Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/903

 rather less than 20 fathoms. The coast-line, especially near the base of the Cumberland mountains, appears to have been surrounded by an ice-foot, which in winter not only caught up the beach formed in summer and before its formation, scratching the pebbles in every direction as the ice was lifted by the tide, but received on its surface vast quantities of lake-district pebbles, and borders, brought down from the interior by land-ice, which, at the breaking up of the icee foot, were spread in confused heap-like masses over the Lancashire and Cheshire lowlands, forming the marine Lower Boulder-clay.

5. Middle Drift.— That at the close of the period of deposition of the Lower Boulder-clay, the climate ameliorated, the subsidence of the land still continuing, the influx of muddy sediment ceasing, owing to the cessation of glaciers grinding the rocks on the land, and that of sand commencing, owing to the pulverizing of pebbles by the action of breakers on the coast-lines of the Middle-drift sea.

6. That the sand and shingle of the Middle Drift, though found at all elevations from 40 to 100 feet between Blackpool and Preston, to 1200 feet on the Buxton Road near Macclesfield, was everywhere deposited in comparatively shallow water of the same depth, being deposited round the ever-sinking coast-lines, on higher and higher ground, in the form of sand-banks, whose crests mark the level of the mean high water of the immediate period of their deposition, — the sinking of the land causing the present elevation of these crests to gradually rise from west to east, or from the sea to the Penine chain, and everywhere (in West Lancashire) to show a marked uniformity of level in a north and south direction. In the Manchester district, owing to the curving round of the high hills of the Penine chain, the Glacial sea extended further east than in West Lancashire, though it does not appear ever to have passed over the ridge dividing it from the Yorkshire area, the eastern edge of which ridge is shown in fig. 1 of Mr. S. V. Wood's paper on the " Boulder-clays of Yorkshire," in the February number of the Quarterly Journal.

7. That if the Middle Drift was thrown down in the form of sand-banks surrounding a gradually subsiding coast, it follows that though the Middle-drift beds at Blackpool, at an elevation of 80 feet, and the Middle Drift at Macclesfield at 1200 feet, must have been formed during one set of conditions, and in the same geological epoch, yet a considerable period of time must have elapsed between the formation of the two deposits ; or, in other words, the Macclesfield beds must be newer than those of Blackpool.

8. That the extreme lamination and current-bedding in the Middle Drift would, without the littoral character of the shells found in it, point to shallow- water conditions; but this current -bedding is often so intense as to preclude the idea of its being entirely due to ordinary tidal streams, and to suggest rather the sudden currents which would be caused by the occasional melting of ice altering the temperature of the water. That ice was occasionally present in the Middle-drift sea is proved by the fact that beds with scratched pebbles are occasionally found in it, both in the Blackpool, Preston, and Chorley districts ; and the contorted and folded Middle Drift