662 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 22, 
the case of the Birket plain in Wirral), leaving a second cliff formed 
of Boulder-clay, in some places several hundred yards in front of that 
of preglacial age. The Boulder-clay cliff itself, however, is seldom 
visible, being concealed in the district between the rivers Alt and 
Douglas by a range of old sand dunes, formed along the postglacial 
sea-margin, as the land began to rise, after the formation of the clay- 
cliffs referred to. This sand is at once distinguishable from the sand 
of the modern sand dunes by its large grain, and by the absence of 
black specks of some hard substance invariably “ound in the latter. 
I have called it the ‘“ Shirdley-Hill sand,” and have described it at 
length in the Geological-Survey description of the quarter sheet 
between Liverpool and Southport. These old sand dunes form a line 
of more or less detached hillocks (fig. 3), the chief of which are known 
as Pye Hills and Shirdley Hill, as well as hills in Haskayne, near 
, Hightown, and several places in Halsall. In the latter parish, near 
_ the village, the sand forms the floor of the peat, which varies from 10 
_ to 2 feet in thickness, resting on about 10 feet of sand, which contains 
' many small pebbles, and occasionally a few marine shells. Its sur- 
face is here about 4 feet lower than the base of the old sand dunes, 
and must therefore be the actual strand of the postglacial sea when 
it covered what is now the peat-moss plain. The sand can be traced 
northwards as far as a hill, or bank of Boulder-clay, about 50 feet 
high, occurring on the south bank of the river Douglas, near Tarle- 
ton. It can also be traced eastwards, along the low lines of this 
river, in the direction of Wigan, everywhere underlying the main or 
thick peat when existing as a marine deposit. A bed of sand occurs 
in the surface of the upper, or 125-feet, plain, in the neighbourhood 
of Aintree, Maghull, Orrell, and Ormskirk, which is apparently of 
the same age as the Shirdley-Hill sand, having every appearance of 
having been blown from*the line of old sand dunes referred to above. 
It is an extremely variable deposit, occurring at the top of a hill and 
not at the bottom, or vice versd, on the one side of a valley and not 
on the other, and often thinning from 16 feet to 2 feet in less than 
a quarter of a mile. On the upland plain it contains minute frag- 
ments of marine shells, apparently Cardiwm edule and Turritella 
terebra, and in one instance a perfect freshwater Limnea; and it 
‘invariably rests upon a bed of peat, varying from 4 feet to an inch 
in thickness. It would therefore appear probable that, after the 
elevation that heaved the surface of the Upper Boulder-clay into land, 
the sea-margin stood much further out in the Irish Channel. The 
gradual wearing back of the Boulder-clay cliffs, probably during a 
fresh subsidence, produced the low plains between the Dee and 
Mersey and Alt and Douglas, on the surface of which the Shirdley 
sand was deposited, which, in the latter plain, on fresh elevation, 
was blown into sand dunes, and from them far into the inland 
country. In the former plain the denudation was so complete, that the 
Boulder-clay cliff was worn back and back until all vestiges were 
destroyed; and the edges of the postglacial deposits rest against the 
base of the old preglacial cliff, which extends from the one river to 
the other. No sandy beds appear to be here present; but they were 
