfarshire [Angus] must slope at an extremely sharp angle. In hollows called [18] giant cauldrons in Sweden five feet deep or pot-holes often three feet deep or more, in Scotland, we see that the water can excavate a pear-shaped hollow by whirling round the pebbles & sand and then taking up the finer sediment suspended in the water & so preventing the hollow from filling, even though the neck of the pot-hole is usually much narrower than the bottom. Is it possible that motion can be imparted to the ice which has filled a tarn capable in like manner of voiding the earthy or solid matter derived from attrition[?]

I used to suppose that the more [19] rapid the descent of a valley the greater the grinding power of the ice, but I rather conclude that they who scoop out Lago Maggiore by a glacier ascribe the chief efficacy to the thickness or weight of the glacier. If I may Refer^ing again to the section of the earth pillars ("Prin[ciples]" vol I p337)¹⁴. I may observe that I ascertained that vast moraine to rest directly on the porphyry and I endeavoured to explain the accumulation of moraine matter by imagining that the glacier kept the bottom of the valley clear while it was at work & ^but that [20] the moraine was left when the glacier receded, after which came the cutting action of the torrent. But the accumulation of 700 f^t of moraine at the bottom of a lake ^glacier long before it was bored to the depth of 2600 f^t forming a mass protecting the rock below from further waste, would seem to me more natural at the bottom of Lago Maggiore than in the lateral valleys of the Rhone in Switzerland, or at the Eisack in the Tyrol.

I think your strongest case is the radiation of valleys & the small lakes or tarns radiating [21] [p.6] in N. Wales & around Helvellyn & in Scotland & some low regions of the northern States of America. But these are all of small dimensions when you come to the great lake region of Canada & the United States, & find Lake Superior to be 350 miles long by 150 broad, besides Lakes Huron & Michigan, Erie & Ontario, to say nothing of a set of minor lakes rivalling those of Switzerland in size, & when we find that the bottom of Lake Superior is 165 f^t below the level of the sea its depth being 792 f^t we cannot [22] but feel that the magnitude of the basins render the hypothesis of the ice scooping more hazardous. If it were now a land of glaciers & Alpine heights & if the thickness of the ice imparted to it great perforating & ejecting power, we should have no right to oppose this theory on the score of a deficiency of time, for we know nothing certain as to the duration of the supposed sheet of continental ice. It is of course very possible as we find marine shells of Post Glacial date upraised to the height of 450 feet at Montreal in the valley of S^t Lawrence, other [23] preceding movements may have lowered the Continental [sic] several thousand feet & have helped to cause the intensity of the cold.

But the admission of the occurrence of such great movements in Glacial & Post Glacial times would suggest another theory independent of ice grinding, for the origin of the great basins. The study of the horizontal platform separating lake Erie & Ontario suggested to me a former period of great marine denudation, & a subsequent one of land ice. I will also refer you to what I have said [24] of the great succession of terraces near Toronto [⁽]^{Travels in N. America Vol II page 103}[⁾] ¹⁵. If they imply great continental movements during & since the Glacial Period we are bound to consider how much these ^singular movements may have had to do with the origin of the lake basins. But I grant that the great Canadian lakes have not the appearance of Alpine valleys eroded by rivers which were afterwards blocked up at their lower end, an aspect which the lakes of Maggiore, Zurich & others convey to my mind. Still I hesitate to believe that Lake Superior [25] [p.7] & the rest were scooped out by ice — & if we could refer their origin to any other cause the same explanation would probably go far to make us understand the larger Swiss lakes, though I am not sure we can prove the Canadian ones to be as modern as those of Switzerland which are of Post-Miocene if not Pliocene date. I suppose you attribute the absence of Lakes in the region of extinct glaciers on the south side of the Himalaya to the filling up of the basins with sediment, which would go on [26] rapidly in such a region. In Cashmere [Kashmir] we may I think affirm that lakes formed by subterranean movements & not by glaciers have even in historical times been greatly effaced by the depositing power of rivers which ^whereas if ice hadnot been present larger lake basins would exist, although ^and in that case they might have erroneously been ascribed to the ice-scooping power[.]

If the bed of part of the sea having an unequal bottom be converted by a general movement into land, we must be on our guard against being induced [27] to ascribe the cavity to ice action merely because the surface has been subsequently modified by ice or polished by it and striated.

Swiss Lakes

In the case of Lago Maggiore[,] Zurich & other lakes, it is admitted by Ramsay that the vallies¹⁶ in which they are situated & their tributaries were formed or eroded by running water as in short they were & are now united[,] the glaciers gave the finishing stroke to the work by scooping out a basin which should be land locked after the melting of the ice. You are right therefore in thinking that it is by comparing glaciers where a [28] great glacier failed to produce a lake as at Ivrea with another place where it scooped out a very deep & wide one as in Lago Maggiore that we shall probably get a key to the enigma. If the thickness or weight of a column of ice be the great secret of its perforating power then this power would increase with the augmenting depth of the lake. But I should think that as the motion imparted to the lowest ice must also constitute an essential part of the grinding power, this motion would diminish regularly with the increased depth, because even if the increased slope helped the onward motion [29] [p.8] & the force from behind, yet the principle of the decreasing rate of progress of the lower strata both of glaciers & rivers would cause the propelling force to grow weaker. You have read what I have said of the periodical escape of the waters of the glacial lake called the Märjelen sea [Märjelensee] ("Principles 10 Ed[.] Vol I p 377)¹⁷ — you see at p.376 what changes are said to take place in the internal structure of the glacier which allow the waters to pass through; certain it is that no water overflows the surface of the Aletsch glacier & the [30] question is whether the water beneath the ancient or extinct glacier of Lago Maggiore, Geneva & others may have had a somewhat similar head of water or reservoir in a higher part of the valley which exerted a hydrostatic pressure forcing up the melted ice mixed with fine sediment at the lowest part of the basin & so cleared out the loose matter which would otherwise protect the rock from being further abraded below the level of the sea.

Believe me [31] ever truly yours | Cha Lyell [signature]