When the lava cooled and became trap-rock, it was at once attacked by frost, by wind, and by rain. Then by a very slow process of colonization, vegetation slowly and gradually crept over the trap-rock and rich mould and plant remains accumulated. At a much later date, there was another wholesale destruction. This time, it was the great Ice Sheet coming down from the Highland hills. Probably it drove heavily over the top of Pennell Brae and worked up into fine mud and powder every vestige of the miocene vegetation.
The very rocks themselves would be scratched, polished, and rounded off. When the glaciers melted away and left the surface free, it would consist of these rounded rocks alternating with clay-filled hollows. The trap-rock below would be covered by a subsoil due to particles of trap, of Highland and other mud, with remains of the miocene vegetation. Upon this surface, frost, wind, sunshine, and rain would again begin to perform their work.
But the subsoil, thus wonderfully formed by fire in the miocene, by frost in the glacial, and by weather in our own geological period, very soon felt the protecting and sheltering effect of a plant-covering.
First a green herb rooted itself every here and there amidst the desolate boulder-clay or perhaps in a crevice where good earth had accumulated. Then the scattered colonists began to form groups; soon patches of green moss united them. Then a continuous green carpet could be traced over a few yards here and perhaps on a few feet somewhere else. But when things had got as far as this, progress became much more rapid, and soon the whole site of the future cornfield was covered over by a continuous green carpet. Only, every here and there, hard stones and uncompromising trap-rocks remained still protruding from the green covering.
In another chapter this first covering of the soil will be described at length.
So far it has been subsoil and underlying rock, but now the roots begin to disintegrate and work up the subsoil; the earthworm has his chance, and forms true soil. On this particular hillside, the water would drain away and there would be no danger of mosses strangling and choking the Blaeberry and the Heather. The worm flourished and multiplied, and the soil became rich and black. Here and there a Sloe or a Rowan, or Poplar, or perhaps Alder and Birch, began to appear. In certain places Whins and Brooms, Brambles and Briers, diversified the hillside. Then a few Scotch firs began to push their way up, through the thickets. At first they were very small and stunted, but as each one formed a dense, deep-going mass of hardy roots, they were able to investigate the riches of the subsoil. Every year the amount of leaf-mould above increased, until the original moss-covering was utterly destroyed and a pine forest (see Chap. XXVIII.) occupied Pennell Brae.
About this time, a paleolithic family may have encamped on the side of the cliff near a little stream which can still be traced. The camp was only a few sticks and branches, with a skin or two for shelter from the north wind. The women lopped down fir branches for firewood, and cut up the young trees. The children set fire to the shrubs on dry days and paths ran here and there through the forest. This would be about 198,000 B.C.
Every year meant a further very gradual, slow destruction of the pine forest.
About 60,000 B.C., our paleolithic hunters with chipped-stone weapons would be obliged to travel further to the north. New savages with round heads and polished-stone weapons would make life in Renfrewshire too uncertain and too diversified by massacres. These last possessed seed corn, a few fruit trees as well as goats, cattle, and perhaps a few hardy, shaggy ponies. At first these settlers would be obliged to live in a lake dwelling, say in Linwood Moss, which is close at hand. They would then drive their cattle over the surrounding district, and camp in slightly-built villages. Near at hand, probably on the hill, they would build a (round) camp or fort, where they could fly for safety in the continual fights and invasions of the period.
Sooner or later a village would be built near Pennell Brae. One summer day the villagers attacked the wood that covered it; they cut down all the small brushwood and hacked through the bark of every big tree. After a few weeks, when the trees were dead, the wood was set on fire. Then a rough fold made of rude wattle and daub was formed, and every night the cattle and sheep were driven in.