Quarterly Journal of the Geological Society of London/Volume 35/On a Section of Boulder-clay and Gravels near Ballygalley Head
At a point in the road along the west coast of Ireland, between Larne and Cushendall and near Ballygalley Head, is a gravel-pit giving a section which may help to determine the classification of the Irish Drift.
The exposed face is, as nearly as I could judge, about 30 feet high, the pit being cut into a grass-covered slope lying against the hilly ground which mostly flanks the coast-road landward.
The following is a sketch which I took on the spot last autumn; and although I did not measure the thickness of the beds, it may be relied upon as giving their proportionate disposition.
The base of the excavation (A) is formed by a bed of current-bedded gravel containing shells and shell fragments. The actual base of the bed is not disclosed, so it is impossible to say how thick it is, or whether it rests on a Boulder-clay or on the natural rock.
It is capped by a perfectly straight and nearly level bed of sand, about 4 inches thick (B), on which rest irregularly disposed masses of clay and sand containing boulders, and above this a band of red clay, C, which is again overlain by a mass of unstratified Boulder-clay, D, the surface of which is covered by a bed of subaerial detritus, E, forming the subsoil of the grassy slope.
Coming fresh from Lancashire, where some geologists, following
Section in Gravel-pit near Ballygalley Head.
A. Current-bedded gravel with shells. B. Sand. C. Red Clay.
D. Unstratified Boulder-clay. E. Subaerial detritus.
I had not many minutes to examine the section, but I picked up during the time I was there a few of the shells and shell fragments. These I submitted to Mr. R. D. Darbishire, F.G.S., of Manchester, who has determined the following species:—Astarte elliptica, A. compressa, Leda pernula, Mactra elliptica, Natica, and Mytilus.
On comparing these with my list of shells from the Boulder-clay about Liverpool[1], I find that during several years' close search Astarte compressa only occurred in great rarity in the numerous localities I examined; and according to Mr. Darbishire's list it is "very rare" at Blackpool. Astarte elliptica was frequent in some localities and rare in others. Leda pernula, supposed to be a typical arctic shell, was very rare in the localities where it was found.
There is thus, according to shell-evidence, nothing to give an "Interglacial" character to these gravels as compared with either the so-called Upper or Lower Boulder-clays of the north-west of England. My opinion has long been against this tripartite classification, and I have pointed out more than once that it rests upon no intelligible basis[2]. The examination of the Irish Drift still further confirms me in the opinion that the marine Boulder-clays of the north-west of England and Ireland are but phases of one long sequence of events uninterrupted by changes of climate.
Prof. Hull has applied the same classification of "Lower Boulder-clay," "Middle Sands and Gravels," and "Upper Boulder-clay" to the Drift of Ireland[3]. It is quite evident, however, if the Boulder-clays of Galway Bay, as represented by the sections at Blake Hill, and the Fermoy valley in county Clare on the opposite coast, as well as the innumerable islands of Drift in Clew Bay, are members of the "Lower Boulder-clay," it is quite a different thing from the Lower Boulder-clay of Lancashire. The latter is marine, the former contains no evidences whatever of marine conditions.
On the north shores of Belfast Lough, beneath the celebrated raised beach, are to be seen sections of a Boulder-clay that corresponds in appearance with the marine Boulder-clays of the north-west of England. Large glaciated blocks of travelled stone, some of which I could have matched with erratics taken out of the Bootle Dock excavations, are to be seen on the beach washed out of the low cliffs of purple Boulder-clay, which is evidently, as in Cheshire and Lancashire, largely reconstructed from the Triassic Marls. I did not notice any shell-fragments in the Belfast Boulder-clay; but my time was limited, and in Lancashire it often requires close examination to detect them.
The Boulder-clay of Galway and Clew Bays is, on the contrary, made up almost wholly of triturated limestone,—so much so that it is even now a loosely consolidated limestone and stands in vertical cliffs. Imbedded in it are blocks of limestone and granite—one of the latter, at Galway, now on the beach, being from Moycullen and containing about 1600 cubic feet of stone. But neither here nor in Clew Bay, nor elsewhere in Ireland where the same kind of drift occurs, could I see on the contained stones those splendidly planed and grooved surfaces which distinguish most of the largest and smallest stones of the Lancashire Drift. The material and its imbedded stones are all distinctly local.
The explanation of these distinctions I propose to defer until I publish Part II. of the "Drift Beds of the North-west of England," having written this short description of the Irish Drift for record as well as to elicit, if possible, further information.