THE 



aEOLOGICAL MAGA.ZINE. 



No. CXIII.— NOVEMBER, 1873. 



J. — 'On the Origin of the Estuaky of the Fleet in Doksetshire. 

 By the Rev. 0. Fisher, F.G.S. 



ME. MANSEL-PLEYDELL'S Memoir on the Geology of Dorset, 

 contained in the last two Numbers of this Journal (pp. 402 

 and 438), refers to a theory of the formation of the Fleet proposed by 

 Messrs. Bristow and Whitaker in 18G9 (Geol. Mag. VoL VI. p. 483). 



At the time of the appearance of that paper I felt that it was very 

 difficult to conceive how the small streams which run into the Fleet 

 could have excavated a channel between the Chesil Beach and the 

 land, especially when it is considered that their abrading power 

 must cease at the level of the Fleet water. A different explanation 

 of the matter then presented itself to me; and since Mr. Mansel- 

 Pleydell has recurred to the subject, I am induced to forward my 

 notions upon it to the Magazine. 



There can be no doubt that the formation of a beach, or spit of 

 shingle or sand, depends upon the set of the tides, as governed by 

 the configuration of the coast and the prevailing winds ; the con- 

 ditions of depth, and those for the supply of materials, being at the 

 same time favourable. In the present case it is the Isle of Portland 

 which mainly determines the position and form of the Chesil 

 Beach. The land on the opposite side of the Fleet has really nothing 

 to do with it, excepting that the sea-bottom shelving from it furnishes 

 the "fleetness, " or shallowness, necessary for the detention of the 

 pebbles. If this land were removed, the Beach would still remain 

 where it is, for it simply occupies the position where, on the whole, 

 the causes which bring up the pebbles, and the causes which sweep 

 them away, are equally balanced. 



If land were to be placed to the westward of the Beach, then, under 

 the present neighbouring configuration, the destructive agencies 

 would exceed the conservative, and the land would be washed awa}'-, 

 and the beach carried forward until it found a place of rest in its 

 actual present position. This is analogous to what I believe has 

 occurred. I look upou the Fleet as the eastern half of a submerged 

 valley, similar to, though on a larger scale than, that one Avhich now 

 forms the Weymouth Backwater; its former western side havino- 

 been encroached upon, and destroyed, by tlie waves of the West 

 Bay. 



If we could take up Portland, and plant it a mile or so to the east- 

 ward of the position it now occupies. I believe that the sea would 

 directly begin to advance eastwards, pushing the beach bef'oro it, 



VOL. X. — NO. CXIII. 31 



