"That floods, probably arising from the causes I have mentioned, did sweep down our valleys seems proved by the fact, that particularly in the narrow valleys of the Avon, all the hill-sides against which the stream would have impinged present bold escarpments, whilst the hills at the sides of the valley not exposed to this wearing action have the usual swelling outline so characteristic of a chalk district.
"But we have not only to take into account this far greater mechanical excavating action of the Quaternary rivers. The surface of the chalk beneath the gravel is extremely uneven, and deep 'pot-holes' are of frequent occurrence; some of these extend downward into the very substance of the chalk to the depth of from 20 to 30 ft. This wasting of the chalk is due, not to any mechanical force, but to a powerful chemical eroding action arising from the presence of carbonic acid in the water, and this chemical action was doubtless a very active agent in deepening our valleys. The rain that falls upon our downs by contact with decaying vegetable matter becomes charged with carbonic acid, and, in consequence, acquires the property of acting as a solvent of chalk or any other calcareous rock with which it may come in contact. Nor is the quantity thus removed year after year inconsiderable, for every gallon of spring water, in a chalk district, contains about seventeen grains of carbonate of lime, and by calculation it has been found that in each square mile of such a district upwards of one hundred and forty tons of chalk are thus dissolved, and carried away by our rivers, annually.[1]
"But when due allowance has been made for the great excavating power of the Quaternary rivers, there still remains the fact, that, since the Palæolithic implements found in the valley-gravels were fashioned, a period has elapsed of sufficient duration to deepen our Wiltshire valleys some 70 or 80 ft.
"There are also other indications of the antiquity of the Fisherton beds, and consequently of the flint implements found in them. 'Rivers in a state of flood, or passing even at a moderate speed over soft or incoherent soil, are always turbid, owing to the presence in their waters of earthy matter which they are transporting towards the sea.'[2]
"This solid matter, being only held in suspension by the velocity of the current, sinks to the bottom, and forms shallows or banks, when the waters from any cause become still. Here and there, at the sides of our Wiltshire valleys, are patches of brick-earth which were deposited, in the manner described, by the Quaternary rivers.
"The streams of the Avon and Wiley unite at Fisherton, and the very extensive deposit of brick-earth that occurs there indicates the position of the still water which in the Quaternary period existed behind their actual point of union. This brick-earth attains a maximum thickness of no less than 30 ft. in Harding's and in Baker's pits. And yet this deposit is evidently not the result of cataclysmic action, for delicate and minute shells perfectly uninjured occur throughout the deposit. Moreover there is distinct lamination in the strata, showing that if was a
- ↑ Evans, "Ancient Stone Implements," p.429.
- ↑ Ibid., p. 583. If the velocity of a river be 300 yards per hour, it is sufficient to tear up fine clay; if 600 yards, fine sand; if 1,200 yards, fine gravel; and if a little over two miles per hour it is capable of transporting shivery angular stones of the size of an egg.
