PHYSICAL FEATURES OF THE DISTRICT.

The Purbeck Hills, commencing at the chalk cliffs of the Foreland, form at Handfast Point the south-easterly promontory of Studland Bay. Their range proceeds from this point W.S.W. for rather less than a mile, forming the high chalk cliffs of the eastern half of Ballard Down so far as Punfield Cove, where they bound the northern side of Swanage Bay. At Punfield Cove the Purbeck Hills strike inland, continuing as the high ridge of Ballard Down, which rises to a height of 528 ft. About two miles from the Foreland their range curves W.N.W., and thence extends in a wide sweep ten miles in length, having its concavity to the South and terminating at Worbarrow Bay. This is a point one mile beyond the stream known as Luckford Lake, which is considered geographically the western boundary of the Isle of Purbeck. The eastern portion of the ridge is known as Nine Barrow Down, which rises to s height of 655 ft. This terminates at about the centre of the range at Corfe Gap. West of this, the ridge is known as Knowle Hill, 481 ft. in height. Two miles from Corfe, on the northern flank of the hill, is the well-known eminence of Creech Barrow, which in height is only slightly less than the Purbeck Hills. Geologically Creech Barrow is of extreme interest, as it consists of Tertiary beds, probably of Bagshot sands, which have been shown by the late Mr. W. H. Hudleston to be there capped by limestone of the Oligocene age. From this we conclude that the Tertiary beds were formerly overlain by Oligocene strata, which now occur nowhere else in this district. In Section I., Plate I., their probable position has been indicated in the extreme right of the diagram. As Creech Barrow does not come strictly within the subject of this paper, it will not be further considered.

At Creech Barrow the ridge of Purbeck Hills bends slightly southwards, about W.S.W., and is there known as Flowers Barrow, which is 567 ft. high, thence it terminates geographically in Warborrow Bay.

GEOLOGICAL STRUCTURE OF THE PURBECK HILLS.

The geological structure of the Purbeck Hills can best be studied at Ballard Cliffs, and especially the adjacent coast of Studland Bay. The great cliff section at Ballard Downs shows the structure to consist of a central axis of nearly vertical chalk strata which projects upwards, so that its edges form an escarpment near the summit of the hills. On the northern slopes this vertical edge is flanked by a sheet of chalk beds which incline rapidly from the vertical to the horizontal, where they are overlain by Tertiary strata. On the southern slopes of the chalk ridge is a thin strip of the edges of Greensand strata comprising Upper and Lower Greensand, and Punfield beds, which strata are almost vertical, conformably with the chalk. These are succeeded by Wealden clays, which, being highly inclined against the Greensand at first, become gradually horizontal southward, and form the lowest slopes of the southern flanks of the hills. This structure is shown in each of the three sections in Plate I.

The disposition of strata on the northern slopes of the hills can best be studied in Studland Bay, from the opening of the little dell, whence Studland village is approached, to near the Foreland. From the former spot Bagshot sands occupy the coast line. These are the lower beds of the Bagshot series, and are of fluviatile origin, although here unfossiliferous. They consist of variegated and coloured sands, with thin partings of clay, dipping at a slight angle of 5° to 7° N.E., and are continued to a point about 400 yards beyond the little coombe leading to the village, where they are replaced by London Clay. This in a short distance gives place to Woolwich and Reading beds, which here consist chiefly of white sand, some 40 ft. thick, with a bed of flint gravel 6 to 10 ft. thick between it and the London clay. The Woolwich and Reading beds occupy some 100 yards of the foreshore, which here curves round to the east. At a point half a mile from the Foreland the chalk appears at the shore level. Its upper surface, on which the

Tertiary beds rest, is deeply piped and eroded, as it is in the same point of junction in Alum Bay, with which it was formerly continuous, before the sea broke through the chalk ridge between the Needles and the Foreland to carve out Bournemouth Bay. The Woolwich and Reading beds are here described by Mr. Strahan (Memoirs of the Geol. Survey, Isle of Purbeck) as consisting of hard brown grit with rolled flints and some red or mottled clay. This description also well suits the same beds lying against the chalk in Alum Bay. The Tertiary beds here commence ascending from the shore on the chalk, and some 250 yards to the east they thin off at the top of the cliff, which, beyond this point, consists wholly of chalk. The chalk here consists of the upper beds of the Belemnitella Mucronata zone, having the usual bands of black flints. As far as the Foreland the beach follows the line of strike of the beds, which are nearly horizontal, and continue so until near the great fault. The right-hand portion of the section in Fig. 1, Plate I., illustrates the strata here described, and is typical of the structure of the northern flanks of the Purbeck Hills throughout their range, though nowhere shown so well as in Studland Bay.

THE SECTION AT THE CLIFFS OF
BALLARD Down.

This magnificent section cuts the southern half of the Purbeck Hills nearly at a right angle, giving as perfect a view of their structure as could be desired, and completing the view obtained in Studland Bay. From the Foreland or Handfast Point, vertical chalk cliffs extend S.S.W. for a mile and a-quarter, increasing gradually in height from 50ft. at the point, to 250ft. at Ballard Head. From Ballard Point to Punfield Cove the cliffs turn west for 400 yards, the chalk there giving place to Punfield and Upper Greensand beds. From the Foreland southwards the strata, corresponding with the beds on the other face of the cliff in Studland Bay,

are nearly horizontal, dipping about 8° a little west of north; but half way along the section they suddenly bend upwards in a magnificent curve, forming a quarter of a circle at the great fault, which here interrupts their continuity. This fault is indicated in Plate I., Fig. 1., but this section, being taken some distance from the cliffs, does not show the structure of the fault, which, however, is exhibited in Plate III., Fig. 3. So many excellent photographs of these cliffs and the fault have been published in various works that we have not thought it necessary to reproduce one; but the best is probably that in Dr. Rowe's "Zones of the White Chalk of the Dorset Coast," p. 35. On the southern side of the fault the beds are vertical, but they gradually decline from this angle to their junction with the Punfield beds. Thence are found in succession the Upper Greensand, Gault (which, however, cannot be identified distinctly) and the Lower Greensand, which are found in some 220 yards, each dipping N. at a gradually diminishing angle, until succeeded by Wealden strata which, at half a mile from Punfield Cove, appear to have escaped the disturbing effect of the fault, and resume the nearly horizontal level of the chalk north of of the rupture towards the Foreland. The total thickness of the chalk in this section has been estimated at 1,300 ft. It gradually decreases throughout the range of Purbeck Hills westward, being 1,200 ft. at Lulworth, beyond the Purbeck area, and about 500 ft. at Blackdown, where the chalk vanishes to seaward. The thicknesses of the three great divisions of the chalk in this section are given by Dr. Strahan as upper chalk,1,049 ft.; middle chalk, 102 ft., and lower chalk, 149 ft. The chalk rock, Melbourn rock, and chloritic marl, which severally divide these sections, can each be traced in their usual positions, and the zones into which each division is subdivided have been exhaustively worked out and described by Dr. Rowe in his paper quoted above. Reference to the three diagrams in Plate I. will show that the geological structure here described is continuous throughout the range of the Purbeck Hills.