Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/897

 so on, rising to an elevation of 500 feet at Dunsear Bridge, being 225 feet in about nine miles, or at the rate of 25 feet per mile *.

It has already been remarked that in the Preston district the Middle Drift is generally found outcropping along the sides of the deep-brook valleys, and capped by Upper Boulder-clay; in addition to this it is occasionally found capping the tops of hills of various elevations, generally at far higher levels than in the banks of the adjoining brooks. When these patches of sand are examined, they are found not to be, as might be supposed, outliers of a sheet of Middle Drift once overspreading the country and resting upon the Upper Boulder-clay, but, on the contrary, they are " inliers" coming up through the Upper Till, forming knolls often of considerable elevation. But in addition to this the sand and gravel are found to form knolls, so to speak, under the surface of the Upper Boulder- clay ; or, in other words, the surface of the Middle Drift is a series of rolls and hollows filled in and covered up with Upper Boulder- clay, since denuded into other hills and valleys, which occasionally follow the old lines of hollows of the Middle Drift, in which case we have the latter at the top of the plain and the Upper Boulder-clay extending right down to the bottom of the valley, 50 or 60 feet beneath it. The level of the top of each knoll of Middle Drift, whether at the surface or found in pits or borings beneath the Boulder-clay, is found to dip steadily towards the west and south-west ; and the top of each is flattened, resembling in form a truncated dome.

When examining the very large sandbanks in the estuary of the Ribble, which are forming at the present time, I was much struck with the general resemblance to the knolls of the Middle Drift. The surface of these banks is a slightly undulating flat, rather hollowed in the centre, the level of the surface of each being a little below ordinary high-water mark, the action of the waves playing at the surface of the water preventing the deposition of the sand brought down by the Ribble and other rivers at the top of the banks, sweeping it into the narrow channels between the sandbanks, from which it is carried by tidal currents far out into the open sea, or cast up on the seacoast, and blown by the winds into ranges of sand dunes.

If we assume the sand-knolls of the Middle Drift to have been old sandbanks whose crowns were flattened by the sea, which appears only to take place a little below high-water mark, then as the level of the tops of these knolls rises inland or sinks towards the sea, it would appear that those occupying the lowest ground were formed first, and that as the land sank another series of banks were deposited, whose crowns were denuded by the waves ; these, again sinking below the action of the breakers, were succeeded by another and another series of banks, until the maximum elevation, in the neighbourhood of Macclesfield was reached. If these deposits were formed in this way on a gradually subsiding surface, the water would be of nearly the same depth at all the points of deposition during the whole time ; and thus, though Blackpool

series), p. 457.
 * " Drift Deposits near Manchester," Trans. Man. Lit. Phil. Soc. vol. ii. (3rd

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