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. An upfold like that met with in the Pennines is called an anticline (see Fig. 1) and a corresponding valley-shaped fold (or depression) would be called a syncline. Now it is a striking fact that the mountain summits very often represent the remains of a syncline. Naturally this is only in areas where great erosion has taken place. The reason for the persistence of the synclinal folds as mountains is that when folding takes place as a result of lateral pressure, the synclinal folds will be compressed and so will tend to become harder. Anticlinal folds, on the other hand, will come under tension and so will tend to crack.
Thus when weathering and erosion takes place, the anticline, being shattered, is more easily attacked and suffers more, while the syncline, being compressed and hardest, therefore tends to be more slowly affected. It is thus logical, if somewhat unexpected, to find that great peaks or perhaps particularly ridges often represent the remains of a syncline, though the synclinal structure may not always be evident because the main ridge of the mountain often represents the long axis of the synclinal fold.
The classical example of synclinal mountain structure, of which a fine picture exists in Lord Avebury’s Scenery of England, is that of Y-Wyddfa, the main peak of Snowdon, as seen from between the Crib Goch and Crib y Ddysgl under suitable conditions—with a powdering of recent snow. This face is usually in shade and not easily photographed to bring out the rock structure, but the essential features are shown in Fig. 5.
Another fine and well-known section illustrating synclinal structure is exposed on the Clogwyn du’r Arddu, to the north-west of the main
FIG. 5.—Rock structure showing syncline on Y-Wyddfa—the Snowdon summit.
summit, where a great synclinal fold makes up the whole of the precipice. These rather simple illustrations serve to illustrate a very important fact that where great earth-movements have taken place the contortions of the rock strata may greatly affect their hardness and resistance to erosion.
Snowdon itself represents the bottom of a great fold whose crest lay somewhere to the south-east. In that locality some 20,000 ft. of rock must have been removed by erosion. The human mind can hardly appreciate the length of time, not less than hundreds of millions of years, which erosion on this scale must have taken. The rocks now exposed belong to two ancient systems which we have already encountered in discussing an earlier illustration (see Pl. 3b). They are in geological terminology of Ordovician and Silurian age (see Britain’s Structure and Scenery by L. Dudley Stamp). The central core of Wales, as of the Lake District, consists of Ordovician rocks which are solidified volcanic ashes and stones (tuffs) and lava flows, with interbedded marine strata indicating a submarine origin. These make up some of our boldest mountain scenery, though there is nothing to suggest that the individual mountains such as Snowdon, Cader Idris or Scafell have ever been volcanoes. Associated with the Ordovician tuffs and lavas are extensive sedimentary rocks of later Silurian age which are mainly fine grits or shales, and these, though generally softer, are as a rule rather poorer in bases like lime. They form somewhat more rounded hills (sometimes described as moels, their Welsh name), to-day almost always grass-covered like the lower slopes of the Ordovician crags. The general appearance is well shown in Pl. XXIII. Together, the Ordovician and Silurian rocks make up some of the most extensive areas of British upland country, characteristic not only of Wales and the Lake District, but also of the Southern Uplands of Scotland and Southern Ireland.
The mention of volcanic action should not necessarily suggest an identification of parts of a particular mountain with the cone and crater of an extinct volcano. The correct interpretation of signs of volcanic action among British mountains is usually possible only if one keeps clearly in mind the fact that most mountains are likely to be the remnants of larger structures. Usually then it will be vain to look for anything so obvious as the cone and crater of a Vesuvius or a Stromboli. The nearest approach to this sort of structure that we are likely to find in Britain is seen in some of the Laws of Southern Scotland.
FIG. 6.—A Scottish “Law”—eroded remains of ancient volcanic vent. The shaded areas are basalt (lava flows)—the laminated areas are volcanic tuffs (ashes).
These usually represent the vents of small volcanoes which have become plugged with solidified lava whilst the surrounding cone has been more or less completely removed by erosion. A simplified section is given in Fig. 6. One of the most complete examples, Largo Law in Fife, is essentially similar but has two main vents. The figure shows the position of the vents and the lava flows which are marked by “basaltic” rocks. Around these are the remains of the cones formed by tuffs or solidified volcanic ashes and stones. The mineral composition of these volcanic tuffs is characteristic, so that they can be recognised where no volcanic cone is evident. It is this type of identification that is used in the case of the Ordovician tuffs already mentioned, where the scale of output was immeasurably larger and no certain vent can be found.
Igneous rocks apart from volcanic lavas more usually fall into one of two main morphological types. The largest areas are occupied by plutonic rocks, representing enormous masses of molten rock which has solidified without reaching the surface. There are secondly “dykes” and “sills,” both representing intrusions of molten rock among other pre-existing strata. In the case of dykes the intruded material runs through cracks or planes at right angles to the general stratification—in the case of sills the molten rock follows between the bedding planes and therefore runs parallel to the general “dip” of the rock. Sills are often more resistant than the rocks into which they have been intruded, and when this is the case they may form striking cliffs. Especially well-known examples are some of the sills in the Edinburgh district, of which perhaps Salisbury Crags are the most impressive. In Northern England the Whin Sill not only forms natural escarpments on which part of the Roman Wall stands, but it is associated both with a remarkable flora and with a series of majestic cascades in and near Upper Teesdale. Far away on the western side of the Pcnnines, it outcrops again on the great western escarpment, particularly
