which divides the Avon from the Bourne. These gravels consist of stones washed out of the geological deposits which occur UP-stream, the upper greensand and the chalk. Blocks of 'sarsen' and beach pebbles are also present, indicating the former existence of Tertiary beds which rested upon the chalk.
"The flints, which form such a considerable part of these gravels, are sub-angular, that is, they have not been subjected to sufficient rolling- action to reduce them to the condition of beach-pebbles, their general appearance being that of stones to be found in a river bed. Indeed, it appears that these gravels at Fisherton and Milford actually formed the beds of rivers which long since flowed in the same direction as our Avon and Bourne, and that the present valleys have been excavated to the depth of from 70 to 80 ft. by the eroding action of these streams.
"The excavating power of these rivers was doubtless formerly much greater than it is at present, and, in attempting to measure the requisite period which has elapsed for the excavation of these valleys to their present depth, this circumstance must be taken into account. From the character of the fauna, the comparative abundance of the reindeer, the presence of the musk-sheep, etc., we know that the climate of the Quaternary Period was of great severity, and, consequently, that the accumulations of ice and snow upon our Wiltshire downs must have been considerable.
"Probably, towards the spring of each year, torrents, like the Shrewton flood of 1841, resulting from a sudden and rapid thaw, swept down our valleys with almost resistless force. But another cause of floods has to be considered, and this is the formation of ground-ice, which played an important part in the transport of some of the larger blocks of 'sarsen,' and indeed of the gravels themselves. In rapid streams, when the water becomes sufficiently cold to reduce the temperature of the bed of the river to the freezing point, ground-ice is frequently formed. The gravel of the river-bed becomes coated with ice, and this ice, being lighter than water, after acquiring certain dimensions, rises to the surface, carrying with it large stones and the gravel to which it adheres. Ground-ice is a great cause of floods in the upper part of the Rhine and the Danube. These rivers have a rapid current, and do not freeze over their entire breadth, but large blocks of ice float upon the surface. These blocks are hurried along by the stream, impinge upon each other, become heaped together, and ultimately barricade the river. This accumulation of ice-drifts, however, is not itself the immediate cause of floods; these take place when a thaw 'commences in the upper part of the river, above the point where the latter is completely frozen, the masses of ice, drifting with the current and unable to pass, are hurled upon those already soldered together; thus an enormous barrier is formed, which the water, arrested in its course, cannot pass over, and hence overflows to the right and left, breaking the dykes, inundating the plains, and spreading devastation far and near.'[1]
"It is at such times that the ground-ice greatly increases the mischief, for, becoming detached from the bottom and rising towards the surface, it unites itself to the under side of the masses already in place, and renders the barrier more difficult to remove.
- ↑ Engelhardt, "Annales de Chimie et de Physique," 1866. Translated in "Smithsonian Report" for 1866, p 425.
