The Ancient Glaciers of the Midland Counties of England

40 0 W. W. WATTS AND W. j, HARRISON ON chief constituents are characters linking the two types, while the freshness of many of the minerals, and especially of the olivine, gives the rock a very new aspect. It would be scarcely possible for a petrologist to discriminate the coarse varieties of rock seen at Hailstone Hill from that of Killala Bay, in Sligo. on the one hand, or from the coarse dolerites of Portrush and Fair Head on the other> Again the finer grained dolerite lavas of the Giant's Causeway, and the Tertiary dykes of Lisnaskea and elsewhere in Ireland would be very difficult to distinguish from the normal rocks of Pouk Hill and Rowley. When we recollect that some of the faults which bound the South Staffordshire Coalfield traverse Jurassic rocks, one is tempted to ask whether it may not be possible that those move ments which post-date the Rowley Rag in the South Stafford shire Coalfield are in reality of Tertiary date. If Dot, the similarity of pre-Tertiary and post-Tertiary volcanic rocks is closer even than Allport suspected, when he was struggling for a recognition of the identity of the rocks of these different ages. THE ANCIENT GLACIERS OF THE MIDLAND COUNTIES OF ENGLAND. By W. JEROME HARRISON, F.G.S. As far as their glacial phenomena are concerned, the Midland Counties offer to the geologist one of the most interesting and important fields of research in the British Isles. And this for three main reasons: (I) Three great glaciers met here. (2) The district contains examples of the terminal and lateral moraines, and also of the" fringe "t of these g;aciers. (3) But little detailed work has as yet been done in mapping out the precise courses and limits of the great streams of ice by which the district was formerly invaded. No SUBMERGENCE. It may be said at once that there is no evidence in the Midlands of any great depression since the close of the Mesozoic Era. The so-called " sea cliffs," " coast lines," etc., can be shown to be but the ordinary results of sub-aerial denudation; the finely-stratified, often false-bedded, gravels, sands. and loams of the surface deposits are for the most part the results of the sub glacial, englacial, and supra-glacial drainage-systems of the glaciers. Some loams were deposited in ice-dammed glacial lakes, while * Guide to fhe Collections of Rocks, etc., belonging to the Geological Sun'ey oflreland. Dubtin, 1895, pp. 51 and 78. t I apply the term l4 fringe" to the dubious region which lay just at or in front of the original margin of the ice. It is usually occupied by gravels and sands, the results of the melting of the ice. The glacier-ice sometimes made temporaryexcursions into the "fringe" in Romeregions, but it was never permanently glaciated. THE GEOLOGY OF THE BIR:\IINGHAM DISTRICT. 401 extensive sheets of gravel were formed along the "fringe," especially at the time of the final disappearance of the ice. The broken and fragmentary sea-shells which have been found in some sections never lived where they now occur, but were scraped off the bottom of the Irish Sea by the advancing ice. Of the earlier deposits formed by the old rivers, which appear to have crossed the Midlands from west to east during Tertiary times, only scanty traces remain. They were largely interfered with, and destroyed or covered up by the action of the glaciers. LOCAL ICE. During the Glacial Period several of the Midland hills-such as Charnwood Forest, the Rowley Hills, the Hartshill Range, etc. appear to have been permanently capped by ice. This ice enabled them to more or less successfully resist the invasion of foreign ice, which consequently glided round them. The result is that we find little or no foreign drift upon the summits of these hills, although each such elevation has a stream or tail of boulders, which can be traced proceeding from it to the southward. And the upper portion of the glacial ice-by which alone these hill-tops would in any case be traversed-would be comparatively" clean" ice, con taining but little debris. THE THREE GREAT GU.CIERS WHICH INVADED THE MIDLANDS. There is clear evidence of the entrance of "rivers of ice" (glaciers) into the Midlands from at least three external sources. These glaciers may be named: (i.) The Arenig Glacier. (ii.) The Irish Sea Glacier. (iii.) The North Sea Glacier, 1. THE ARENIG GLACIER.-The Arenig Hills of North Wales lie south-east of Snowdon, and attain a height of 2,817 ft. They are very favourably placed to arrest the south-west winds, and the precipitation of snow upon them during the Glacial Period must have been enormous. There is the clearest evidence that a glacier descended eastward from the Arenigs, passing down the Vale of Llangollen and debouching into the Shropshire plain near Ruabon, Overton, and Ellesmere. Great moraine heaps occur here. The Welsh ice then pursued a south-easterly course across Shropshire and Staffordshire, and terminated its career in the tract of country lying between Bromsgrove and Birmingham, some of its boulders (felspathic Arenig ashes and felsites) occurring on the top of Romsley Hill (one of the Clent Hills, in North Worcestershire) at the remarkable elevation of 897 ft. This Arenig Glacier presents several remarkable features, and 402 W. j, HARRISON ON THE appears to have retained the characteristics of a true" Alpine " type glacier throu ghout its course . It did not follow what might have been thought its normal course on emerging from the Vale of Llangollen, either along the present Dee Valley to the north , or the Severn Valley to the south .. The former was blocked by the Ir ish Sea ice ; while Welsh ice from Plinlimmon, the Malverns, etc., occupied much of the Severn Valley to the south. The Welsh ice had, as we know from many sections on the coast of North Wales, attained a conside rable extension before the arrival in its distri ct of the Irish Sea Glacier. It is thus probable that the Arenig Glacier was the first of the thre e great glaciers ment ioned above to enter the Midland s. But it is doubtful if it would have obtained tbe extension which it actually did (the distance from Bromsgrove to the Arenigs is abo ut 100 miles), or if its boulders would have attained the remarkable height on the Clent Hills of 897 ft. had it not been for the extraneous impelling force which its ice received from the ice of the Irish Sea Glacier, which must have united with the Welsh Arenig ice and forced it onward and forward. Welsh boulders (often accompanied by basalt boulders from the Rowley Hills) occur along a line about 15 or 20 miles in length, extending from the south-west of Bromsgrove to the north-east of Birmingham. But this indicates a comparatively small amount of "fanning-out" as compared with the length of this glacier. A fine Arenig bould er lies in Cannon-H ill Park, Birmingham, close to the spot from which it was dug out (Fig. 1 2 ). Dur ing the excursion to the Lickey Hills several Areni g bould ers will be noted near Rubery, and a fine group of similar boulders lies close by upon till capping Frankley Lower Hill. At this point is .the section which revolutionised the ideas of the late Prof. Carvill Lewis, when he visited it not long before his death. Th ere is also a classical section of the till, gravels, etc., of the Arenig Glacier exposed in the brickyards at California, a spot about four miles south-west of Birmingham. And the railway cutting at Moseley (a southern suburb of Birmingham) shows a magnifi cent section of false-bedded sands and gravels, 80 ft. in thickness, constituting an esker and proba bly mark ing an important point of discharge of the waters draining the glaciers which lay to the north-west (PI. XII, Fig. 2). The railway cuttings excavated in recent years at Soho, :K ing's Norton, etc., near Birmingham, have revealed extensive sections of boulder-clays containing Welsh rocks. But in no case has any trace--not even a fragment-of a shell been found. The reason evidently is, that the Aren ig Glacier did not cross allY portion oj a sea bottom during its course from Wales to Birmin gham. The Keuper Sandstone at Weoley Castle (near the California brick pits) showed striations running south-west; and the Bunter Sandstone ridge, adjoining Icknield Street; Birmingh am, bears GLACIAL GEO LOGY OF T H E BI RI\H NGH AI\! D ISTR ICT. 40 3 ~ ~ ~. ~ <, ~ es ~ ~'" '"<::' '"I1 ~.;: ?i {. .... ;:,. "" -e<>- I ..,. :J> ::: ::0 :;;. '"z ~ B ~ t::l 0 " c: ::t: r- ::l'" e ~ . 1"'1 ::0 ?' 2 l::l o <:;- :>- ;;. z ~ z ::. 0z <>- :r: "~ t=: ~ t""' '-> "1:l ~ > i:l:l r-'" ;;. '";;:. if ~ <, W. ]. HARRI SON ON THE along its course the signs of enormous ice pressure acting from the north-west, which has forced Arenig boulders into the sand stone, and torn off great slices from it.'*' The Rowley Hills (highest point about 890 ft.) of South Staffordshire lie directly in the path of the Arenig Glacier. I have found no foreign drift upon their summits, but a train of basalt bould ers can be traced from them to the south and east, extending to the Clent Hills, to Harborne, and to the north-western suburbs of Birmingham. Where the surface soil on the Rowley hi11s has been removed during quarrying opera tions, well-striated platforms of the basalt have been exposed, the striations running from N.W .

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