river. It is difficult to account for its occurring in this position, unless we are to suppose that at an early period before the complete denudation of the Fen country, and while the Boulder Clay still covered the surface of the Chalk, and the level of saturation was higher in the latter than at present, a tributary stream, possibly the old representative of the Lark, flowed into the Little Ouse near this spot, and the gravel was deposited on the tongue of land near the confluence. The country drained by the Little Ouse seems at one time to have been almost covered by Glacial deposits, including beds of shingle, composed for the greater part of quartzite pebbles. The beds at Brandon Down are nearer the sea than any analogous beds towards the source of the stream, and occupy a higher position relatively to the existing river, being 90 feet above it. If they resulted from river-action, they would, in accordance with the hypothesis, be among the oldest of the river- deposits; and would, as indeed they do, consequently contain a far larger proportion of the quartzite pebbles than those of somewhat later age and farther up the valley.
At Bromehill, where the drift is but a few feet higher than the present level of the stream, and would, in accordance with the hypothesis, belong to a later period, there are but few of these quartzite pebbles, but the gravel contains a very large proportion of rolled fragments of chalk, which, so far as I have observed, are absent in the probably older beds, at Brandon Down; the implements also are frequently much rolled and water-worn. This fact is also in accordance with the hypothesis, for the river at the time of the formation of these lower beds would, in the lower part of its course, have completely cut through the Glacial deposits above the Chalk, and would have been attacking the Chalk itself. There is also an abundance of rolled chalk in the Shrub Hill beds, which seem to be of much the same age. In the valley of the Lark, the rolled chalk pebbles occur in gravels at a somewhat greater elevation. Higher up the Little Ouse, the gravel at Santon Downham occupies the slope of a hill on the inner side of a great sweep of the river, while at Thetford, the beds form a long terrace by the side of the stream, with a rather abrupt slope towards it. Here also, land and freshwater shells have been found in the gravel, but neither these nor implements have as yet been observed in the gravels of the valley of the Little Ouse, or of its tributaries, above Thetford.
Tracing the main stream back to its source, we find that both
