(see here), but it lies also in their possible bearing on the origin of the high-level corries (or cwms or cirques) so often found in the larger British mountains. In the extreme form these are rock basins and undoubtedly relics of the small high-level glaciers and nevé which must have lingered on for long after the main ice-sheets had passed away. Corries seem to be most frequent on the east of a main summit or ridge, and it may be that in the first place their position was the result of a semi-permanent snow-bank which started an erosion system. In the later stages it has been supposed that the upper nevé exerts a plucking action on the frost-shattered mountain face, through the periodic filling and downward contraction of the bergschrund, if one may use this term in such a case. In this way continuous over-steepening of the head of the erosion system may have resulted in the formation of the crags encircling the corrie. It seems probable that corrie-formation was most vigorous during and just after the Ice Age, but as it usually lies above the other main erosion effects of the ice-sheets it may be appropriate to regard it as an extreme effect of persistent snow-lie.
In attempting to summarise what has been obtained from this survey, it becomes clear that physical instability is the most noticeable feature of upland surfaces, and it is equally evidently a chief characteristic of the high-level or montane region—although it also accompanies any steep slope as well as the borders of active erosion systems such as streams. Physically stable areas in the uplands differ little from lowland areas, except in other features such as those of climatic origin.
We also see that British mountains are often likely to show an upper zone of comparatively gentle slopes, representing the ancient land forms, moulded long ago, but often kept alive or unstable through the agencies we call nivation. The lower slopes have often been over-steepened in comparatively recent times as a result of glaciation or of the extensive erosion which must have been associated with the melting of the ice. This common plan, if we may call it so, results in the appearance of numerous rather round-topped mountains, although it is modified in innumerable ways as a result of the varieties of rock which make up the mountain blocks and of the different sorts of bedding planes which may be found in different areas.
There is still another way of looking at these matters. The present cycle of erosion as it affects the upland surfaces may be considered to have started at the end of the Ice Age. The upland surface at that time, except where covered by drifts or morainic materials, must have been very different from what it is to-day. It must have been mostly exposed rock which, presumably under the sub-Arctic post-glacial conditions, quickly developed frost-shattering and the characteristic erosion forms found to-day in the Arctic and at high altitudes. To-day much of the corresponding surface is soil- or peat-covered, and only the montane or unstable areas preserve what must have been a widespread condition in the immediate post-glacial period. It will be seen, therefore, if this argument is correct, that the study of the montane areas is likely to be of especial interest. We shall expect to find that the biological character of the unstable areas is widely different from that found elsewhere and perhaps in some respects reminiscent of a condition that was more widespread in post-glacial times.
CLIMATE
THE differences between upland habitats and those of the lowlands are only partly structural. Partly they are climatic and this aspect must now be considered. British mountains are only of moderate size but they lie near the sea and across the path of the strong Atlantic breezes from the west. For this reason, wind and cloud and rain play a large part in the weather conditions and they combine to give a characteristic “atmosphere” to British mountain scenery, something of which is conveyed in the photograph of Glen Einich in Pl. VIII. Equally familiar to inhabitants and noticed by many visitors is the building up of evening cloud after sunset (see Pl. XXIX), while even in the finest weather the day is likely to break beneath a curtain of morning mist, well shown in the charming photograph of Llyn Padarn, (see Pl. 10). The visual impressions we thus carry with us can readily be confirmed from the precise data collected by meteorologists, and to them we may now turn.
We are fortunate in having detailed records which enable us to assess these effects over long periods and thus to present them as the main features of mountain climate in Britain. They were made between 1884 and 1903, when an observatory was maintained near the top of Ben Nevis (4,406 ft.), and though they thus give the extreme climatic limits for British mountains, they enable other more scattered observations to be checked and utilised.
In the first place, the records confirm the impression that strong winds are frequent. During thirteen years, an average of 261 gales a year with wind velocities exceeding 50 miles an hour was recorded at the summit of the mountain. This large number should be compared with the conditions at sea-level, when, even on the exposed western seaboard, few places average annually more than forty winds of such a velocity. The comparison between montane and lowland conditions may, however, be made in another form. A more recent estimate of wind-speeds has been made on Crossfell (2,930 ft.), a much lower summit in the Northern Pennines. There, it was estimated that the average wind velocity was at least twice that prevailing in the adjacent lowlands, a result comparable to similar estimates on Ben Nevis.
The Ben Nevis records also serve to illustrate the cloudiness of the mountain sky, for during the years of observation the summit was clear of mist and cloud for less than 30 per cent of the time and, as the table shows, had correspondingly low figures for exposure to sunshine (Table 2). These are, however, only different aspects of a more fundamental feature, the great humidity of the atmosphere. The average relative humidity of the air on Ben Nevis was 94 per cent of saturation with water vapour, showing little variation throughout the year, except in June, when it fell temporarily to 90 per cent, still an exceptionally high average figure.
As might be expected, this high atmospheric humidity was associated with high rainfall. Over a long period this averages 161 in annually at the summit, and it was rather higher during the thirteen years of comparative observations given in Table 2. The maximum recorded was 242 in. in 1909, and as much falls on Ben Nevis during the three “dry” months, April, May and June, as would represent the whole annual rainfall in Eastern England. High rainfall is, of course, a general feature of British mountains. Thus there is the well-known example of the Seathwaite District in Cumberland where Stye Head Tarn, east of Great Gable (2,900 ft.) has an annual average of 153 inches with a recorded maximum of 250 inches in 1928. The computed average for Glas Llyn (2,500 ft.), 500 yards north-east of the summit of the Snowdon ridge, is 198 inches. The Snowdon summit, Y-Wyddfa, in fact, competes with the head of Glen Garry (in Western Inverness), east of Sgurr na Ciche (3,140 ft.), for the distinction of being the wettest place in the British Isles. Both are considered to have an average annual rainfall of some 200 inches. Ben Nevis or Scafell and its Pike, have more of the character of isolated peaks, so that the prevalent winds can slip around them and less rain results.
The last feature of the Ben Nevis records to which attention must be directed is the range of temperatures, also given in Table 2, where they are compared with those at Fort William (at the base of the mountain).
In this table, the figures given at the foot of the columns for the year are averages in the case of temperature, and annual totals for hours of sunshine and rainfall. As there are many summits between 2,000 and 2,900 ft. to the south and west of Fort William, the rainfall there is already much higher than it would be on the outermost seacoast, and sunshine records are accordingly lower, so that the contrast between the lowland and montane conditions is much diminished.
Table 2 METEOROLOGICAL
