354 



B. MACKINTOSH ON HIGH-LEVEL MARINE DRIFTS. 



blown sand, and, when I last saw it, was in course of being blown 

 by the wind, from the brink of the quarry-excavation, over the grassy 

 flat above. On the right-hand side of the tunnel, where there is no 

 clay above the sand, the latter presents a peculiarly involved 

 appearance. 



On the left-hand side of the tunnel (1880) the sand inter- 

 laminated with gravel had evidently been contorted to a great extent. 

 The shell-fragments principally occurred in layers of fine gravel, in 

 one place chiefly made up of angular flint chips. There the average 

 thickness of the sand and gravel was about 10 feet, the clay above 

 being about the same thickness. When Darbishire saw these drifts 

 about thirty years ago the clay was only about 1 ft. 9 in. thick. In 

 different quarry-excavations at different levels up the hill-side, sections 

 more or less differing in their character have been revealed. In 

 1880 the brink of the drift-cliff was only about 30 feet below the base 

 of the abruptly rising rocky crest of the hill ; and it is more than 

 probable that the crest once rose as an isolated mass of rock above 

 the surface of the glacial sea. The shelly sand and gravel (so far 

 as aneroids can be trusted) extend up to quite 1350 feet above 

 the present sea-level ; and they have been found as low down as 

 1170 feet. 



5. Position of Boulders. — One may go for miles along a railway- 

 cutting without seeing a single boulder in situ ; but where a large 

 clay- or gravel-pit has been excavated by the side of the railway, 

 many boulders may be seen on the floor of the pit, the explanation 

 being that boulders exposed in a railway-bank very soon tumble 

 down and are covered up, blasted, or removed. It is therefore wrong 

 to conclude that because there may be many boulders on the floor 

 of a quarry in drift-covered rock, they were (as boulders) originally 

 situated at the base or towards the base of the drift-deposits. It is 

 probable that many of the boulders of the Moel-Tryfan deposits 

 which now lie on the quarry floor, were once dispersed at various 

 levels in the overlying drift ; and in 1871 I saw a boulder at least 

 3 feet long high up in the sand. This accords with the fact that at 

 lower levels, including Anglesey, very large boulders may sometimes 

 be found imbedded in sand or fine gravel containing shell-frag- 

 ments, as well as in clay. 



6. Bent and shattered Edges of Slaty Lamince. — In 1871 I noticed 

 that the edges of the vertical slates were bent in the direction of 

 Mynydd Mawr, or from about N.W. to S.E. Darwin, as already 

 stated, observed somewhat similar phenomena in 1842, which he 

 attributed to the impact of floating-ice. In 1880 I happened to see 

 what may be called a magnificent display not only of the bending 

 but likewise of the shattering of the edges of the slaty laminae, and 

 of the extreme contortion of the laminae of sand by which the slates 

 were covered (fig. 1) . From the section (which from the quarry upper 

 floor to the top of the clay is at least 20 feet thick) it is perfectly clear 

 that the sand and overlying clay must have been deposited before 

 the derangement of the clay, sand, and slaty laminae took place. 

 The only explanation which appears sufficient to account for this 



