Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/895

 a tributary of the Mersey, can be proved historically to have been once the chief estuary of that river. The southern bank of this gorge extends westward to the river Dee, forming an abrupt termination to the various ranges of hill and valley, which, coinciding with the strike of the component Triassic rocks, run in a series of north and south parallels. Most of these valleys are traversed by faults, also running in a northerly direction, which, generally throwing hard rocks against soft, have caused the initiation of the lines of denudation of the latter, the soft Keuper marls and upper mottled sandstones being denuded away, while the hard beds of the Keuper sandstone and Bunter pebble-beds form long lines of escarpment, whose steepest sides face the western gales. This, however, as has been pointed out by Mr. Hull, is the characteristic feature of the Triassic scenery of this part of England.

With the exception of the crests of a few of the highest hills, the whole district is covered with glacial deposits — the slopes of the hills, equally with the bottoms of the valleys and plains, and even the bed of the Mersey itself.

The low cliffs of Triassic pebble-beds between Eastham and the mouth of Bromborough Pool are capped with a thin coating of glacial deposits ; at one point the Upper Boulder- clay is seen resting on some sand, which appears to represent the middle drift, about three feet in thickness, resting directly upon the rock, the Lower Boulder-clay being absent. On the Lancashire side of the river, opposite Eastham, the glacial deposits come down to the level of the beach ; it will therefore be seen that the cliff on the south (or Cheshire) side of the river, capped with drift, must have been formed since the glacial epoch, the river now flowing through an old preglacial valley since filled up with glacial debris and reexcavated by the present river Mersey.

The Middle-drift sand at Egremont thins out in the direction of Liscard, probably terminating against the concealed cliff, or, to speak more correctly, the slope of the old valley. No Boulder-drift appears to be found in the bed of the Mersey between Egremont and Liverpool ; and the rocks form a low terrace in the lower part of the town ; indeed a portion of it is reclaimed from the river itself. In this low district, however, certain postglacial deposits occur, hereafter to be described. These are about 20 feet thick, and rest directly upon the rock.

I have described the marine Lower Boulder-clay as probably formed in water of a maximum depth of about twenty-five fathoms, in unequal heaps of deposition, upon a sloping sea-bottom — its surface, below the base of the Middle Drift, rising from 5 to 15 feet per mile from the sea towards the watershed. On its surface the Middle Drift is piled up to a thickness often of 60 and even 70 feet ; but at elevations of about 300 feet above the sea it generally rests upon rock, above that occasionally on the stiff blue-coloured Lower Boulder-clay. It is chiefly composed of sand, with beds of gravel dovetailed, so to speak, into the mass. The gravels are much current- bedded, apparently by a current moving from the north-west to the