• Abstract and Figures
• Public Full-text

110 Rev. J. Clifton Ward—Geology of the Lake District. also wanting, unless we regard the presence of large boulders at high levels, as the diallage blocks of Crousa Down for instance, as the unremovable debris of an old glacier system, and ascribe the presence of large boulders, at some distance from their parent rocks, in river gravels, to the relics of moraine, carried down to successively ^ lower levels in the excavation or deepening of the present lines of drainage. However, if, as I agree with Mr. Godwin-Austen in thinking (op. cit), the land stood at a much greater elevation during the Glacial epoch, a great and constant snowfall may have given rise to local glacier systems; and as the present area of the county would offer little more than the generative sources of the (imaginary) glaciers, all traces of pre-existent deposits and of moraine matter, except very large boulders, would be swept down by the flood waters of the succeeding period of subsidence to levels now submerged. But as all such glacial theories are purely hypothetical, it behoves one to fall back on the probability that Cornwall, during the Glacial epoch, stood at a much greater elevation, and that its highlands were crowned with constant snows, the melting of which during the succeeding amelioration, accom- panied by subsidence, caused the liberation of great quantities of surface water with torrential power carrying off the pre-existing detritus to lower lands, now submerged. (To be continued in our next Number.) I beg leave to correct the following errata in Part I. of my paper. On p. 33, line 40, for "10to20" read " 10 to 12;" on p. 34, line 20, for " B.c. 400" read "B.C. 440," and for "e/ctras" read " iovaat," also for " yriaat" read " " and on p. 79, line I, for " on " read " or."—W. A. E. U. III.—ON THE PHYSICAL HISTOET OP THE ENGLISH LAKE DISTEICT. WITH NOTES ON THE POSSIBLE SUBDIVISIONS OF THE SKIDDAW SLATES.1 By the Key. J. CLIFTON "WARD, P.G.S., F.E.M.S. Post- Carboniferous Periods. As the Carboniferous rocks form a framework around the Silurian mountain country, so do the Permian rocks for considerable distances lie as an outer flat and bevelled edge to the slightly raised Carboniferous frame, the edge indeed overlapping the frame itself along the west coast-line between St. Bee's Head and the Duddon Estuary. It is not purposed to give a detailed description of any Post- Carboniferous formations lying outside the mountain district. The two thick Permian sandstones (the Penrith and St. Bees), separated by a set of shales and marls, are believed by some to indicate the conditions of an inland sea, and the formation in some parts of its English development to suggest cold or glacial conditions. How- ever this may be, it seems more than probable that during the whole of the Permian Period the present Lake District was dry land, and undergoing that subaerial waste which, long continued, was to give 1 Concluded from the February Number, p. 61. Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 Rev. J. Cliflon Ward—Geology of the Lake District. Ill rise to the present beautiful forms of mountain and valley. If the Permian were partly a cold period, then an early set of glaciers may have found their home among the early Cumbrian mountains, and any amount of ice-smoothing and polishing of the rocky surface then produced must, to a certain degree, have hindered the work of denudation for a long period afterwards. Indeed, it is more than likely that the very large amount of denudation produced by glacial means during a glacial period is more than counterbalanced by the retarded action of subaerial forces working upon a generally smoothed and rounded surface. In post-Permian times, when extensive north and south movements and dislocations occurred (as along the foot of Cross Fell range, and the Pennine Anticlinal), sym- pathetic movements may have taken place in the mountain nucleus, and very likely the majority of the N. and S. faults ranging through the Silurians, and frequently shifting the previously formed E. and W. ones, were produced at this period. As we pass in review the periods succeeding the Permian, nothing more can be said with regard to the history of the Lake District than that in all probability the Cumbrian centre remained an area of dry land, more or less lofty, though slowly being eaten down and into by the forces of denudation. From the mountains as they then existed one could have looked northwards and southwards into areas of water beneath which the New Bed deposits were being formed. Subsequently the Liassic sea with its marine monsters probably skirted the mountain district at no great distance. Onwards in time through the Jurassic Period, with its Australian- like climate, during which the Cumbrian mountains may have experienced many a storm of semi-tropic violence, through the long Cretaceous Epoch and Tertiary times must the District have been suffering wear and tear, at times doubtless nearly or quite united to the neighbouring high ground of the Pennine Range and of Wales. No wonder so much has been carried away by apparently weak sub- aerial powers of denudation, rather the wonder is that the mountains should have been preserved at all, instead of all being levelled to the sea. This will again be commented on in following pages, when the question of the numerical equivalents of periods will be dis- cussed. That the District experienced many vicissitudes of climate throughout this enormous length of time seems certain—now tempe- rate, now semi-tropic, and now again probably glacial, and the last marked geological change which it underwent was during the so- called Glacial Period proper. Glacial Period. This period having been treated of in full, as regards the Lake District, in my former Papers, and in much detail in the Survey Memoir, chap, xiii., nothing but a mere necessary outline will be now given. The glacial phenomena of the district are as follows:—Till, • mainly the product of a confluent glacier-sheet. Drift gravel, and Stratified sand and gravel, often occurring in the form of eskers, due Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 112 Rev. J. Clifton Ward—Geology of the Lake District. principally to marine action. Boulders carried far from the parent- rock by glacier-ice at one time, and floating-ice at another. Glacial scratches partly produced by a confluent glacier-sheet, partly by small separate glaciers; also occasionally made by floating-ice during the period of submergence. Moraines left by an early set of glaciers, much modified by subsequent aqueous action; and more perfect moraines left by the most recent local glaciers. Many years study of these phenomena has led me to draw the following genera] conclusions, which I take in part from my Survey Memoir, p. 97 : " At the commencement of the cold period small glaciers occupied the heads of the various valleys, and, as the cold continued and increased, they became larger and larger, until, in many cases, they united, overlapping the lower ridges parting valley from valley, and forming one great confluent ice-sheet, the movement of which was determined to the north and to the south, or east and west—as the case might be, in different parts of the district—by the main water- parting lines. A great quantity of rocky debris was moved onwards and left scattered over the country, partly by the first formed moraines being pushed forward, and partly by the ice overriding the same and dragging on and triturating the fragments beneath it. In this way the Till was formed, sometimes left in rock-sheltered upland hollows, but most largely deposited on the lower and less inclined ground. " Whether this first land-glaciation was interrupted by one or more mild periods, the deposits in this district do not prove. As the final close of this epoch of intense glaciation drew on, moraines were left by the retreating glaciers plentifully scattered in every valley, but the glacial streams and rivers must have made much havoc amongst them, cutting them up and bearing away their material to lower levels. "Then, when the cold had disappeared, began a submergence of the district to a very considerable extent As the land sank, the old moraine material was sifted, sorted, and partly rounded. At the ends of some of the fiords or straits, sand-bars were formed; but, as there was no floating-ice during the earlier stages of submergence, these sand and gravel deposits enclosed no large boulders. The district became gradually converted into an archipelago, and currents circulated among the islands. When depression had gone on to the amount of 1,000 feet or less, the cold returned and ice-rafts bore blocks from one part to another.1 In many cases the direction in which currents swept the floating-ice was the same as that of the old glaciers, and thus boulders were transported along the same course at different periods and by different means. Sometimes, however, or at certain parts, marine currents bore floating ice, with its boulders, in directions opposed to, or much at variance with, the old glacier courses.

Load more

  16

Copy Link