The Geology of Snowdonia - A Collection of Historical Articles on the Physical Features of the Peaks of Snowdonia

By Read Books Ltd.

Snowdonia is one of the most fascinating areas in the United Kingdom for its stunning peaks, stark beauty and ancient rock. Here are collected some of the articles that helped us understand how and when these breathtaking landscapes were created and are a

• Language: English
• Publisher: Read Books Ltd.
• Release date: May 31, 2013
• ISBN: 9781473390430

Book preview

North

THE AGE OF THE MOUNTAINS OF SNOWDONIA

BY EDWARD GREENLY, D.SC. F.G.S.

Read November 3rd, 1937

The Development of Opinion on the Problem.—Half a century ago, it seems to have been widely held that the mountains of North Wales were, as mountains, very ancient; much older, it was remarked, than some which are loftier and more famous. Yet by 1867–1870 it had come to be generally recognized that the mountains of the Inner Hebrides (which are very nearly as high) are, as mountains, of Tertiary age (Geikie 1897, pp. 114–5). Why did not this arouse speculation with regard to the mountains of Wales?

Some ten years later it was urged by Goodchild (1880; repeated, more fully, in 1888 and 1889) that the mountains of Lakeland had been sculptured out of a high plateau which was a sub-Cretaceous floor, so that, as mountains, they must be of Tertiary age.

About the same time, Ramsay (1881, p. 319) expressed an opinion which approached Goodchild’s somewhat nearly: During the deposition of the chalk, there is no proof that the higher parts of Wales were altogether submerged, but rather the opposite. He does not (so far as I know) specify the grounds for this opposite, and seems indeed not to have thoroughgoing confidence in it, for he says on p. 320: It thus appears that since the beginning of the Permian epoch, the higher ground of Wales has formed land well raised above the level of the sea, and even if there be some doubt about this while the chalk was being deposited, that must have formed merely an episode when compared with the whole of the terrestrial period indicated above.

Finally, Professor Fearnsides (1910, p. 819) suggests that the whole region may have been covered by the Chalk. But that paper being a summary, highly condensed, it afforded no space for the setting forth of evidence.

Thus, the moulding of British mountains: first those of the Scottish Western Isles, then of Lakeland, then of Wales, was gradually coming to be regarded as a work of Tertiary time.

In the memoir on Anglesey (Greenly 1919) I adduced evidence that the sea-floor to the east must have the structure of a gentle synclinal fold, taking in Mesozoic rocks, with an outlier of Chalk. Such a syncline must be of Tertiary age. Anglesey, accordingly, must be the core (greatly cut down by the planing of the platforms) of a complementary anticline. This, however, I ventured to suggest, was subordinate to a much greater anticline.

The mountains of Snowdonia are visibly sculptured out of a lofty platform which, seen from the north-west, assumes the aspect of an exceedingly flattened dome. Was not this the core of the major anticline? If so, it also must be of Tertiary age, and the erosion of its valleys (in other words, the moulding of its mountains) must be a work of Tertiary time. Far more cogent evidence is now available.

The Dykes.—In Anglesey there are Tertiary dykes, most of which are olivine-dolerites of a type readily recognized. Several are now known in Arvon. But all of these outcrop at somewhat low levels, the highest being the Henborth dyke at Holyhead Mountain, about 220 feet, and the dyke of Nant-y-garth in Arvon, 330 feet. They tell us that the Menaian and other platforms are late Tertiary, but tell us nothing about the mountains.

Some years ago, however, Dr. David Williams (1924; 1930, pp. 224–6) discovered a group of five dykes (Fig. 1), of precisely similar nature and with the same trend, high up in the mountain-land, at Marchlyn and Bwlch-cywion, which attain heights of 2400 and even 2500 feet.¹ Recently, while remapping the Penrhyn quarry, he has found (among a plexus of Palæozoic dykes) yet another of the same character, at a height of 1200 feet.

It becomes obvious at once that the yawning valleys adjacent cannot possibly have existed at the time of these intrusions, and that the excavation of these valleys (or, put conversely, the moulding of the mountains) was a work of Tertiary time.

A Snowdonian leaf-bed would have been welcome, but these dykes are hardly less convincing.

Certainly, the valleys are deep. But it may be more than a coincidence that the valleys of the Scottish Western Isles, excavated in rocks not a whit less obdurate, are of just about the same depth.

Their Period.—To what period of Tertiary time are these dykes to be assigned? In any case, to some pre-Pliocene period, for the Menaian and other platforms are assignable to the Pliocene. The age of the great Tertiary volcanic episode does not appear to be known with precision. It seems (Bailey and others 1924, pp. 55–89) to have begun in the Eocene period; though Geikie (1897, p. 462) was inclined to think that it might have lasted beyond that period. The volcanic pile, indeed, is of great thickness. But in view of the fact (Williams and Bulman 1931) that the Ordovician volcanic series of Snowdon, which is of comparable thickness, was confined to the duration of a single graptolitic zone, the Eocene period may well have been long enough.

Their Cover.—These dykes must of course have had a cover. What was its depth? The Marchlyn dyke in the corrie, at about 1700 feet, is 40 feet in width, but on the ridge of Bwlch-y-marchlyn, at 2400 feet, it has narrowed to less than six inches, and has become compact, so that it seems on the point of thinning out, requiring no great depth of cover. The Bwlch-cywion dyke, however, at 2500 feet is about 25 feet wide and is an olivine-dolerite. But these dykes emerge almost on the crest of Snowdonia, for Carnedd Dafydd and Carnedd Llewelyn, only just across the Ogwen, rise to 3426 and 3484 feet, less than 100 feet short of the height of Snowdon. They have doubtless suffered some degradation, so the local level of the platform can hardly be less than 3500 feet. That gives 1000 feet of cover, which, in view of the Marchlyn case, would appear to be sufficient.

The Significance of the Dome.—Let us now go out into Anglesey some seven or eight miles, gaze at the mountain-land, and note the form of its dome-like outline, which subtends to the horizon an angle of hardly more than 1°. Peak after peak just rises to that line, touches it but does no more. As for the valleys, they are mere local interruptions; the line sweeps right across them. In fact, this aerial curve has a singular evenness.

FIG. 1.—Sketch-map of the Marchlyn (M) and Bwlch-cywion (B) dykes. F = Foelgoch. Scale: 3 inches to 1 mile.

Now, did it date from the Permian or some other long pre-Tertiary period,¹ would such evenness have been possible? Would it not have been far too irregular to have looked like a dome at all? That is, if it could have so much as survived, which, in view of the known rates at which land is lowered by waste, seems quite incredible.

Can it then be very much older than the valleys which have been cut in it? These we have found to be Tertiary. Is it not then ascribable to the last great base-levelling which preceded Tertiary time; which was the work of the sea of the Chalk. Of this there is a confirmation: chalk-with-flints lies on the sea-floor 20–25 miles away, indicating a northward slope nearly as gentle as that of the visible dome.¹ It is true that even now there is not a demonstration, but in matters of this kind demonstration is rarely attainable; we almost always have to be contented with a good measure of probability. If that be admitted, we seem to be justified in regarding that 50-mile aerial curve as a sub-Cretaceous base, lifted in the course of the Tertiary period into a gentle anticline.

FIG. 2—Sketch-map of the Holland Arms (H) and Marchlyn (M) groups of dykes, showing the single line of intrusion along which lie all the dykes. P = Plasnewydd; V = Vaynol; N = Nant-y-garth; B = Bwlch-cywion; Q = Penrhyn quarry. Scale: 1 inch to 4 miles.

Dates of Intrusion and Elevation.—Had the lofty platform attained its present height at the time of these intrusions? There are two points to consider.

First, the Anglesey anticline we have regarded as a lobe, which could never have been at anything like the height of the other.

The second point is this. The dykes of Holland Arms, Pont-y-crug, and Plas-newydd (see the Anglesey one-inch map) are manifestly on a single line of intrusion. This has recently been found to be continued across the Strait by dykes from Vaynol to Nant-y-garth, while the dykes from Bwlch-cywion to Marchlyn are likewise on a single line of their own. The important relation is that the Bwlch-cywion to Marchlyn line points directly towards the line from Nant-y-garth to Holland Arms (Fig. 2). It is manifest that the whole series is on one and the same line of fissures, a line 12 miles in length. The dykes are a volcanic unit.

Consequently, had Snowdonia stood at its present height at the time of the injection of this unit, with its magma driven up to more than 2500 feet, how could the dykes of Arvon and Anglesey have failed to flow out to the surface as lava? We may even venture to suggest that the pre-Tertiary floor (which seems to have been the Cretaceous base) may then have been little, if any, higher over the site of Snowdonia than it was over the site of Anglesey.

In any case, the rise of the anticline of Snowdonia must have been subsequent to the intrusion of these dykes. If they be Eocene, then the anticlinal movement would, at the earliest, have been at the close of the Eocene. Probably it was a good deal later, for it seems reasonable to connect it with the other Tertiary movements of Britain, in which, on the southern coast, Oligocene beds have been involved.

Date of the Valleys and the Mountains.—If this be so, then the age of the valleys can be determined somewhat more precisely. For since their erosion could not begin until the anticline was rising, that must have been accomplished in the course of the Miocene period (and perhaps the close of the Oligocene). Accordingly, we must assign the development of the mountains of Snowdonia to the Miocene period.

There is a small allowance to make. With the elevation of the Pliocene platforms, the mountain-land has been raised about 550 feet. Its valleys have been a little deepened, and some of its rivers rejuvenated. The 50-foot beach elevation and the Forest-bed depression nearly neutralize each other. So, unless there were some movement whereof we have no record, the net effect is that Miocene. Snowdonia was a trifle less lofty than the Snowdonia which we see.

LIST OF WORKS TO WHICH REFERENCE IS MADE

BAILEY, E. B., & others. 1924. Tertiary and post-Tertiary Geology of Mull, Loch Aline, and Oban. Mem. Geol. Surv. Scotland.

FEARNSIDES, W. G. 1910. North and Central Wales. Geology in the Field (Jubilee Vol. Geol. Assoc.), p. 786.

GEIKIE, A. 1897. The Ancient Volcanoes of Great Britain. II. London.

GOODCHILD, J. G. 1880. An outline of the History of the River Eden. [Reprinted from newspaper reports of an address given to the Carlisle Society. Recast and reprinted in 1889 under the title given below.]

——. 1888. The Physical History of Greystoke Park and the Valley of the Petteril. Trans. Cumberland & Westmorland Assoc. XIII (1887–8), p. 89.

——. 1889. The History of the Eden and some Rivers adjacent. Trans. Cumberland & Westmorland Assoc. XIV (1888–9), p. 73.

——. 1891. An Outline of the Geological History of the Eden Valley or Edenside. Proc. Geol. Assoc. xi, p. 258.

GREENLY, E. 1919. The Geology of Anglesey. Mem. Geol. Surv. pp. 688–9, 777–8, 895–6.

RAMSAY, A. C. 1881. The Geology of North Wales. 2nd edition. Mem. Geol. Surv. Gr. Br.

WILLIAMS, D. 1924. On two Olivine-dolerite Dykes in Snowdonia. Proc. Liverpool Geol. Soc. xiv, p. 38.

——. 1930. The Geology of the Country between Nant Peris and Nant Ffrancon (Snowdonia). Q.J.G.S. lxxxvi, p. 191.

WILLIAMS, H., & O. M. B. BULMAN. 1931. The Geology of the Dolwyddelan Syncline (North Wales). Q.J.G.S. lxxxvii, p. 425.

DISCUSSION

The PRESIDENT (Professor O. T. JONES) said that it was instructive to hear Dr. Greenly’s view regarding the age of the Snowdonian mountains and to learn of the new evidence of the extension of dykes of Tertiary type into the mountain area. Mr. Lake many years ago regarded the Welsh drainage as radiating from a dome in North Wales, and this would agree with the author’s views. There was some difficulty in the way of accepting their conclusions freely: if the dykes of North Wales were to be attributed to the same series as those of the west of Scotland, then it would appear that the valleys of North Wales had been excavated at a later period. This would appear to demand that they were eroded since late Oligocene or even Miocene times. The mountains of North Wales stood out, however, conspicuously above the high plateau of central Wales, which also extended into South Wales. The speaker had given some reasons for concluding that the Miocene movements of the south of England were continued into South Wales and had caused modification of a drainage system crossing the high plateau. This would suggest that two major movements had occurred, the first of which gave rise to the radial drainage of the high plateau and the second of which caused modifications of that pattern. If the latter were accepted as of Miocene age, then the previous movement would have to be assigned to a considerably earlier period, possibly late Cretaceous or Eocene.

Professor W. G. FEARNSIDES said that his inspired guess of 1910 that the high Snowdonian hill-tops were relics of a stripped pre-Cretaceous peneplain was in fact a re-statement of the views of Ramsay modified to accord with the physiographic evidence assembled by Mr. Lake, and he saw no reason to abandon the position. Carboniferous Limestone wrapped round Lower Palæozoic hills in Denbighshire, Coal Measure red rocks overstepped older rocks against a rough topography, even on to Pre-Cambrian rocks, in Shropshire; and neither these nor the base of the Trias of the Dee–Severn area or the Vale of Clwyd had the appearance of having transgressed regularly on to the even peneplain which truncated the slate hill-tops around Snowdonia, and out of which in rounded monadnocks the highest Ordovician volcanic rock outcrops rose. Towards the Irish Sea, Welsh mountain streams had bitten back into the pre-Cretaceous peneplain by stages and were graded to successive platforms, each also independent of the structures (a) below 1000 feet, (b) below 400 feet, and (c) below sea-level; and as of these the middle one agreed with the Pliocene platform of Cornwall, the upper one might well be Miocene or Eocene. He accepted the early Tertiary age of the Menai dyke swarm and its continuation in the dolerites with olivine high up on the hills which overlooked the Carnarvon slate quarries. He thought that the gingerbread rocks found in NW—SE. trending gullies high up on Y Foel Ddrug, east of Trawsfynnydd, should also be accepted as of Tertiary age.

Professor W. W. WATTS said that, in his opinion, none of the evidence put forward by the author with regard to the Tertiary dykes precluded the possibility that much of the Snowdonian land sculpture had been effected before the area had been covered by Secondary rocks. There were many cases of landscapes covered with Triassic, Carboniferous, and even Silurian sediments, and Snowdonia, so far as the evidence went, might belong to any one of these.

Professor H. L. HAWKINS, while welcoming the presentation of a paper of a speculative character, was inclined to doubt the necessity and probability of a Cretaceous age for the reputed peneplain of Snowdonia. He thought that the stratigraphical record showed frequent indications of an upstanding region there from Silurian times onwards, and suggested that the present topography of the area represented a newly re-exposed Triassic surface. Farther to the east, the region of Siluria was of a similar nature yet more recently laid bare, while Charnwood Forest showed an early stage in the same process. If the Snowdonian valleys were filled to overflowing with New Red Sandstone at the time of intrusion of the early Tertiary dykes, the problem of their failure to extrude would be solved. He inquired whether the exceedingly gentle inclination of the peneplain was sufficient to bring it below the assumed Cretaceous outlier in Liverpool Bay without severe faulting.

Dr. L. HAWKES remarked that the formation, elevation, and deep dissection of the extensive erosion platforms in the lower Tertiary igneous rocks of Iceland and the Færoes was a geological work equal in magnitude to the preparation and dissection of the platform of Snowdonia. Considering only the period adequate to produce the effects, the topography of Snowdonia could well have been developed since early Tertiary times.

The AUTHOR said that the dykes might be of early Tertiary age, since the vulcanism of the Scottish Western Isles was assigned, provisionally, to the Eocene. It was suggested that the Snowdonian base-level might be of Triassic age. In a measure, it might, for these lofty platforms might have been base-levelled more than once. But its evenness pointed to the last base-levelling having been of Cretaceous age. An ingenious method had been suggested by which the dykes might reach such different levels, and yet all remain dykes. But the interval being only five miles, that would appear to be very unlikely.

[Note added January 25th, 1938.]—The Author regrets to find, on reading the Abstracts of Proceedings of the meeting, that he