CHAPTER XII. GEOLOGICAL EXCURSIONS ALONG PART OF THE SOUTHERN COAST OF DORSETSHIRE. VOYAGE FROM YARMOUTH TO SWANAGE-SWANAGE BAY-THE PURBECK STRATA-FOSSILS OF THE PURBECK STRATA THE PORTLAND OOLITE GEOGRAPHICAL DISTRIBUTION OF THE STRATA-THE SOUTH-EASTERN COAST OF PURBECK-THE SOUTH-WESTERN COAST --WORBARROW BAY-LULWORTH COVE-FOSSIL TREES-DURDLE COVE-CHALDON DOWNS-RINGSTEAD BAY-THE ISLE OF PORTLAND -PETRIFIED PINE-FOREST-FOSSILS OF PORTLAND AND WEYMOUTH -RETROSPECT-COROLLARY. VOYAGE FROM YARMOUTH TO SWANAGE.-At the distance of about fifteen miles across the channel, from the western extremity of the Isle of Wight, the south-eastern part of the county of Dorsetshire forms a peninsula, which is called the Isle of Purbeck. This district is of an irregular oval form, its greatest length from east to west is between thirteen and fourteen miles, and its average breadth from north to south, seven miles. It is composed of cretaceous, Wealden, and oolitic deposits, and a brief notice of its geology will form an interesting and highly instructive sequel, to our survey of the corresponding strata in the "beautiful island." At Yarmouth a cutter may be engaged to convey the visitor to Swanage, a little town on the eastern coast, situated near the once celebrated quarries of Purbeck marble. But during a great part of the year, steampackets ply twice or thrice weekly from Portsmouth and Southampton, to Poole and Swanage, calling for passengers at Ryde, Cowes, and Yarmouth; so that the Isle of Purbeck may be included in our geological excursions with but little inconvenience. This short voyage is very delightful in fine weather, for with a telescope the principal lines of strata may be traced along the distant coasts, and a comprehensive view obtained of the relations, and original continuity, of the now dissevered masses which form the dry land. In sailing from Yarmouth towards the Isle of Purbeck, our course, after passing by the Needles, is to the west, and the chalk cliffs of Dorsetshire soon become distinctly visible ; and we at once perceive, that if the vertical chalk range we are leaving behind were prolonged in a direct line to the west, it would unite with that towards which we are steering our little bark. Mr. Webster has graphically described the most striking features of the varied landscapes which successively appear in this voyage: "The Island we had just left assumed a very interesting appearance. The gradual rise of the northern side above the sea-the coloured strata of Alum Bay—those singular masses of rock the Needles with the majestic chalk cliffs behind them of the most dazzling whiteness—and the precipitous face of the southern side of the downs above Freshwater-composed altogether a picture of the most romantic character. On the north lay the coast of Hampshire (see p. 119), consisting of tertiary clay cliffs, above which were seen the town of Christchurch, and several lesser places. As we approached the Dorsetshire side, I perceived that the chalk cliffs were of great height, forming the eastern end of the promontory called Handfast Point; on the north side of which lay Studland Bay, and on the south Swanage Bay. In these cliffs, which present a vertical section of the Isle of Purbeck from north to south, were seen, while we were yet some miles distant, several very large caverns; and towards the north end four or five detached masses of rock rising out of the sea; these were evidently isolated portions of the chalk, which like the Needles had resisted longer than the rest of the cliff, the destroying effects of the waves. When sufficiently near, I made the vessel lay to, that I might examine the stratification by a telescope; and I was surprised and delighted to find the south end composed of strata of chalk and flint quite vertical."* Swanage Bay. Punfield. Ballard Chalk Downs. Handfast Point. N. LIGN. 28.-SECTION FROM BALLARD DOWN TO DURLSTONE HEAD, ACROSS SWANAGE BAY. S. Durlstone Bay. SWANAGE BAY.-We land at the little town of Swanwich, or Swanage, where excellent accommodation may be obtained, and vehicles and boats hired for excursions by land or water. The exportation of stone is still so considerable as to give the place an aspect of cheerful activity. The cliffs that extend across Swanage Bay, expose a transverse vertical section of the strata from north to south, and afford a key to the geological structure of the Island; as shown in lign 28. In this line of coast, the entire series of deposits from the Chalk at Ballard Downs, to the Portland oolite at Durlstone Head, is displayed in the natural order of succession. This section will remind the reader of that from Shalcomb to Brook Point (lign. 18, p. 160); but in the latter, only the upper Wealden beds appear on the south, the lower group not being exposed to view. The general dip of the strata is to the north, but the angle of inclination is not uniform, and some of * Sir H. Englefield's "Isle of Wight," p. 164. the cretaceous and Wealden deposits present flexures and contortions of a very remarkable character. Before we stroll along the shore to collect specimens, and examine the strata in detail, it is desirable to engage a boat, and sail across the bay, that we may obtain a general idea of the geological structure of the country. From Swanage the cape or headland of Handfast Point, a chalk cliff 350 feet high, appears on the north; and to this spot we will direct our course. Doubling this promontory we enter Studland Bay,* and perceive that the coast is formed of the upper chalk, which emerges from beneath the eocene strata on the north at a very low angle, and continues round the extreme point of the cape, where several pinnacles, that may be regarded as representative of the Needles, guard the coast. A little to the south of this point, the layers of chalk and flint, which are as strongly defined as in Culver Cliff (p. 244), suddenly turn upwards in a curve, forming nearly a quarter of a circle, and abut against vertical strata of the same kind; "the vertical layers of flints meeting the bent part of the chalk, as so many ordinates would meet a curve, and decreasing in height as they get more under it." (See lign. 28, Handfast Point.) This appearance, as Mr. Conybeare has clearly shown, is attributable to a fault or dislocation, which in part of its course coincides with the lines of stratification, The chalk in the curved and vertical strata is extremely indurated, but in the horizontal, is of its usual consistence. The vertical flints, like those in the Isle of Wight, are, for the most part, shattered (ante, p. 147), and the fragments separated from each other by the chalk. These flints do not fall to pieces in the hand, like those of Freshwater, Culver, &c., because the severed portions are firmly held together by the chalk in which * If the section, lign. 28, were prolonged, Studland Bay would appear on the north of Ballard Downs. "Geology of England and Wales," p. 110. they are imbedded; but there is the same variety in the size and form of the fragments, from large angular pieces to fine powder.* Southward of Handfast Point, near the foot of Ballard Down, there is a small cove or recess termed Punfield, in which strata of Firestone, Galt, Greensand and Wealden clay, are seen in a highly inclined position. The middle series of the Wealden deposits, the Hastings Sands, which are not visible in the Isle of Wight, form the line of cliffs from Punfield to Swanage. These beds consist of sands, clays, and calcareous grits, enclosing seams and disseminated masses of lignite, and containing bones of reptiles, &c. The sea-shore, as in Brook and Sandown Bays, is often strewn with fossil trees, and rolled bones of the Iguanodon and other animals, that have been washed out of fallen masses of the strata. The tract on which the town of Swanage stands, is on the line of junction between the Hastings beds, and the Purbeck group; the latter comprises clays, sands, and limestones, with bands of shelly marbles, calcareous shales, and coarse limestones, full of small paludinæ, cyclades, and other freshwater shells. The spaces respectively occupied by these deposits, as measured along the foot of the cliff, are given by Dr. Fitton as follow: *These appearances were first pointed out by Mr. Webster, whose descriptions will be found as faithful as they are lucid, by those who visit the scenes he has so exquisitely portrayed in the beautiful illustrations of Sir Henry Englefield's work. These phenomena may be most easily examined on the south side of the cliff at Handfast Point, where the shore is generally strewn with large masses of fallen chalk. |