This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
One marked characteristic of upland areas is the preponderance at high altitudes of bare rock surfaces. Moreover, an outcrop where soft and hard rocks are contiguous is as a rule diversified, and the hard rocks are exposed as hills or escarpments, whilst soft clayey or shaly beds are weathered down rapidly to lowland conditions. It is usually where quarries have been made in the lowlands that rock surfaces are to be seen, and even these are less frequent than along a hillside itself. Rock may be exposed in a valley or level region, but except in the case of coal-pits and granite quarries this is unusual.
It has already been mentioned that a hilly tract may, in spite of a higher rainfall, have really less moisture relatively than a region with a smaller rainfall. The wind is a very important factor in this connection, its drying effect at certain seasons, when not itself moisture-laden, tending to counteract the condensing effect of the mountain itself.
It is only where hollows are formed in a hillside by the oozing out of underground water through porous strata or other causes that water remains upon a hill permanently. It is aided in this by the effect of bog-mosses such as Sphagna, and the rapid growth of moorland plants that help to retain the water in such pools or bogs.
In the case of hills such as those formed by chalk or limestone, where streams do not carry the water away, the whole surface acts as a sort of sponge and water collects in underground reservoirs. In both cases the effect of carbonic acid gas in the water in dissolving the lime is to form pipes, fissures, and cavities in the rock, and to produce caverns, as in Derbyshire and elsewhere.
As one ascends from the lowlands a noticeable feature is the absence of, or decrease in, the areas given over to cultivation. This is due largely to the same causes that control the existence of wild plants at high altitudes, such as increased cold, moisture, wind, a high degree of insolation, exposure, shallower and in general more barren soil. Also fog and mist may be prevalent.
Watson indeed established a zone, called the Agrarian Zone, within which cultivated plants would grow and flourish, and above which they are unprofitable. This more or less corresponds with the limits of growth of the chief deciduous tree types, as the Oak, etc, or 1000 ft. above sea level.
This has a decided effect upon wild plants that are addicted to a mountain habitat. For up to that altitude both soil and vegetation have been more or less disturbed. At the same time also the soil conditions, where cultivation has been carried on, in many cases have undergone considerable change, making the return of natural vegetation, when the land relapses into an uncultivated stage, less likely, and its substitution to a great extent by the followers of man and the plough the more probable.