THE GEOLOGY OF THE PURBECK HILLS. 145 



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 con- 

 tinue 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, 



