The Post-Glacial Geology and Physiography of West Lancashire and the Mersey Estuary

T. M. Reade—Post-Glacial Geology. Ill temperate conditions ; the periods becoming less intensely contrasted towards the close.1 This was the time of great continental ice-sheets, but in the later glacial periods of the same cycle the ice- sheets were less extensive. Oscillations of the relative level of land and sea took place, but to what extent is not known.2 2nd. A period of unknown duration, when the ice-sheets withdrew from all the low grounds, leaving behind them piles of rubbish. Climate becoming temperate. 3rd. A period of subsidence, during which the moraines profondes and terminal moraines were much denuded and their remains heaped up into mounds and ridges by the action of the sea. Climate temperate. 4th. A period of emergence, characterized throughout by arctic conditions; much floating-ice dispersing erratics over the submerged land; accumulation of clays with arctic mollusca; pauses in the upward movement marked by " raised beaches." 5th. A period of local glaciers in Great Britain and Ireland; continued elevation of land; continental condition of our island followed by partial submergence and re-elevation. Climate becoming gradually ameliorated. Having now considered the glacial and interglacial deposits, I pro- pose to take up some of those difficult problems which are suggested by the phenomena of the cave deposits and older river gravels of England. It will be seen in the sequel that the facts already adduced bear strongly upon this subject; indeed, without a clear conception of the succession of events revealed by our glacial deposits, it appears to me that we run some risk of getting into confusion when we seek te decipher the history of post-glacial accumulations. V.—THE POST-GLACIAL GEOLOGY AND PHYSIOGRAPHY OF WEST LANCASHIRE AND THE MERSEY ESTUARY. By T. MELLARD READB, C.E., Associate of the Institution of Civil Engineers. Y attention having been directed, during the construction of the Main Outfall Sewer at Birkdale, to the Post-Glacial deposits underlyinM g the great plain between Waterloo and Crossens, and having, through the execution of numerous other works in the dis- trict, peculiar advantages for prosecuting investigations in Post- Glacial Geology, I am induced to lay before your readers several interesting facts, and also, as I venture to think, some important de- ductions therefrom. With the exception of the superficial sands and gravels and river 1 The evidence derived from the Glacial deposits of East Anglia would make it appear that at the beginning of the cycle the arctic and temperate periods were also less strongly contrasted than they subsequently became. 2 I have stated in the text my belief that the Macclesneld beds and those of Moel Tryfan ought to be referred to the period of great submergence, during which the karae series or esker drift was accumulated; they are in all probability the equivalents of the high-level shelves of drift met with in Ireland and Scotland, and likewise in North America. 112 T. M. Reade—Post-Glacial Geology of valley deposits, desoribed by Binney, Trimmer, and others, the great mass of Post-Glacial deposits of Lancashire and Cheshire lie on the margin of the sea-coasts, and of the rivers Mersey, Dee, and Eibble, and follow with remarkable regularity the 25-ft. contour line below which the bulk of them lie. The Lancashire deposits between Waterloo and the river Douglas have a coast-line of about 24 miles long, are 4 miles wide at Formby, measuring from high-water mark, and run inland in the valleys of the rivers Alt and Douglas about 8 miles respectively. They also extend into the valley, in which was formerly Martin Mere, about 7 miles, joining the Douglas valley deposits at Bufford, through a narrow neck. Between Martin Mere and the river Douglas the Boulder-clay rises through the Post-Glacial deposits to a maximum height of 68 feet above Ordnance datum, and occupies an area of about 8J square miles. The total area of the Post-Glacial deposits lying inland, from high-water mark at the coast-line, forming what I call the Formby plain, is about 75 square miles. Nearly the whole of this plain, though much of it lies below high water, is, through the drainage works of the Alt and those at Crossens, now under cultivation. In Cheshire the equivalents of these deposits form another plain drained by the Birket, a brook falling into Wallasey Pool, the lowest and major part of which pool is now occupied by the Birkenhead Docks. This plain is about 6 miles long at the coast, and has an area, inland of high-water mark, of about 7J square miles. In addition to the Formby and Birket plains, there are numerous equivalent formations, such as the Inoe and Helsby Marshes in Cheshire, and alluvial or tidal flats fringing the Mersey, and reaching above Warrington; with similar formations on the Dee and Eibble, occupying together many square mites. Base of the Deposits.—The base upon which the deposits rest is nearly everywhere Boulder-clay, under which occur the sandstones arid marls of the Trias. A careful comparison of borings and sink- ings at various points has convinced me that at the base of the deposits, and forming the surface upon which they rest, k a Post- Glacial beaoh cut in the Boulder-clay, and that the valleys of the Alt, the Douglas, and the Eimrose brook pass under the deposit* seaward, to a considerable depth below low-water mark ; the whole being filled and levelled up with the marine formations and old land- surfaces now under consideration. Borings taken on the Waterloo shore show the clay from 6 to 9 feet below the surface, and other borings show a gradual deepening of the Post-Glacial strata north- wards, until at Birkdale it reaches 80 feet; the depth of the strata bearing a pretty constant proportion to the recession inland of the 25-feet contour line. A well-sinking at Seaforth Station showed 40 feet of peat and 10 feet of silicious silt, without reaching the Boulder- clay or the rock, showing that the Eimrose brook formerly, when the land was elevated, flowed down a deep and narrow gulley, now levelled up. Again, at Eufibrd, piles were driven 70 feet into the Bilts without bottoming them, proving that the Douglas, at the same the Estuary of the Mersey. 113 period, also flowed down a deep valley now silted up; and, as its mouth now bottoms on the Boulder-clay, and the river is nearly empty at low water, after the silting up, it must have worked its course to the westward and cut out the cliffs of Boulder-clay forming its west or left bank at Hesketh. The valley of the old Wallasey Pool, that of the river Weaver, and numerous tributaries along the Mersey, now silted up, all go to prove that the land was, previously to the Post-Glacial deposits, considerably elevated, as these valleys could only be cut down vertically by sub-aerial action; though, I . believe, they have been considerably widened and modified by marine denudation. Washed-Drift Sand.—Evidences of a very considerable submergence of the land are tolerably abundant in districts adjacent to the one under consideration, marine shells having been found in stratified beds of sand and gravel up to 400 feet high on the Eibble, 1200 feet at Macclesfield, and 1300 feet on Moel Tryfaen in Wales; on the opposite side of the Irish Sea on the Three Eocks Mountain, at Kingston at 1200 feet high, and I believe, also, in elevated positions in the Isle of Man. These shells consist wholly of recent species, but some of them are of a northern character, indicating colder conditions than at present obtain. Their exact position in relation to the Boulder-clay of Lancashire has not yet, that I can ascertain, been clearly made out, and demands much closer attention than has hither- to been given to the question. Judging from the physical appearance of Lancashire and Cheshire, I have slowly arrived at the opinion that since the laying down of the Boulder-clay, it has been elevated and again submerged, as its surface presents what can only be planes of marine denudation.1 While, on the other hand, the valleys before alluded to, now filled up with Post-Glacial deposits, and also the main valley of the Mersey, had, undoubtedly, been excavated their full depth before any of these Post-Glacial depositions took place. The width of these valleys is too great, and the slopes of their sides too gradual, to be due to sub-aerial influences alone. Nor could they, in my opinion, have been formed during the first emergence of the Boulder-clays from under the sea, as I fail to understand how, in that case, they could have been cut down to their present depth. A considerable Post-Boulder-Clay submersion of the land, previous to any of the deposits I am about to describe, has, therefore, I venture to think, taken place. Whether this interval of elevation and submergence is accepted or not, the Boulder-clay has, undoubtedly, suffered much marine denudation, and the effect has been the elimina- tion from it of what I have termed, in allusion to its derivation, washed-drift sand, under which term I include the Shirdley Hill sand of De Bance2 and the upper drift sand of local geologists. Possibly the forest sand of Binney3 and the superficial sand described 1 Chambers calls them terraces. See Ancient Sea Margins, p. 224. 2 Explanation of Geological Map, 90 S.E.

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