Coal Measures; b, Millstone Grit; c, Yoredale Shales, etc.; d, Carboniferous limestone; e, Ordovician and Silurian.
FIG. 2.—Dissection of Craven Pennines by river valleys: 1, Yoredale rocks; 2, Carboniferous limestone; 3, Ordovician and Silurian. (Diagrammatic.)
The three parallel summits of the mid-Pennines, from east to west—Great Whernside (2310 ft.), Penyghent (2273 ft.) and Ingleborough (2373 ft.), but also, to the north-west, Whernside (2414 ft.) and Grey Gareth (2250 ft.), reproduce almost identical characteristics in structure and altitude. They are separated by a series of river valleys, Wharfdale, Littondale and Ribblesdale, then also by Chapel-le-Dale and King-dale, which have very obviously been cut down through the original rock formations, here almost horizontal (see Fig. 2). A reconstruction of the mid-Pennines on an east-to-west section thus shows these mountains as the surviving elements of the Pennine fold, resting on a mass of the still older Silurian rocks, upon which also the rivers now run. North and south of these Craven Uplands, the Pennines have commonly the character of a high moorland plateau (see Fig. 28). It is only when these plateaux have been greatly dissected by erosion and by river action, that distinctive mountain peaks are frequent. This has happened not only in Craven but also at the southern extremity of the Pennine range where dissection has also split up the plateau into peaks such as Kinder Scout and Bleaklow.
Now whatever their origin may otherwise be, it is extremely common to find that mountain masses have the character of dissected plateaux. There is perhaps no better example of this in Britain than the Cairngorms as a whole. The observer standing at any considerable distance from these mountains (so as to be able to see most of the major summits) will inevitably be struck by the fact that the group as a whole presents a nearly level or gently domed-shaped profile. This may be seen particularly well from near Aviemore (see Fig. 3), and it is suggested by the skyline in Pl. 5. In other words, these peaks, so impressive at close quarters, are due to the cutting up of the high plateau by deep and steep valleys. Even on isolated peaks like Lochnagar, the vast summit plateau clearly indicates the remains of one still more extensive.
FIG. 3.—Silhouette of the northern face of the Cairngorms—a dissected plateau.
Imagine the processes of erosion and dissection proceeding over many square miles of nearly horizontal strata, until much more has been removed than is left, and it will be possible to understand the origin of the extreme examples of mountain or plateau dissection to be seen in Western Ross and Sutherland. Here, formerly, nearly horizontal layers of Torridonian sandstone covered an ancient surface of hard and resistant crystalline rocks. To-day, such mountains as Suilven and Canisp represent the last remains of these sandstone masses, most of which have long since vanished. In this category also must no doubt be placed Lugnaquilla (3039 ft.) in south-eastern Ireland—the last remnant of rocks overlying a large boss of granite.
There is one other point about the effects of erosion which is worthy of brief mention. If a mountain mass or ridge were composed of uniform materials and if it were equally eroded on all sides, the shape of the mountain would tend to approach more and more closely, as time went on, to that of a perfect cone. This generalised type of mountain is not perhaps very common in Britain—though isolated hills like Muckish and Errigal in Donegal are of this general type as well as many of the rather lumpy mountains in the Scottish Highlands, especially perhaps Schiehallion. The Paps of Jura, illustrated in Pl. 3a, show the disintegration of a quartzite ridge in this way. A common British variant of this simple type is one in which the summit is distinctly flat-topped or tabular. This is particularly to be seen in some of the examples already mentioned. The three most prominent Pennine summits, Cross Fell, Ingleborough (see Pl. 17) and Kinder Scout all have this form as do the Sutherland mountains Suilven and Canisp, and MacLeod’s Tables, west of Dunvegan in Skye. It is due to the presence at the summit level of a horizontal stratum of hard and resistant rock, usually Millstone Grit in the Pennines and Torridonian Sandstone, capped by Cambrian quartzite in Sutherland, the latter containing so much white quartz that the rock may be mistaken for a snow-cap when seen from a distance (see Pl. III).
The Craven Uplands show in a particularly striking manner the dependence of mountain scenery and vegetation on the geological structure. The rocks are horizontally stratified and they consist of an upper zone, mainly of Yoredale sandstones and shales, below which lies a great thickness of Carboniferous Limestone, once called the Mountain Limestone from its association with upland areas in Britain. Where the overlying rocks have been removed by erosion, the hard limestone may form extensive plateaux, and because it is almost pure calcium carbonate, it yields practically no soil on weathering. It is traversed in all directions by deep vertical fissures and is consequently dry (see Pl. XXII). The surface, aptly called “limestone pavement,” is usually devoid of vegetation except where traces of glacial drift occur, but a luxuriant flora lives in the shelter of the fissures. The limestone plateaux are often bounded by almost vertical “scars” (see Pl. XVIII). A very striking type of scenery is thus produced, a feature not only of the Craven Uplands and mid-Pennines in general, but also of large areas in Western Ireland (Clare and Mayo).
In contrast, the Carboniferous Sandstones (including Millstone Grit) and shales are non-calcareous and are almost always covered by the moorland vegetation which is so characteristic a feature of the high plateaux of the northern and southern Pennines. In Craven, where these rocks are exposed along with the limestones, the contrast between the two sorts of rock is often very striking, and is well illustrated in Pl. 4. Thus both the physical and chemical qualities of the rocks may affect the scenery and vegetation.
The simple conical form that is to be expected where rocks of approximately uniform texture are equally eroded on all sides, is lost not only when the harder rock strata occur, but also wherever the mountain is composed of strata that are not horizontal. Thus both Blencathra and Dow Crags in the Lake District show one gently sloping aspect (see Fig. 4) which is that of the “dip” or slope of the rock strata, while on their steep faces the rock weathers into blocks, more or less at right angles to the dip of the strata, so that a
FIG. 4.—Some types of mountain form. A, Symmetrical weathering of uniform rock; B, Recent oversteepening below ancient upper form; C, Ridge with softer interbedded rock; D, Dip and scarp slopes.
steep angle tends to persist. In Wales, Tryfan also shows this type of structure in a still more spectacular manner (see Pl. II) and it is very generally to be seen in the different mountain areas, often recurring, again and again, wherever the rock strata act as guiding planes for the inevitable erosion.
Sometimes hard rocks arranged in this manner overlie much softer ones. Such is the essential structure of Mam Tor, the “Shivering Mountain” in Derbyshire, and also of Alport Castles not far away. In both cases, hard sandstones and grits in the upper part of the escarpment have below them soft shales which are constantly washing and weathering away. Thus, on Mam Tor, the upper parts of the escarpment is constantly being undermined and so are constantly falling. Alport Castles, in contrast, represent an immense wedge of the mountain detaching itself from the face behind and falling outwards with infinite slowness, pushing before it into the valley a great wave of earth, as is well shown in Pl. 6. There are no better examples in Britain of the instability of mountain structures than these two Derbyshire hills.
In other British mountain areas, the comparatively simple arrangements of rocks seen in the examples already discussed are obscured and other considerations become important.