196 TRANSACTIONS OP SECTION C. 



the Carboniferous Limestone on the Welsh Border, where these strata were 

 first studied in the early days of stratigraphical research. It was here that 

 the limits of the Old Red Sandstone were originally fixed, and as a consequence 

 also those of the contemporaneous rocks of marine origin elsewhere deposited 

 which were a little later grouped together to form the Devonian. 



Types of Faults in the Coal Measures (Yorkshire and Cumberland). 

 By Professor P. P. Kendall and Dr. A. Gilligan. 



4. The Erosion of Bowrnemoidh Bay and the Age 'Oj its Cliffs. 

 By Dr. William T. Ord, F.G.S. 



The fii'st step in the erosion of Bournemouth Bay was the breaking through 

 by the sea of the range of chalk hills which were formerly continuous from the 

 Needles to Ballard Down. There is evidence that the gap occurred at a point 

 due south of Bournemouth pier. It is thought that the Swanage Eiver would 

 here have flowed through the hills to join the (Solent Eiver (which then crossed, 

 what is now the bay, from Poole Harbour to the Solent), following the examples 

 of the Corfe streams, and the Yar in the Isle of Wight, each of which cuts north 

 through the chalk. The break through must have taken a long period, though 

 once accomplished the friable Tertiary beds were rapidly washed out of the bay. 

 Dui'ing Miocene times the Hampshire* Basin was formed by upheaval of the two 

 southernmost chalk anticlines, as were the North and South Downs and Chiltern 

 Hills. The sea was then far to the south, and busy eroding the Purbeck and 

 Portland barrier, which still shows in fragments along the Dorset coast. The 

 Frome-Solent River flowed across what is now Bournemouth Bay, through the 

 Solent, to enter the sea at Spithead. Its tributaries were the Bourne. Stour, and 

 Avon, and others from the New Forest. During the Pliocene subsidence the sea 

 advanced to the southern flanks of the chalk anticline between Pui'beck and the 

 Island. Then occurred the Raised Beach Period, when the land was stationary 

 at 30 ft. above O.D. for a time sufficient to form the beaches, remains of which 

 are found at Portland, Torquay, Cornwall, and the Bristol Channel. It was 

 probably at this time that the sea first gained access to Bournemouth Bay 

 through the gap cut by the Swanage River. Rapid erosion occurred on both 

 sides, but chiefly eastward towards the Needles. Admiralty charts show a 

 shallow bank which indicates the base of the destroyed chalk ridge. The sea 

 destroyed the old drainage system, and received the Rivers Frome, Bourne, Avon, 

 and Stour directly. As it advanced it formed a low line of cliffs around the 

 bay some distance south of the present cliffs. This period closed by a re- 

 elevation of the land, when the sea receded to the south, leaving Bournemouth 

 Bay once more dry ; this probably continued during the Glacial Period which 

 immediately followed. There is nothing to show what changes occurred during 

 the Glacial Period, but we know that, towards the end, vast floods from melting 

 ice washed flint gravel from the high ground northward and deposited it in 

 sheets (containing palaeolithic implements) as the Plateau Gravels that cover our 

 cliffs to-day. The Neolithic Period was ushered in, according to Mr. Clement 

 Reid, by a re-elevation of some 80 feet. Since then a gradual subsidence has set 

 in, and the sea has again invaded Bournemouth Bay. Then grew the forests which 

 have gradually become submerged. Remains of these, far to the south of the 

 present pier, were observed by Sir Charles Lyell in 1825, and forty years pre- 

 viously by Mr. Fisher. A century ago Sir John Evans records that a stretch of 

 .swampy ground separated the foot of the cliffs from the sea. It seems then that 

 only within the last century has the sea reached the foot of the old line of cliffs, 

 which were formed in pre-Glacial times. As the talus which protected them 

 was washed away and the new and old cliffs became one, erosion would be more 

 rapid, and the cliff face more perpendicular. There is plenty of evidence to 

 confirm this in recent measurements by the Borough Engineer, which give 

 accurately the present rate of erosioii of the cliffs at various points. 



