Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/912

 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 found 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 surface 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 Tarleton. 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 versa, 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 fragments of marine shells, apparently Cardium edule and Turritella terebra, and in one instance a perfect freshwater Limnoea ; 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