[From Isle of Man, Cumming 1848]
The Sheading of Rushen.-General view of the southern area of the island.-The Eve of the Calf.-Spanish Head.-Port Erin.-Port St. Mary.-Ballasalla.-Rushen Abbey.-Castle Rushen.-King William's College.-Langness.-The great natural features of this sheading.-Ellipsoidal hills of the boulder formation.-Great driftgravel platform.-Valleys of denudation-Estuary deposits - Notice of the agriculture of the island.-Hints on drainage.
A HUNDRED yards south of the junction of the old and new road to Castletown and the road leading up to Harrisdale Farm, we reach the top of the last hill on the road from Douglas to the metropolis of the Isle of Man. Six hundred yards further brings us to a cottage on the righthand side of the road, a most desirable spot for taking a bird's-eye view of the structure of this southern area, and we may very well at this point enter upon the geology of the Sheading of Rushen. The stream we have just crossed (the Santon-burn) forms its north-eastern boundary from the sea inland to the very source of the stream at the Foxdale Mines; thence to the north and north-west it is bounded by a curved line running over South Barrule (a little to the westward of its summit), and thence over Irey na-Lhaa into the sea. It thus takes in the loftiest mountains of the southern division of the island, the great masses of schist in all the varieties here existing, the remarkable granitic boss near Foxdale, the old red sandstone fringing and underlying the basin of the carboniferous deposits, the limestone itself (the older beds of Ronaldsway and Port-St.-Mary, and the newer series of Poolvash), the traptuff of Scarlet Head, and the trap dykes and greenstone dykes which in different directions cut up the area; it includes the rounded hills of the boulder formation, the terraces of drift-gravel, and the still newer and most interesting series of marine and freshwater alluvia which occupy the southern valleys. In fact we have the entire geology of the Isle of Man itself brought within a reasonable compass in this single sheading. And here let us see what it is made of.
We are looking down upon an elliptical area of somewhat irregular outline, the extremities of whose major axis we may consider to be Coshnahawin Head in the northeast, and Port St. Mary in the south-west. The outline of the area to the north-westward of this axis is formed by the mountain chain, which from its general direction from N.E. by N.to S.W. by S. curves round towards the south as it approaches the Calf of Man. We cannot see the Calf of Man from this point, but we catch a glimpse of the Eye of the Calf, which appears as if united to the main island. Spanish Head presents a bold front to the sea, and was till within the present year an overhanging precipice of more than 300 feet ; it is surmounted by the Mull Hills. A gap (in which lies Port Erin) intervenes between these hills and Brada Ilead, which has a front to the west not less imposing than Spanish Head. Port Erin itself is hidden by one of the nearer eminences, over which peeps Brada Head to the south-westward. A second gap to the northward (in which lies Fleshwick Bay) separates Brada Head from the great mountain chain which terminates in Irey-na-Lhaa. Port St. Mary is conspicuous with its whitewashed houses and the smoke of its limekilns to the northwestward of Spanish Head in a snug corner of Poolvash Bay. It is the most southern hamlet on the island, situated in the parish of Kirk Christ's Rushen, whose village church, if it had a very tall spire, we might perhaps distinguish over the hill in a line between us and Port Erin; and the same line would pass nearly over Arbory Church, and rest upon the venerable abbey of Rushen, the ruins of which lie down in the valley here about half a mile from us amidst that mass of foliage; we can just see two of its gray and ivied towers peeping out from amongst the elms which surround it. It is a spot full of solemn associations, the resting-place of kings, bishops, abbots and holy men.
O ! quis verendorum admonitus sacros
This at our feet is the village of Ballasalla, the largest in the island, and even within the last fifty years of sufficient importance to have a Deemster's Court held in it. And there two miles beyond on the western edge of its bay is Castletown itself, clustering round the ancient pile of Castle Rushen, which the Danish Guttred 1 erected nearly nine hundred years ago,-a noble specimen of fortification, scarcely inferior to that of Elsinore, which it greatly resembles. Wisely did that Scandinavian hero choose the material of his castle from the crystalline limestone of this immediate locality. After more than eight centuries and a half of war with the elements, after the sieges 2 with which by man at various times it has been beleaguered, it looks as fresh and entire as its neighbour, King William's College, raised from the same quarry not the seventh of a century ago. To be sure the latter has undergone the ordeal of fire, which has left somewhat of a more sombre and gray tinge of time upon its tower than would otherwise have been the case, yet even before that calamity appearances were not greatly in favour of the junior building. The reason in fact why both have suffered so little from the severe tests through which they have gone, is that the stone has been (so to speak) annealed in the quarry in ages long gone by, the great mass of molten trap with which the limestone is in contact at Scarlet-point having in cooling imparted to the latter rock (of which the College and Castle are built) a crystalline and enduring nature. It is strange that with such a specimen as Rushen Castle before them, the architects of Douglas Pier and Castle Mona should have gone across the water to England and to Scotland for a much worse material, a material too which does not harmonize so well with the character of the scenery with which Douglas is surrounded.
But to return to the landscape now within the field of view. Malew Church peeps forth in a gap between the Creggins and Ballahot, and St. Mary's Church in Castletown is seen to the south-eastward between the castle and the bay. To the north of the town on the rising ground stands Lorn House, the present residence of the Lieut. Governor. The old and new pier, the latter unfinished, are hid from view by the intervening buildings. Altogether the view of Castletown from this northern point is imposing, and gives the idea of greater extent than is the reality. The foreground too is good and sets it off to advantage; it appears to stand almost at the opening out of a finely cultivated valley into a noble bay, penetrating three miles inland, and being two wide at its mouth ; the northern shore (the head of the bay) is relieved of the tameness it would otherwise have by the College, which forms, with its central tower rising more than 100 feet from the ground, a conspicuous object of a cathedral character. The basaltic pile of Scarlet Stack at the western side, and the peninsula of Langness on the eastern side of Castletown Bay, with its round tower near the southern extremity 3, will next catch the eye; and then to the north of the narrow neck of land which joins Langness to the main island, lies the quiet fishing hamlet of Derbyhaven, with. its retired and lovely bay. Ronaldsway, an ancient mansion and battle-field, and a favourite resort of the geologist, to the north of Derbyhaven, is hardly hid by the trees and rising ground of Ballahick, a property which lies down here to the left-hand on the opposite side of the valley from Ballasalla.
But the great natural features of this southern area have as yet been hardly dwelt upon.
The immediate neighbourhood is a low undulating country, presenting a series of rounded hills 4 rising from 100 to 200 feet, whose seaward front is a curved line from Coshnahawin Head towards Kirk Arbory. The most prominent are the Brough and Ballahick Hill on our left, Ballahot on our right, westward of Rushen Abbey, and directly in front and trending westward, the Creggins Hills, Skybright (near Malew church) and Ballown. If we examine closely into the general direction of the axes of these ellipsoidal hills (the osar of Swedish naturalists), and of the short valleys or depressions between them, we shall see that it is parallel, or nearly parallel to the direction of the great mountain chain. It is also coincident, or very nearly so, with the direction of the scratches and groovings on the surface of the subjacent rocks whenever they can be discovered. The composition of these hills is the gravel, sand, and clay of the boulder or pleistocene formation ; the points where it is best seen being in the banks of the Silverburn, below Ballasalla, near the Creggins, and again to the westward of Skybright. The lower portion will be found the more loamy, and of the colour of the subjacent rock, of which also it generally contains not very rounded but much-scratched fragments. The middle portion is generally sandy, with small rounded pebbles and boulders chiefly of foreign rocks; the upper portion is still more gravelly, but it is on the surface generally, and also on the higher ground that we fall in with the largest boulders, whether single or in patches.
Direct attention again to the further-off landscape between these rounded hills and the sea as far as the eye can reach. It may be described as one great terrace of gravel rising gradually inland from twenty-five to sixty feet above the sea-level, with an indistinct appearance of a series of lower terraces or raised sea-beaches down to the present sea-level. But the whole area of these terraces is cut up by a succession of valleys of denudation, which have evidently been occupied at one time by the sea as estuaries 5, but which are now receiving constantly the alluvial deposits which the streams from the mountains bring down and spread forth in them. If we examine the outline of the coast and the materials of which it is composed, we shall discover certain causes which have operated in producing the present physical appearance of the lowlands. We shall see that Langness Point, Scarlet Head and I3alladoole Hill, present to the south masses of tough, igneous and crystalline rocks which very powerfully resist denuding action, and which have acted as breakwaters against that action coming up the Irish Channel. We shall see also that the driftgravel lies along and is preserved over the ridges of the undulations which have been impressed upon the subjacent limestones. Indeed so much so is this the case, that we might easily fall into the error that the limestone has been thrown into these wavy ridges since the deposition of the boulder clay series and drift gravel; and some of niy earlier sections across this basin favoured that supposition. A closer examination, and the finding rolled fragments of the trap, whose eruption caused the undulations of this area, in the boulder formation, showed such an hypothesis to be incorrect 6.
When the sea flowed over these low barriers of the hard palaeozoic rocks before the land had risen to its present elevation, it would wash the base of the rounded hills of the boulder series which have been spoken of as lying on a curved line from Coshnahawin to Arbory, beating up into the valleys between them, and doubtless causing the removal of large portions of the looser materials of which they arc composed, transporting them to a greater or less distance according to their different specific gravities and the strength of the currents. The great terrace of driftgravel was evidently the sea-bottom of that period, and its contents have in great part been derived from the boulder formation. The great features of these lowlands are therefore plainly attributable to the denuding action of the seawaves during the elevation of the island, and the resistance presented at various points by the superior hardness of the rocks subjacent to the tertiary formations; and the rounding also of the lower hills of the boulder formation may in part, perhaps, be attributed to a similar action. Yet it seems not improbable that their contour had been impressed upon them previous to the elevation, when they lay as banks at some depth beneath the surface of the glacial sea, which we have evidence on the Isle of Man was relatively with the land 400 feet at least higher than it now is 7.
Presuming that the action of the currents would then be influenced as now by the coast-line, and that they would have the greatest force in directions parallel to it, tideways, i. e. submarine valleys, would be originated and maintained in those directions, and thus the apparent connection between them and the great axis of the island (which gave the form to the coast-line) would be established and exhibited on the subsequent elevation of the sea-bottom. It is further to be observed that the scooping out of these valleys would be aided materially, were the currents passing through them charged with icebergs, which there is good reason for believing to have been the condition of the sea at that period.
Of the agricultural character and capabilities of this southern area, it may be well to say a word or two before we leave this point d'appui. It is evidently the garden of the island, and altogether under the plough, and the advantages of soil and position for obtaining cheaply the most valuable ingredients for its improvement are extremely great; yet it is only of late years that any attempts at systematic and scientific farming have been made. We may attribute much perhaps of this state of things to the fact of the farms having been broken up into so many small holdings, and the great deficiency of capital. To carry on therefore any general and effectual system of drainage was next to impossible, even if individual farmers had been so disposed. The gravel terraces and the lower rounded hills of the boulder formation admit of very easy drainage; but this is not the case with either the cold clay uplands which rest upon the schist and granite, or the alluvial valleys which have been scooped out of the drift gravel ; and the consequence is that these are even yet under very partial and imperfect culture. The uplands are not very promising 't is true, yet individual instances of industry have shown that a profitable return for labour may be obtained even from them; and this more especially by the simple act ofdraining carried on with judgement through a series of years, even with very limited means. In every part of this sheading lime may be had within a distance of four miles ; the great wonder is that it has not been more largely used.
But the case is different with respect to the valleys which open out immediately upon the sea. Their extreme moisture is owing to several causes, each of which requires a separate treatment.
The denudation of the drift-gravel has originally taken place down to the lowest bed of the tough boulder clay, in many cases through the clay down to the inferior lime- stone. The dip of the limestone towards the sea is generally at an extremely low angle, and in some places again it is horizontal, at others raised up into bosses.
The valleys have again been in part filled up with estuary deposits of marine sand and fine gravel, as well as with the detritus brought down by freshes from the mountains. These deposits are in most instances only a few feet thick. fence the waters from the uplands after excessive rains accumulate in the valleys, overflow the river courses, saturate these sands and gravels, and are only ultimately carried off to the sea by an extremely sluggish natural drainage.
But there is another additional cause of their moisture. The rains which fall on the gravel terraces which inclose the valleys, sink through the gravel and sand till they come down to the ,boulder clay. This is almost impenetrable, and the water prevented from descending further is thrown out on each side into the valleys, and the consequence is that just at the foot of the gravel terraces, where they fringe in a manner the valleys, there is a constant accumulation of moisture producing peat, moss, and rushes. One deep drain on each side of the valley would take off the springs at their head, but I am not aware of any instance in which this system has been adopted. Should the present hint be acted upon, it will afford an opportunity of testing the soundness of my views as to the structure of this area, and be an instance of the practical application of geology to the interests of agriculture in this particular locality.
1 In Johnstone's Jurisprudence, page 13, it is stated that Castle Rushen was built by Guttred in 960, but in some repairs of the Castle in 1815 an old oak beam was discovered, bearing the following characters---
The central letter appears to be a combination of the Męso-Gothic (o) and x (ch,). The date 947 is supposed to be the ovra of the reign of Guttred, second prince of the line of Gorree, who acquired the island in the tenth century; Guttred is said to be buried within the Castle. Cardinal Wolsey, who was guardian to the young Earl of Derby, is generally supposed to have planned and caused to be erected the glacis, of which there is a portion remaining on the east and south sides.
2"In the year 1313 King Robert Bruce beseiged this Castle," Feltham, page `?74.
3 The fort and ruined church at the northern extremity of Langniess (or rather in the island of St. Michael, which is connected with t by a breastwork of stone) are concealed at this point by the hill :above Ballahick.
4- See the Map of the Southern area, including the tertiary formations, Plate II. and Plate VII, section 1.
5 See the Map of the Southern area of the island, including the tertiary formations, Plate III. and Plate VII, section 1.
6 This remark must not be taken so as to exclude the probability of any disturbance having affected this area since the boulder clay deposit, as there is very good evidence of some disturbance, but in a different direction.
7 See my memoir on the " Geology of the Calf of Man," published in the Quarterly Journal of the Proceedings of the Geological Society of London, in May 1847, p. 184.
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