WCP2220

[1]

73, Harley St

London W.

16 — March '[18]69

Dear Wallace

I have been reading your letter with much interest,¹ and, if Tyndall² does not answer Moseley³ & Croll⁴, I should like to see your letter in print. It was Darwin⁵ that called my attention to the controversy, and unless I hear from you to the contrary, I shall show him your letter when I next meet him.

For my part I hoped, when I wrote what I have said on the motion of Glaciers in my 10^th Edition, vol.I, p.370,⁶ that we might rest satisfied with the Regulation Theory with which I believe your view coincides pretty well—

I have been thinking much over your explanation of the [2] absence of a great lake below Ivrea, no doubt when the Alpine ice reached that vomitorio [Latin: vomitory], reaching the open plain of the Po it was able to spread laterally, & its thickness would diminish rapidly, whereas when the Rhone reached that part of its valley where the lake basin of Geneva now exists, it would still be confined between walls of Great height, although they would not be so high & so near one another as above or, in other words, the valley filled by the Glacier would not be so narrow as it had been between the present head of the lake and [3] Martigny. Before the scooping out of the lake basin of Geneva had begun, a thickness of ice very much greater than that which spread itself over the area which is now represented by the surface of the lake ought to have hollowed out a much deeper basin between Martigny & Port Vallais [Port-Valais]; so in regard to the valley of the Dora Baltea the two Great Glaciers coming from Mont Blanc & Monte Rosa, & meeting between Aosta & Ivrea ought to have scooped out the deepest of all the Italian lakes. Perhaps you [4] may say "How do you know that they did not"? If such an excavation did take place below Martigny or below Aosta, such basins would have been filled up with alluvium.

Mortillet⁷ describes, in the great valley which ascends to the pass of Mont Cenis, a succession of lake basins which got filled by alluvium after which the rivers cut gorges through the rock which the glaciers left as the boundary dam at the outlet of the lake basin. I very much doubt however that such a great thickness of stratified alluvium [5] [ p.2] has filled up any such imaginary lake basins, when the lakes are of very large size, from the dimensions of the Lake of Geneva to Lake Superior, I prefer the calling on another well known & ever active cause, namely those subterranean movements which seem to me to have converted a few of the old Alpine Valleys of denudation[,] the Lago Maggiore among others[,] into lakes. To suppose that no pre-glacial valleys were converted into great receptacles of ice is to make the monstrous assumption as I have somewhere [6] said, that there was a coincidence between the great watersheds & the lines of maximum upheaval. I have never for a moment denied that a glacier may wear out its channel or even a mountain basin (though I cannot exactly understand how it performed this ^latter feat) but I have no more attributed glacier lakes to local subsidences than I have ascribed valleys to such a cause. It is precisely because I believe that subterranean movements are wholly independent of those minor inequalities of [7] the earth’s surface which are connected with the great lines of drainage that I think those lines must occasionally be converted into lake basins, wherever there is an excess of upward movement in the lower country as compared to the upland region, there being at the same time a sufficient quantity of ice to fill the new basin & prevent a ravine or outlet being cut by running water at the lower end of the basin. The absence of this ice explains of course why there are no lakes in tropical countries for these, there has been [8] no ice to fill up the cavity. The same movement which would give rise to the Lake of Constance or of Geneva and subsequently to the lacustrine formations by which they must in time be filled, could not in any tropical country produce similar formations, save in some exceptional cases, i.e. where the movements have been so rapid & considerable that the river channel was turned into a lake. If it be said that I require a vast period for ordinary subterranean movement to convert valleys into deep lakes, I reply, [9] [p.3] not as long as would be necessary for scooping out by ice such deep and large basins as the Theory of Glacier erosion requires. Along the foot of the Maritime Alps there are proofs of Post Pliocene upheaval & raised beaches, & if you must call in partial movements to explain the upheaval of massive glacial shelves to a height of 1400 f^t in Moel Tryfane[Moel Tryfan], why may they not have acted on the land intervening between some of the old Alpine valleys & the sea.

Our surveyors are discovering faults of 6,000 f^t vertical height which were produced in the [10] coal measures between the close of the carboniferous period and the beginning of the deposition of the lower Permian. It is chiefly in the coal formation that proofs of such displacements are found & they are augmenting daily, clearly because the coal is so much more carefully studied than other formations. Although I do not appeal to such abrupt inequalities of movement in the case of the Alpine lake basins, I still think that it is reasonable to impute even to very slow movements of the Earth’s crust a power of causing lakes from 1000 to 2000 f^t deep & of such [11] length as those north & south of the Alps, rather than to ascribe such vast hollows, in great part below the level of the sea to ice action. Marine Eocene strata have been uplifted in the Alps to the height of 12,000 feet in Post Eocene times, Miocene brackish water strata 5000 or 6000 feet in Post Miocene times. There are also proofs of thousands of feet of Eocene and Miocene subsidence. The cretaceous rocks in the Alps subsided [12] hundreds of feet before they were upraised 12000 feet. The same is true of the Jurassic formations and of some of the antecedent Palaeozoic strata. It has been the area of the greatest European upward & downward movements. It would therefore be strange if during & since the Glacial Epoch the drainage of the old valleys had never been interfered with in those glacial times, when the [13] [p.4] force of running water was prevented from removing any obstructions caused by an excess of depressing movements in the higher, or of upheaving movements in the lower country—

ever truly yours | Cha Lyell [signature]