4. — On the Chalk of the Southern Part of Dorset and Devon*. By William WHITAKER, Esq., B.A. (Lond.), F.G.S., of the Geological Survey of England.
As my rambles through Dorset and Devon (in 1867-68) were made from east to west, the same course will be followed in transcribing my notes, a course that will also have the advantage of starting from the point nearest to the Isle of Wight, the Chalk of which has been described in a paper of which this may be taken as a continuation†.
At the northern side of Swanage Bay, where the rocks are almost vertical, the Upper Greensand, consisting of green-grey sand with layers of nodular stones, is capped by evenly bedded Chalk Marl, made up of alternations of lighter-coloured thicker and harder beds, with darker thinner and softer, and forming a sort of ridge-and- furrow foreshore, as in the Isle of Wight. The Chalk Marl has a thick grey bed at top, and seems to be about 60 feet thick. It is succeeded by hard bedded Chalk without flints, which again is soon succeeded by a thin layer of the Chalk-rock, hard, with the usual irregular-shaped green-coated nodular lumps (chiefly at the top) and iron-pyrites. Above this is Chalk that weathers to a rough surface, and higher up contains flints. Further east, at the highest part of the cliff, the Chalk is less rough, and not so full of flints as in the Isle of Wight.
I was not able to get at the section between Ballard Hole and the Foreland ; but enough has been already written on that part‡. I may remark, however, that two of the isolated pinnacles of Chalk still have a little turf on the top, and so show the former continuation of the land-surface, with its smooth sloping contour, due to subaerial denudation, and greatly differing from the abrupt cliff against which the sea washes. The cliff does not cut through the highest part of the escarpment, but seems here to be along the flank of an old pass or gap.
In Studland Bay the junction of the Beading Beds and the Chalk is piped ; but this is hardly- enough to prove unconformity between the two formations.
At the gap in the escarpment between Ballard and Nine-Barrow Downs the almost vertical bedding is marked in part by distinct even and parallel lines in the turf, caused by difference of growth on harder and softer beds.
A small pit on the flank of the escarpment about a mile and a half eastward of Corfe Castle shows a northerly dip of about 60° in the following beds : —
- The district referred to is represented in Sheets, 16, 17, & 22 of the Map
of the Geological Survey of England.
† Quart. Journ. Geol. Soc. vol. xxi. p. 400.
‡ Rev. W. D. Conybeare, ' Outlines of the Geology of England and Wales,' p. 110 (1822) ; Rev. W. B. Clarke, Mag. Nat. Hist. vol. x. pp. 414, 461 (1837) ; Dr. J. Mitchell, ibid., p. 587 ; T. Webster in Englefield's ' History of the Isle of Wight. '
