These Silurian areas give rise to hummocky regions, where small hills abound, without much relation to the trend of the axis of elevation.
Farther to the south-west, in the shires of Perth, Inverness and Argyll, they give place to the ordinary hummocky crested ridges of Highland scenery, which, however, in Ben Nevis and Aonach Beg reach a height of over 4000 ft.
The influence of wind and tide breaks up the frozen surface of the sea, and sheets yielding to the pressures slide over or under one another and are worked together into a hummocky ice-pack, the irregularities on the surface of which, caused by repeated fractures and collisions, may be from 10 to 20 ft.
A hummocky irregular country spreads southward, where the Silurian axis is encountered, in continuation of the southern uplands of Scotland.
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