Mountains and Moorlands. W. Pearsall H.

John Ray and his associates were no strangers to the high northern hills, but the first record of an ascent of a British mountain we owe to another botanist—stout Thomas Johnson—whose account of the ascent of Snowdon in 1639 all naturalists will enjoy, especially perhaps the concluding sentence: “Leaving our horses and outer garments, we began to climb the mountain. The ascent at first is difficult, but after a bit a broad open space is found, but equally sloping, great precipices on the left, and a difficult climb on the right. Having climbed three miles, we at last gained the highest ridge of the mountain, which was shrouded in thick cloud. Here the way was very narrow, and climbers are horror-stricken by the rough, rocky precipices on either hand and the Stygian marshes, both on this side and that. We sat down in the midst of the clouds, and first of all we arranged in order the plants we had, at our peril, collected among the rocks and precipices, and then we ate the food we had brought with us.”

      Johnson lived in troubled times and he was later to die of his wounds as a Cavalier soldier. If interest in the Alps may be said to have started as a result of de Saussure’s scientific expedition to Mont Blanc in 1787, we may perhaps fairly regard Johnson as a British de Saussure at a far earlier date, though on a more modest and unassuming scale.

      FIG. 1.—Map of areas discussed, showing some of the Nature Reserves in Highland Britain.

       CHAPTER 2

      STRUCTURE

      THE British Highlands are composed of blocks of hard and old rocks that occupy the north and west of these islands. While the biologist is not primarily concerned with the manner in which these rocks originated and attained their present condition, the geological structure of the uplands is a matter of some importance to him because it determines the character of the soil and the nature of the habitats available for living organisms. Consequently, a slight acquaintance with geological structure and processes forms part of the necessary background of the present subject and one, moreover, which is of interest in helping us to understand the great scenic and biological diversity of different parts of Highland Britain.

      Almost every mountain observer has been struck by the evidence of decay which centres around the larger peaks. There is shattered rock round their summits (see Pl. III) while below every crag we find scree and from every gully there runs a stone-shoot (see Pl. 2b) formed from débris coming down from above. In the mornings the ceaseless downward trickle of stones or the occasional rock-fall proclaims the constant attrition to which the steeper hills are subject. Thus to the distant observer the mountains may seem to be permanent, “the immortal hills,” but to those who know and move among them a different impression is formed, in which breakdown and change play by far the most prominent part.

      The causes of this constant weathering of the rock surfaces are primarily the uneven contractions and expansions of the rocks caused by fluctuations of temperature, the action of rain and frost and the force of gravity. Any cracks that develop become filled with water and are expanded when the water freezes and widened when it thaws. The actions of rain and of gravity tend to remove the smaller rock fragments so that nothing accumulates to protect the constantly exposed surface. Thus the surface continues to be weathered away until the slope approaches an angle of rest (usually between 30° and 40°). Rainwash, soil-creep and the gradual downward movement of larger stones continue, long after this angle is reached, to move the materials towards the valley.

      Still more important in the long run are the effects of running water, for this not only tends to move materials downwards, but it may also remove them from the area altogether. In the course of time, therefore, every mountain torrent erodes a gully of its own construction, and the coarser materials eroded are deposited below the gully on a gravel fan or delta (see Pl. 2a) while the finer materials are carried ultimately to the plains beyond the mountain area. It follows that the land-forms in the uplands tend to be those which have survived the processes of weathering and erosion. In terms of these ideas, the uplands, whether moorland or mountain, have survived because the rocks of which they are formed are harder or have resisted removal rather than because they are being or have been lifted up. At the same time there must have been some original mountain-building process.

      Many facts can be used to illustrate this argument. Almost every visitor to the English Lake District becomes familiar with a type of scenery in which there is a foreground of lower and rounded hills, backed by a skyline of larger and steeper mountains. This is well shown in the photograph of Esthwaite Water in Pl. 3b and this type of scenery has a simple explanation. The rounded hills in the foreground are composed of rocks less resistant to weathering and the high mountains of harder or more resistant rocks, in this case the hard volcanic “tuffs” or ashy beds of the Borrowdale Volcanic series which make up much of the mountain core of the Lake District. Both to the north and the south of these hard rocks lie softer slates and grits. Although to the uninitiated these rocks present a generally similar appearance, they exhibit considerable difference in hardness. As the harder Borrowdale rocks have weathered much more slowly, they now generally form much higher ground than do the adjacent softer rocks. The actual junction of the harder and softer rocks is shown in Pl. I where it will be seen that the harder rocks are mountain slopes, the softer being soil-covered and cultivated.

      There are many equally good examples elsewhere of the influence of the hardness or softness of rocks upon land forms. A second illustration may be taken from Scotland, where, north of the Highland line, there are to be found long stretches of steeply inclined and metamorphosed grits, hard and resistant rocks which give the line of the summits, Ben Lomond, Ben Ledi, and Ben Vorlich. The valleys intersecting this area lie on beds of softer rocks, shales, limestones and phyllites, which have suffered correspondingly greater erosion and so have been cut down below the general upland level.

      The general concept of mountain structure thus illustrated can be applied on a larger scale, for it has been pointed out already that the distribution of mountains and moorlands in Britain is essentially that of the older and harder rocks. These lie, as we have seen, to the northwest of the British Isles, and their range covers all the main mountain areas. The causes of this distribution lie in the far-distant past when large-scale earth movements were taking place, and there seems to have remained since a tendency for the north and west of the British Isles to stand as a raised system. While any attempt to trace these mountain building movements lies outside the scope of the present discussion, it may be interesting to indicate something of their effects on upland structure.

      In the simplest cases, of which the Pennine range in Northern England is a good example, the upland area represents essentially a fold in the earth’s crust. In the Pennines, this fold runs approximately north and south and a transverse section through it would show the general arrangement represented in Fig. 2, with the newer rocks (including the Coal Measures) represented on either side of the fold, that is in Lancashire and Yorkshire, but absent from the top of the Pennines themselves. There is evidence of various types which points strongly to the probability that newer rocks, from the Coal Measures upwards, have in fact been removed by erosion from along the crest.

      Thus the Craven Uplands, including Ingleborough at their southern end, are separated from the main block of the Southern Pennines by the great Craven Fault system. Just south of this fault, at Ingleton, coal was formerly mined from strata lying above those which correspond to the rocks on the top of Ingleborough. The assumption seems clear, therefore, that these Coal Measures have been removed by erosion from the area north of the Craven Fault.

      The Craven Uplands are also interesting in another respect.

      FIG. 1.—General character of Pennine anticline.