Quarterly Journal of the Geological Society of London/Volume 33/Note on the Red Crag
6. Note on the Red Crag. By William Whitaker, B.A., F.G.S. (Read November 8, 1876.)
(Communicated by permission of the Director General of the Geological Survey of the United Kingdom.)
My work on the Geological Survey in the Crag District has led me to think that previous observers have made a slight error in the classification of a certain ferruginous sand that is often to be seen above the shelly Red Crag, the line of junction being mostly very irregular.
This sand has been described by Prof. Prestwich as an "upper division" of the Red Crag, or, to quote his own words, "owing to the want of all fossils in the neighbourhood of Ipswich, as the 'unproductive sands' of the Red Crag"[1]; and he goes on to speak of the "erosion of the lower division" underneath this, afterwards classing the upper with the Chillesford sands[2].
Mr. S. Y. Wood, Jun., has referred the ferruginous sand in question to various horizons in the Glacial Drift, I believe with a constant tendency to lower its horizon; but his former views need not be dwelt upon, as that tendency has continued until he has accepted my classification, and now regards this sand simply as Red Crag, not separated stratigraphically or palæontologically from the shelly mass below.
The so-called "eroded" surface of the shelly Crag, noticed by various observers, is, indeed, apparently so only; but I must say that in many sections there is little or nothing to throw doubt on the reality of the appearance, which is somewhat analogous to the "mimicry" sometimes seen in insect life, though in our case one cannot see any object to be served by the delusion, unless it be the bewilderment of geologists. An examination of a large number of sections, however, and an attention to mere local details that could hardly be expected from any one but an observer who is obliged to note them as a matter of business, has conclusively shown that we have in this case not an eroded surface, worn out in a lower before the deposition of a higher bed, but an occurrence akin to the "pipes" of sand &c. so often seen piercing the top of the Chalk, and which, too, have also been taken as evidence of erosion, though their origin is now well understood: we have, in fact, an irregular underground surface, caused by the dissolving action of carbonated water in permeable beds, a surface formed after the deposition of the upper beds by the dissolving away of the shells that they once contained.
That the above is the true explanation of the irregular removal of the shells was first suggested to me by the fact that the apparently unfossiliferous sand above is, for the most part, exactly like the sand of the shelly Crag below, differing only in the absence of shells. Confirmatory evidence was given by the not uncommon occurrence in the upper sand of layers or masses of ironstone or iron-sandstone, which were often found to contain impressions and casts of the shells that had been dissolved out; and these, as far as could be told, were of Crag species. They occur in the area in which the sand has been thought to be unfossiliferous; and I believe they have not been described before from thence; but in a paper read to the Society in 1874 I have noticed like impressions of shells near Sudbury and Hadleigh[3].
Absolute proof, however, of the correctness of the explanation advanced was got by the observation of a few sections in which the lines of bedding, or even of false-bedding, in the shelly Crag were continued into the sand without shells. In some cases a marked gravelly layer was clearly seen to be at one spot in the Crag and at another in the sand; and in one pit a hand-specimen could be got of such a bed, half containing the actual shells, and the other half with casts only. In some pits large masses of shelly Crag are to be seen quite surrounded (in section) by the shelless sand—a fact difficult of explanation on any other view, but comparatively easy to understand as brought about by irregular dissolving action, checked in places by local hardness or slight decrease of permeability in the beds.
In many cases the dissolving away of the shells would seem to have been followed by some destruction of the lines of deposition in the sand; and, indeed, we should expect such a thing to occur on the abstraction of so much material. It is, of course, in these cases that the appearance of erosion is most deceptive.
It may be asked "What becomes of the carbonate of lime of the shells?" Some of it is doubtless carried away in the water of the many springs from the bottom of the Red Crag, thrown out by the London Clay beneath; but in places some of it is again deposited in the lower beds of the Crag as whitish marly streaks in slight fissures or open spaces. Very possibly, however, great part of the dissolution of shelly matter may have taken place under conditions somewhat different from those we now see, when perhaps the Crag was more permeated by water, or even water-logged.
Small as this subject may be, yet it is, I think, worthy of notice, and for three reasons:—because it tends to simplify our classification over an important Crag tract; because it shows a greater extension of the Red Crag than has been thought to exist, that deposit occurring in its almost unfossiliferous condition some way beyond where it is known in its shelly state; and because it draws attention to that slow metamorphism which takes place in permeable beds through the agency of water, whether as a dissolver of carbonate of lime &c, or as a depositor of iron-oxides.
Postscript.
After this Note was read, I heard from M. E. Vanden Brock, of Brussels, that he had observed an irregular junction of shelly and shelless sand, like that above described, in Belgium, and that he attributed it to the same cause as that now suggested.
(For the Discussion on this paper see p. 140.)