Chap25

[398]

CHAPTER XXV.

OLD BRITISH GLACIERS CONTINUED.

I SHALL now briefly describe some of the broader features of the glacial phenomena of the western coasts of England, with here and there necessary allusions to and descriptions of the interior of the country.

It is a self-evident proposition, that when cold began to increase sufficiently to produce glaciers in Britain, these in their infancy must have been first formed in the high regions of the north, where precipitation of snow was greatest among the mountains of the Highlands. As the climate got more severe, such glaciers would spread from the upland glens in all directions, and by-andby, as cold and precipation became more and more intense, and at last the whole mountain land, like the interior of Greenland, got smothered in ice, a prodigious on.flow of glacier ice spread from the Highlands west into the Atlantic across the Outer Hebrides, and south into the North or Irish Channel along the iceburied valleys of the Sound of Jura, Loch Fyne, and the whole of the Firth of Clyde, in the midst of which the island of Arran then formed, what it may seem presumptuous to call, only a great roche moutonnée; or if any of is peaks then stood above the surface of this mer de glace, they yielded but a feeble contribution of ice to swell the general mass of the glacier. Escaped from the Highlands, the glacier split upon the island of

[Ice-marks. 399]

Rathlin off the coast of Antrim,¹ and being largely reinforced by tributary ice that descended from the Galloway mountains and all the high lands, the slopes of which, then filled with tributary ice, now send rivers into the Solway, the advancing mass invaded the area called the Irish Sea, where, it was still further swelled by the glaciers that descended from the mountains of Cumberland.

These facts are further confirmed by observations in the Isle of Man by the Rev. J. G. Cumming, who shows that the chief glacial striations in that island trend from NNE. to SSW. as if the ice that made them, travelled from the high ground of Kirkcudbrightshire and the northern borders of the Solway Firth.

If we now go into the interior of the country what do we find? First, it is obvious to anyone with an eye educated in glacial phenomena, that the whole of the mountains of Cumbria and Westmoreland have been buried in ice during the period of extremest cold. Though now somewhat ruined by time, their mammillated forms proclaim it, and in the time that the glacierice attained its maximum, that ice, pressed on by ice coming from the north, must have passed southward into and far beyond Morecambe Bay. East of this mountain-land, between the rivers Kent and Lune, almost all the striations run about SSW. while a very few trend near southwesterly, while on all the high Fells on both sides of the Ribble, the prevailing direction of the stria is either south or a few degrees west of south, as shown by Mr. R. H. Tiddeman in his memoir 'On the Ice-Sheet in North Lancashire and adjacent parts of Yorkshire and Westmoreland. '² I am

¹ Vividly described by Mr. J. Geikie, 'Great Ice Age,' chap. xxiv.
² Jounal of the Geological Society,' vol. xviii., 1872, p. 471.

[400 Glaciers of North Wales.]

well acquainted with the country, and can vouch for the accuracy of his observations, and what makes them of special value in this inquiry is, that such striations range from a few feet up to 40, 100, 200, 300, 550, 775, 1,100, and 1,375 feet above the sea, and at many intermediate elevations.

One great fact which they teach is this, that the broad and thick ice-sheet, urged onward from the north, buried the whole of the region described, and all the ground to the east as far as the sea, and further, that the glacier moulding itself to the shape of the country (after the manner of all glaciers), was pressed right onward with so much force, that the long northern slopes of the east and west valleys offered comparatively no more impediment to its onward march, than an occasional transverse bar of rock hinders the onward flow of a river. Occasionally there are striations that do not quite conform to the rule, but in some cases I feel convinced that these were due to undercurrents in the ice in some of the deeper valleys, and at a later date to minor glaciers that got specialised in the valleys during the decline and disappearance of the ice-sheet.

At Liverpool, and on the opposite side of the Mersey, Mr. Morton observed on the Keuper Sandstone certain ice-grooves, trending S. 35° E.¹, and it seems to me that this direction is connected with the circumstance that when the northern ice-sheet reached the rising ground of Denbighshire and Flintshire, it was deflected to the right and left, and while one part flowed south-easterly across the plains and undulations of Cheshire, another part flowed southwesterly, and, scraping the coast hills of North Wales, overwhelmed Anglesea and the low ground of Lleyn that forms the north horn of Cardigan

¹ Reports of British Association, Liverpool,' 1870.

[North Wales. 401]

Bay. This I shall presently prove, for having brought our ice so far south, it is now time to explain the part played by the mountains of Wales in this glacial history.

When glaciers first began to form among the mountains of the Highlands of Scotland, from 200 to 300 miles north of Wales, though the heights of the latter region may have been far more snowy than they ever are now, yet at first it is probable that the snow did not continue through the year, and therefore no glaciers were formed. But in time, as the great glaciers advanced, and the cold increased and snow in North Wales became perennial, then glaciers began to be formed, first in the high valleys in the upper recesses of the mountains; and as the climate and precipitation of snow grew more severe, these glaciers must have waxed in size, till at length they filled all the valleys, and intruded on the plains and low undulating grounds beyond. How far south they extended from the mountains of Merionethshire I do not know, but probably the ice-flow went far into South Wales. Neither is it. possible to say how far these early glaciers of Snowdonia stretched across the broad undulations of Anglesea, for, if they did so, the marks that they made were afterwards entirely obliterated by the onward march of the great northern glacier which I have already described, and which I have no doubt extended southward into St. George's Channel. In aid of this statement I would quote the opinion of the Reverend M. Close, and the later observations of Professor Hull. The central plain of Ireland forms a great basin, surrounded by the broken mountains of the south from Kerry to Wicklow, and of the west and north-west from Galway to Donegal.

[402 Glaciation of Anglesea.]

Like Greenland of the present day, but on a smaller scale, the whole of this basin of more than 120 miles in diameter was deeply buried in snow and ice, which, in glacier fashion, found vents through the broad gaps in the circling mountains, westward into the Atlantic, and eastward in the direction of what is now the Irish Sea, to join the ice-sheet that worked its way south from Scotland and the north of England.¹

If all this be true, it is easy to see how any older ice-grooves in Anglesea mist have been obliterated, and in aid of this argument I will now give a more particular account of the glaciation of Anglesea, seeing that it has an important bearing on the whole question of the great northern glacier.

The structure of the low ground of Caernarvonshire, within three or four miles of the Menai Straits, in almost all respects resembles that of Anglesea, both in its geology and physical geography. The Menai Straits

¹ With Messrs. Hicks, Homfray, and Etheridge, near St. David's, on the coast of Pembrokeshire, I have seen well marked glacial striations on the rocks on the coast at Trwyn-hwrddyn, Porth-clais, and Pen-dal-deryn. In the first two they point NW. and SE., and in the last NE. and SW. Boulder-clay with ice-scratched stones is common, and chalk flints mingled with stones native to the district, are not uncommon on the mainland and in Ramsey Island. Flints are also found in quantity, mingled with ice-scratched erratics, all along the low ground of Glamorganshire north of Bristol Channel between Cardiff and Bridgend, and Boulder-clay is indeed common here and there all over South Wales. When the Geological Survey was in that district glacial phenomena had just begun to be heard of, and the 'drift,' as it was termed, was chiefly looked upon as a troublesome superficial deposit that concealed the boundary lines of the solid rocks beneath. Till some qualified person surveys the whole of the glacial phenomena of South Wales and the adjacent counties in a connected manner, it would be premature to connect the striations in Pembrokeshire with the northern glacier described in this book.

[Glaciation of Anglesea. 403]

divide the two regions; but Carboniferous rocks form the larger part of either shore, and the Straits may be considered simply as a long shallow valley, the bottom of which happens to lie beneath the level of the sea. The question thus arises—At what epoch and by what means was Anglesea separated from the mainland?

Looking north-west across the country from any of the minor heights a mile or two inland between Bangor and Caernarvon, no one would even suspect the existence of the Straits. The whole of Anglesea is low; and only one steep escarpment, a minor one, occurs in the island—that of the Old Red Sandstone overlooking Traeth Dulas, which rises abruptly above the tidal flat of the Traeth to the height of about 250 feet.

The entire island may, indeed, be looked on as a gently undulating plain, the higher parts of which attain an average elevation of from 200 to 300 feet above the level of the sea; while most of its principal brooks and small rivers run north-east and south-west, in depressions with gently sloping sides; and only one inland valley, with the same trend, is of any markd importance, namely that of Malldraeth Marsh, in which a small coalfield lies. There are, however, a few exceptions to the average levels mentioned above the summit of Holyhead mountain being 709 feet, and Garn, near Llanfairynghornwy, 558 feet above the sea, while the greatest elevation crossed by the sections of the Geological Survey (sheet No. 40) is only about 400 feet high.

On the opposite side of the Straits, the same kind of low, undulating scenery prevails for several miles inland, with the same kind of minor north-east valleys, one marked instance of which occurs in a long, shallow, and

[404 Glaciation of Anglesea.]

narrow valley, in, or alongside of which, the Caernarvon and Bangor road runs for several miles.

The surface of the ground on both sides of the Straits is to a considerable extent composed of glacial detritus, with erratic boulders large and small (from the north), gravel, sometimes sand, and clay, from which any number of ice-scratched stones may be gathered from wellexposed sections, as, for example, in the Boulder-clay coast-cliff of the Mount at Beaumaris, or anywhere else in similar cliffs round the shores of Anglesea, or, inland, in occasional pits and fresh cuttings on both sides of the Straits. Through these glacial accumulations the rocks of the country frequently appear, sometimes in barren tracts of considerable extent, sometimes in small isolated bosses of gneiss or grit, often covered with heath or furze, while the more fertile grounds of the whole of Anglesea consist chiefly of glacial detritus, with here and there small alluvial meadows by the sides of the streams.

When freshly stripped of glacial débris, or even of a mere thin turfy soil, the underlying rocks are often found to be ice-smoothed and marked with glacial striæ, running generally from about 30° to 40° west of south. The larger valleys of Malldraeth Marsh and the Menai Straits (with others of minor note) run in hollows in the same general direction.

I have already shown that in mountain regions where glaciers exist, or have in past times existed, the disturbances of the earth's crust that produced the elevation of the mountains go back to periods long antecedent to the last great Glacial epoch. Thus the first great upheaval of the Alps is of pre-Miocene age, and the last, as far as the Alps is concerned, closed the Miocene epoch, while the mountains of Scotland and

[Glaciers of North Wales. 405]

Cumberland were mountains at least before Old Red Sandstone times, and the last great movement of the rocks of Wales is certainly older than the Permian epoch, and, probably, like the mountains of Oiimberland, very much older.

There was therefore plenty of time, in what is now Wales, long before the beginning of the great Glacial episode, for the more ordinary agents of denudation to have formed deep valleys, down which, when that episode began, the growing glaciers might gravitate, deepening their channels as they pressed forward, and mammillating and striating the rocks over which they slid; for the great original valleys of the mountains were by no means entirely scooped out, but merely modified by the glaciers.

Thus, for example, it happens in Wales that all the striations in the valley of Dolgelly and the estuary of the Mawddach, in Merionethshire, follow the southwesterly trend of the valley, the glacier that filled it when at its greatest being fed by the snows of the slopes of Cader Idris and Aran Mowddwy, and those of the tributary valleys of Afon Eden and the Mawddach that joined it from the north; while from a central low watershed, near the sources of the Wnion, another branch pressed north-easterly, into and far beyond the region now occupied by Bala lake.

The striated rocks exposed among the sands at low tide in the estuary of the Mawddach, and the islet-like heathy bosses of rock that stand out amid the marshy moss opposite Barmouth, are merely roches moutonnées, once buried deep beneath the glacier that pressed forward to join the great northern glacier that then filled Cardigan Bay.

In like manner all the western valleys of the

[406 Glaciers of North Wales.]

Cambrian mountains of Merionethshire, such as those of Afon Atro, Ardudwy, and Afon Ysgethin, are marked by deep grooves and striations pointing more or less westward, according to the trend of the valleys.

In ascending the valley from Llanbedr on the coast south of Harlech to Llyn-cwm-bychan, the experienced eye is at once attracted by the long smooth sweeps of the ice-ground rocks of Mynydd Llanbedr, all trending towards the west, and from the summit of Graig-ddrwg, looking south towards Rhinog-fawr, the same effects of old glacier-ice are seen on a still grander scale. The deep craggy pass of Bwlch-drws-Ardudwy is itself strongly ice-grooved, while the western flanks of Graig-ddrwg are covered with deeply incised striations, up to the very summit of the mountain, all trending westward. The rock-bound hollows of Llyn-cwm-bychan, and other mountain turns, tell in like manner of the effects of thick masses of glacier-ice, as I shall afterwards explain.

The broad flat moors and roughly hilly, but not mountainous country of Cors-goch, Afon Eden, Trawsfynydd, and indeed all the lower ground bounded by the splendid amphitheatre of scarped mountains formed by the Arenigs, the Manods, the Moelwyns, and the Cambrian steeps of Diphwys, Graig-ddrwg, Rhinog-fawr, and Cefn-cam, were at the same time filled to the brim with deep accumulations of snow and ice, from which were discharged radiating currents of glaciers, one pressing southward to swell the ice-stream that filled the valley of what is now the estuary of the Mawddach, another through the Pass of Afon Treweryn between Arenig Mawr and Arenig Bach eastward towards Bala and the valley of the Dee, there to be aided in the work of erosion by the glaciers that descended from

[Glaciers of North Wales. 407]

either flank of Aran Mowddwy. Further west, swelled by all the snows of the Manods and the Moelwyns, a great ice-stream flowed south-west, into what is now the broad flat of Traeth Bach, there to be joined by another tributary which, partly descending Cwm Llydaw and Cwm-llan from the high eastern slopes of Snowdon, filled Nant Gwynant, and debouched into the area now occupied by the marshy flat of Traeth Mawr. In all of these the directions of the striations necessarily conform to the trend of the valleys—easterly, southerly, or south-west, as the case may be. And this must have been the case even though it happened that the mountain valleys and broader amphitheatres were filled to the very brim, and overflowing with ice and snow in such a manner that, had there been human eyes to look on the scene, it would have been impossible to have specialised each individual glacier. In such a case, however, there were many deviations consequent on under and upper ice-currents, the upper parts of glaciers diverging from the direction of the under-flow, and passing across what are now low watersheds, like that of Llyn Cawlyd, which lies between the valley of the Llugwy, and that of the Conwy-a circumstance to which special attention has been called by the Rev. W. T. Kingsley.

On the north-west. slopes of the Snowdonian range,¹ great glaciers poured their ice-streams down. the valleys of Llyniau Nant-y-llef to the west, and of Llyn Cwellyn, Lianberis, and Nant-ffrancon, the last deriving additional power by aid of the tributary ice-flows of Cwm-llafar and Afon-gaseg, the chief gathering grounds

¹ use the word range as a convenient term. There is no range of mountains in North Wales. Taken collectively they form a group.

[408 Glaciation of Anglesea.]

of which, were the cliffy cirques on the western flanks of Carnedd Llewelyn and Carnedd Dafydd, which, with Y-Foel-frâs, formed one great nursery of the glaciers of Caernarvonshire, sending off ice-flows eastward to Capel Curig and the valley of the Conwy, and westward to where Bangor now stands and the Lavan sands.

None of these glaciers, at a certain epoch, quite reached the region now occupied by the Menai Straits, but escaping from the higher bounding-walls of their valleys, they spread out in the shape of broad fans on the north-western slopes of the minor hills that now overlook the Straits. This is partly proved by the northerly curve of the glacial striations at the mouth of the Pass of Llanberis, on the flatter area above the steep slopes of the slate-quarries by Llyn Peris and Llyn Padarn.

If, as I believe, these glacier masses did not cross the Straits into Anglesea, we must look for some other cause for the production of the north-east and southwest striations which mark the whole of that broad region.

These striations point directly towards the mountains of Cumberland, a country which, lying further north, was at one, time buried so deeply under snow and ice, that almost all its mountains look simply like gigantic roches moutonnées. From Cumberland, as already stated, a vast mass of ice flowed southward; and reinforced by the ice-streams that came from the mountains of Carrick in the south of Scotland, and from the basin of the Clyde, it overspread the region now occupied by the shallow sea of Morecambe, Lancaster, and Liverpool bays, that lie between Cumberlaud and Anglesea, nowhere more than 30 fathoms deep.

In its onward course, this mighty glacier buried all the hills and rounded knolls of Great Ormes Head, Little Ormes Head, and Diganwy, which are still on a large

[Glatiation of Anglesea. 409]

scale so strikingly moutonnée, and pressing along the slopes of Lianfair-fechan, the lower end of Aber Valley, the seaward flank of Moel Wnion, and across the lower end of the valley of the Ogwen, it marked its track by long, slightly-inclined terraces, somewhat faintly marked, but still clear to the experienced eye when looked for from the shores of Beaumaris. Beyond this the glacier continued its course across Lleyn, and onward to the region now occupied by St. George's Channel.

Furthermore, in my opinion, so great was the size and power of this ice-flow, that it hindered the glaciers of Y-Foel-frâs, Llanberis, and Nant-ffrancon from encroaching on the territory of Anglesea, and they simply joined the larger glacier as minor tributary icestreams. For this reason it happens that the glacial striations of Anglesea, as we might at first expect, do not point towards the old glacier-valleys of Snowdonia that open on the Straits, but run at right angles to the courses of these comparatively minor glaciers.

If we now turn to the rocks that form the banks of Menai Straits, we find that they chiefly consist of nearly flat-lying Carboniferous strata, and looking at the disposition of these beds from Traeth Melyn, opposite Caernarvon, to Llanfair-pwll-gwyngyll, in Anglesea, and on the opposite shore from Caernarvon to Bangor, there is no reason to doubt that from end to end they once filled the whole of the region now occupied by the Straits. The larger part of this region, as it now exists, is of Carboniferous Limestone age; but it by no means consists entirely of solid limestone. On the contrary, numerous bands of shale and friable sandstones and conglomerates are intermingled with the limestones, together with beds of soft red marl. On the coast opposite Caernarvon, the low cliffs are entirely formed of red marl overlying the limestone; and on the

[410 Glaciation of Menai Straits.]

Caernarvonshire eoast, for three miles north of the town, also overlying the limestone, there are soft shales of the Coal-measures, sometimes red and many, and containing thin seams of coal.

In Anglesea, from three to four miles north-west of the Straits, lies the valley of Malldraeth Marsh, the rocks of which also consist of Carboniferous Limestone, Millstone grit, soft Coal-measure shales, with a little sandstone, beds of coal, and Permian strata; and this valley, nine miles in length, runs almost exactly parallel to the valley of the Menai Straits. Many years ago, at its north-eastern end, I saw deep glacial striations on the Millstone Grit, running straight down the shallow valley towards Caernarvon Bay.

Considering that the south-westerly trend of each of these valleys and of others of minor note, corresponds with the general direction of the glacial striations of Anglesea, and therefore with the onward course of the great glacier that produced them, I have been led to the conclusion that both of the shallow valleys were scooped out in comparatively soft rocks, by the grinding power of the vast glacier coming from the north-east, and that when in the course of time the climate ameliorated, and the glacier disappeared, the sea flowed in where part of the glacier had been, and thus it was that Anglesa got separated from the mainland and first became an island. The islets in the narrower and shallower part of the Straits at the Menai and tubular bridges are merely weathered roches moutonnées, once overridden by the moving glacier, and Menai Strait is merely a long and broad glacial groove, which was first laid bare by the partial removal of the boulder-beds, after the close of the Glacial epoch.