The Parents' Review
A Monthly Magazine of Home-Training and Culture
"Education is an atmosphere, a discipline, a life."
The Connection Between Geology and Scenery.
by the Rev. H. H. Moore, M.A.,
Let us now go on to show that the upheaval and survival of these oldest and hardest rocks have had quite as marked an effect on the inland scenery of the western and north-western and south-western portions of our island as on its coastline. Which districts have the loftiest hills and most rugged scenery? Are they not the Scotch Highlands, the English Lake District, North and South Wales, and the Cornish Peninsula? Well, the characteristic physical features of those districts are due to the presence of igneous rocks, and of the oldest and hardest sedimentary rocks. Taking
first, we find that fully three-fifths of its surface are occupied by rocks belonging to the lower Silurian formation, but by volcanic and igneous agencies and pressure, most of them have been changed, more or less, into hard crystalline rocks, and are traversed in every direction by innumerable intrusive veins and masses of igneous rocks. Ben Nevis, Ben Cruachan, the Cairngorms, the hills about the Dee, which form the romantic scenery so dear to our Queen, the Ochill range, the Sidlaw range, are all mainly of granite. Schiehallion in Perthshire is composed of a very hard quartz rock. The Cheviots are chiefly composed of porphyrite, and in some places of syenite passing into granite. The Highlands, being largely composed of hard igneous and crystalline rocks which do not readily decompose and wear into soil, mainly form a wild moorland and pastoral country, "Sacred to grouse, black cattle, sheep and red deer" (Ramsay). The Silurian rocks in the southern half of Scotland are likewise more or less hard and intractable, but have not undergone such an extreme degree of metamorphism, therefore, although they form similar scenery, and are likewise fit only for pastoral purposes, yet they are not so elevated, rugged, and abrupt as the Highlands. Whereas in the Highlands there are more than a dozen mountains exceeding 3,000 ft., and Ben Nevis over 4,000, in South Scotland there are none over 2,688, and more than half the area is under 2,000 ft. Still, where the scenery is the most striking and picturesque, it is due to the greater development of igneous rocks and veins. Going now to
North and South Wales
we shall find the same scenic effects produced by the same geological causes. North Wales, like the North and North-West of Scotland, seems to have been a centre of great volcanic disturbances. Submarine volcanoes poured out rivers and sheets of lava, and threw up immense showers of dust and ashes, which fell again to the bottom of the sea. The stratified Cambrian and Silurian rocks, of which the district is chiefly formed, are pierced in every direction by masses of igneous rock and have undergone a great amount of metamorphism. But those volcanic agencies did not operate so powerfully in South Wales, and therefore the rocks there were not so extensively metamorphosed, and consequently they were of a softer and more uniform texture, and wore away more quickly and more equally. That is why the peaks and ranges of South Wales are less elevated and, as a rule, offer less wild and rugged scenery than the mountains of North Wales. The greatest outpouring of volcanic products was in the Snowdon district, where the old ashes and lavas form the grandest and wildest region of Wales, including those well-known mountains, Snowdon, Moel Siabod, Carnedd Llewellyn, Carnedd Dafydd, the Glyder-Fawr, the Moel Haboy, etc. Snowdon itself owes its shape simply to the accident of some of the beds towards the summit being especially hard, and thus able to stand the wear and tear of the sea-wave, ice, and rain. It has a mass of volcanic ashy strata on its shoulders 1,000 ft. high. The crescent-shaped ring of hills extending from the south side of the Mawdach estuary, opposite Barmouth, sweeping round northwards to the northern side of the beautiful Vale of Festiniog, are all igneous rocks, and include the well-known heights of Cader Idris, Arran Mowaddy, the Arrenigs, and the Moelwyns, Penmenmawr, and the hills about Bettys-y-coed are Silurian rocks. Great Orme's Head is mountain limestone.
English Lake District.
Coming next to the English Lake District, we find that its highest and most rugged hills are largely composed of igneous rocks, and of the oldest stratified rocks, often highly metamorphosed. The whole of that district, in fact, during the Lowe Silurian age was studded thick with volcanoes, which ejected volleys of shattered rock, clouds of ashes, and streams of burning lava. Where Keswick now stands was once the crater of a land volcano, probably equalling Etna in height, as certainly it did in activity. The crags which look down upon Derwentwater on its east side are nothing but streams of lava which once flowed from a volcano. Castle Head and Friar's Crag, between Keswick and the lake, are protruded masses of igneous rock. The view from these well-known look-out points affords a striking example of the effect of geological structure on scenery. The igneous rocks on the east side of the lake are rugged, craggy, bold, whilst the hills on the west side have soft outlines and smooth grassy slopes, being composed of Skiddaw slate, which weathers away to a fine flaky and clayey shale. The rugged hills on both sides of the Esk and Mite, in Cumberland, for 14 miles from north to south are red granite. Harter Fell, Scawfell Pike, Great Gable, High Stile, Helvellyn, Fairfield, Bow Fell, High Street, Harrison Stickle, Ill Bell, Coniston Old Man, and others, owe their bold jagged peaks, sharp ridges, and steep craggy sides, to the fact that they are mainly composed of volcanic green slates, porphyries, and highly metamorphosed Lower Silurian rocks. Contrast the rugged, craggy, precipitous character of Scawfell Pike with such mountains as Skiddaw, Saddleback, and Black Combe. The least observant tourist can hardly have failed to be struck with the difference. What is the reason that these three have such long smooth regular slopes and soft outlines? Is it not interesting to know that it is because they are composed of the softer and unmetamorphosed Silurian slates? On Shap Fells an area of four or five square miles is occupied by granite, which rises at Wasdale Crag to a height of 1,500 feet.
Hills of the Midlands.
Going next to Shropshire, the romantic character of the scenery about Church Stretton must have struck every observant traveller going from Manchester to Bristol by the L. & N. W. [London and North Western Railway] To what causes is it due? On the east side of the beautiful valley you have the Caradoc range of abrupt peaked hills, Ragleth, Hope Bowdler, Caer Caradoc, and the Lawley, which consist entirely of erupted trap rock. On the west side of the valley you have the huge mound of Longmynd, with its deep picturesque ravines, which consists of L. Cambrian rocks, and again, west of that and parallel with it, but separated by another valley, you have the rock-crested range of the Stiper Stones, composed of Lower Silurian rocks. Both these ranges are highly metamorphosed. The two valleys between these three parallel ranges of hills were originally straits of the sea separating these island mountains. These hills afford proofs of the excavating power of the sea, in the deep narrow gorges with sides rising at angles of 55 to 70 degrees. These gorges begun by the sea have been continued by running water. On the top of the Stiper Stones ridge are detached groups and masses of highly crystalline rock, of very irregular, broken, and fantastic shapes, evidently the remnants of a ridge that has been breached by the action of the sea beating against it on all sides.
The Clee Hills, in Shropshire, and the Rowley Hills, in Staffordshire, have preserved their present elevation only because they have a capping of hard basalt. This is extensively quarried on the Clee Hills to make the roads of the surrounding districts, for it is almost as hard as iron. It is called "Jew-stone," a local corruption of "Dhu-stone," "dhu" being Celtic for "black."
The noted Wrekin is an elliptical hill about a mile and a quarter in length. It is the cone of an old volcano, and consists of igneous rocks with pre-Cambrian strata on its flanks in a highly metamorphosed state. It is one of the most remarkable examples of eruptive volcanic rocks in England. The contour of its summit and flanks, the shape of Raven's Crag, and the Needle's Eye, the narrow pass between it and Ercall, have been caused by the action of the sea. The Severn valley is strewn with stones that have been torn from the Wrekin by the sea and deposited on what was then a sea bed, but is now the beautiful Shropshire plain. Coming next to Worcestershire, the fine range of the Malverns must have attracted the attention of many of you, and caused you to wonder why they rise so high and so abruptly out of the broad level Worcestershire plain, and why they stand so insulated from all other hills. The reason is that they are a mass of igneous and Archaean rocks pushed up by a great upheaval. On their western side, they more recent Upper Cambrian strata were lifted up with them, and are now found reposing on the western flank of the Archaean axis; but on the eastern side, a complete fracture or separation between the Malverns and the newer strata took place, and the latter were left lying at their original low level. That is the reason the Malvern range rises so abruptly from the Worcestershire plain. The hills themselves owe their preservation from Nature's destructive and degrading forces to the fact that they consist from end to end of such hard igneous crystalline rock.
In Leicestershire the fine range of the Charnwood Forest Hills consists of granite and of the oldest of all formations, the Archaean, in a highly metamorphosed state. At Mount Sorrell, their eastern extremity, enormous masses of granite are quarried to make the roads of the Midland Counties. When the Charnwood range was pushed up, it also necessarily lifted up that portion of the coal measures overlying it. These latter strata, together with an enormous thickness of granite and of slates have been swept away, so that the Leicestershire and Nottinghamshire coal-fields, which were originally continuous, are now separated by a considerable distance.
The Wrekin, the Malverns, and Charnwood Forest, are the oldest of our English hills, the first that rose above the sea that originally occupied the site of the British Isles. They are older as hills than even Snowdon and Ben Nevis and the Lake District Hills, and probably only find their coevals among the rocks of the Outer Hebrides and the N. W. coast of Sutherland. The Himalayas, the Andes, the Rocky Mountains, the Alps, the Pyrenees, are all but as babes of yesterday compared with the venerable antiquity of our oldest English hills, which had been dry land for countless centuries, while those grand ranges were slumbering yet unborn beneath the waters of the primeval ocean. Our oldest English hills are now insignificant in height (Charnwood Forest is only 800 feet), but that is no measure of their former dimensions and importance. We must remember their immense antiquity, and the consequent amount of wear and tear to which they have been subjected. In fact they are but the worn-out stumps of what they once were. As a rule the oldest mountains in the world are the lowest, e.g., the Laurentian range of Canada is now worn quite flat, and the whole island of Mull is nothing more than a basal wreck of a vast prehistoric Teneriffe, which once towered into the air with its volcanic cone as high as Etna.
Going next the south-west of England, we find that the peninsula of Devon and Cornwall owes its survival to the fact that granite forms its backbone. If we could strip off the partial covering of stratified rocks, we should find granite everywhere forming the foundation and core of the peninsula. In some places the sedimentary rocks have been worn away sufficiently to expose four great bosses of granite which form the chief eminences of Devon and Cornwall, viz., (1) Dartmoor, which is all granite: it extends over more than 200 square miles, and rises to the height of 2000 feet in Yea Tor, being the highest ground in that half of England which is south of Trent; (2) a second huge mass of granite is Brown Willy, which is 65 square miles in area and 1368 feet high; (3) a third mass lies between St. Columb, Bodmin, and St. Austell; and (4) a fourth between Falmouth and Redruth, Smaller protrusions connect these with the Land's End, which is all granite, and with the Lizard, which is Serpentine. These hills and ranges are the harder portions of the mountain range which have been withstood the destructive, denuding effects of the sea, as each part rose in turn above its level, and afterwards the eroding, degrading effects of rain and frost. The depressions and gaps between these higher points are the softer parts or lines of fracture which have been more easily eaten out by the sea and the streams. Thus we see that the ordinary description of the peninsula of Cornwall and Devon as "four or five islands of granite rising from a sea of clay slates" is a very accurate one. The chief features of the scenery, which is the result of such a geological history and structure, are these--rounded hills with rocky projections at intervals, and crowning tors, and groups of isolated rocks. These tors represent the harder remnants of the rocky summits which have survived the battering, carving effects of the sea and weather. They were once surrounded by the sea, which washed over them and cut through them, separating the different parts of a rocky ridge from one another, and leaving them standing in huge irregular piles, and as gigantic broken pillars, the unscathed monuments of the stupendous denudation by which the adjacent parts have been removed. The extent of the wear and tear and waste which this granite range has suffered will give a faint idea of the enormous antiquity of the range. At Bovey Tracey, near Newton Abbot, South Devon, is an extinct lake, which is filled 300 feet thick with white clay sediment, the waste of the granite crags of Dartmoor. For, strange to say, the china clay that is used in the manufacture of cotton in Lancashire, one of the softest substances, is the product of one of the hardest rocks, granite. The felspar, which is one of the constituents of granite, is acted on chemically by the rain. This region of granite originally extended as far as Brittany.
Proofread by LNL, June 2020
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