FROM START POINT TO PORTLAND. 537 



equilibrium so that one kind is present at one place and others at 

 another. Looking at the matter broadly, we may say that wherever a 

 current is running we find stones, but where there is comparative calm 

 we find sand or mud. 



It has been shown that the Great West Bay is the centre of 

 reciprocating currents in the Channel ; further, that the main tidal 

 stream does not penetrate far into the bay, and consequently that the 

 current inside is less than one knot per hour at full and change of 

 moon, whereas outside it is over two. These causes combined bring 

 about a state of comparative calm within the bay, more especially to 

 the westward side and in the centre, and it is there we find the finest 

 bottom-soil of the whole region. 



At the head of the bay, i.e. off Beer Head, it was pointed out that 

 the tides set round a complete circle within the twelve hours, and the 

 tendency of the currents, therefore, is to carry away from this centre 

 whatever material may be movable. We thus find that along the 

 northern shore the ascent to the beach is comparatively rapid, i.e. 

 deep water approaches close to the shore, and, secondly, that the 

 ground is mostly hard and rocky, with a few patches of mud and 

 sand between the rocks. 



As we pass from the head of the bay along its two arms, it has 

 been shown that we meet with two currents, which gradually increase 

 in velocity and strength as they approach and finally pass round the 

 two extreme points. On the one hand, at Portland this current is 

 made up of the flood-tide and an eddy from the ebb, and is of great 

 velocity — greater than the outside main tidal stream ; on the other 

 hand, at Start Point the current arises from the ebb-tide and an eddy 

 from the flood, and is comparatively weak. Both these currents flow 

 in their respective directions nine hours out of the twelve, and their 

 constancy and duration are considered to be due to the heaping up of 

 the waters by the tide at the head of the bay. 



On the eastern arm towards Portland there can be little doubt that 

 the east-going stream has considerable influence in keeping the bottom 

 clear of sand and mud, and thus leaves the rocks bare and uncovered. 

 Along this arm the rocks of the coast project outwards as far as the 

 fifteen-fathom line. The seas raised by the gales from the south and 

 south-west break heavily on this eastern arm, but their influence is 

 mostly destructive, and the sand and mud would tend to return to their 

 former position were it not for the steady, though at the beginning 

 slow, current which is in reality the effective agent in the transference 

 of sand from west to east. It is to the excess of the easterly-going 

 current over that going to the west that Vaughan Cornish* ascribes 



* "The Grading of the Chesil Beach Shingle" [Proc. Dorset Nat. Hist. Field Club, 

 1898). 



