1870. ] DE RANCE—GLACIAL PHENOMENA. 647 
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 ex- 
tending 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 ordi- 
nary 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 wiih: it 1s 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 de- 
posits 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 
* “Drift Deposits near Manchester,” Trans. Man. Lit. Phil. Soe. vol. ii. (8rd 
series), p. 457. 
VOL. XXVI.—PART I. 3B 
