Image: Wikimedia Commons
At school we were shown how to make a model of the landscape. Using cardboard and papier mâché, and taking our measurements from the contour lines of the Ordnance Survey map, we gradually built up a rather clumsy version of the local hills and valleys.
Even coated with paint (green for the fields and woods, grey for the built-up areas, and an improbable ultramarine for the rivers, canal, and gravel pits) it was a sorry-looking object, and was quickly put out of sight by the geography teacher. I believe he may have hidden it behind the scout hut. In any case it was not in evidence when Open Day came round.
The forces which moulded – and are still moulding – the real landscape make a much more convincing job of it. They are incomprehensibly vast and slow-moving, like the glaciers which were responsible for carving out the valleys of the Gade, the Chess, the Ver, and the Colne.
The last Ice Age ended here about 12,000 years ago. We are presently in a mild interglacial period: another Ice Age is probably on its way.
The origins of this cyclic advance and retreat of the ice are thought to lie in the behaviour of the southern ice cap. The core of the Earth is still molten, and the heat must escape continuously from the planet surface. There reaches a point when the accumulation of ice in Antarctica is so great, the thickness of the ice-sheet so tremendous, that this geothermal heat is trapped. The base of the ice-sheet melts, is freed from the friction of the underlying rock, and begins to spread over the ocean as a floating mass.
What happens next affects us in the north. When the Antarctic ice reaches an area of about 10 million square miles, its white surface reflects back so much sunlight that the net input of solar heat is reduced by the critical figure of 4%. The planet cools, the northern ice cap enlarges (aggravating the reflection loss), and the glaciers start to advance.
The process goes into reverse when the icecaps reach the warmer oceans nearer the equator. Then the ice shelf is weakened by warmth and physically eaten away by the erosion of the waves. The area of white begins to contract, and the input of solar heat increases.
Here in south-west Hertfordshire (not that such names mean much to a glacier), the vegetation at one time was that of a tropical forest. The valley of the Thames was a steaming swamp very much like something to be found today in, for example, Malaysia. This was about 63 million years ago: over the next 61 million years the climate cooled and our flora became more typical of the temperate zone.
The first major glaciation came somewhere between 800,000 and 590,000 B.C. Since then there have been half a dozen more: the third from last, the so-called Lowestoftian glaciation, was the most extreme of all and reached as far south as the Thames basin.
The ice of the Lowestoftian advanced across England in a generally south-easterly direction, flowing and rolling like a slow-motion tidal wave. With it the glacier carried a vast amount of debris, billions upon billions of tons of fragmented rock torn up in its path. Where it met resistance it altered course to some extent through the softer rocks.
In the general trend of our local valleys – from north-west to south-east – we see evidence of the glacier’s passage. Nowhere is this more obvious than at the junction of the Rivers Gade and Chess. Both valleys are quite deep, separated, near the junction, by a wedge-shaped nose of land which we call Croxley Green. The Green itself is at the summit of this particular ridge, but even on the Green the land is beginning to slope downwards. The slope accelerates at Scots Hill, and the two valleys converge just north and east of Rickmansworth.
This was the site of a contest between three giants – the iceflows from the Chess and Gade, and the larger flow making its way south along the modern course of the Colne. The battlefield was the wide expanse of Croxley Moor.
The opposing forces then united and continued south, along the present Colne Valley. The glacier got another twenty miles, as far as the Thames, before the climate began warming up again.
In its retreat, the glacier abandoned the rock debris it had brought. This, washed and pounded further by the torrents of meltwater, was left behind as the gravel which has since been excavated in such quantity along the valley of the Colne.
The ice destroyed almost all living matter in its path, and the British flora is the poorer because of it. The landscape after its retreat must have been utterly desolate, and silent but for the sounds of wind, rain, and flowing water.
Since then the harsh contours have been softened by the gentler forces of the weather; the land has been recolonized by plants and animals (including ourselves) from the south. The effect that man himself has had on the landscape, profound as it is, is nothing compared with the work of the glacier. Its scale makes all human efforts seem puny, and puts into perspective our present worries about politics or the price of bread.
However, there is talk of averting the next Ice Age before it even starts. Technically it would be possible, using nuclear explosives, to break up the ice in Antarctica as it formed, and so change the course of geological history.
Comforting as it may be to know that the next glaciation might be stopped, it would be a pity to feel that we had gained control of such a monster as this. Some things should remain sacrosanct.
Perhaps we ought to put this scheme behind the scout hut, where it belongs.