SUBMERGED PLATEAU SURROUNDING CEYLON. 69 



THE SUBMERGED PLATEAU SURROUNDING CEYLON: SOME 



CONSIDERATIONS REGARDING THE FORMATION OF 



THE COAST LINE. 



By Commander Boyle T. Somervillb, R.N. 

 (Late of H.M.S. " Sealark.") 



With Diagrams. 



I. — General Remarks. 



nnHE Island of Ceylon has the peculiarity of being surrounded by a 

 -^ submerged plateau, which extends to an average distance of 

 twelve miles from the land. The edge is strongly marked : first by 

 a drop, in most places of 400 feet in about 200 yards, and then of 

 3,000 feet more in two miles ; the oceanic depth of 6,000 feet (or 

 1,000 fathoms) being reached at about eight miles beyond that 

 again. (See figs. I., II., and III.) 



To the northward the plateau merges into that surrounding the 

 south-eastern coast of India, off which a similar formation prevails ; 

 but to the southward it gradually deepens, still preserving its shelf - 

 like form along the eastern, western, and southern coasts of the 

 Island, and is both narrowest and deepest (as regards soundings) in 

 the vicinity of Dondra Head. 



The 100-fathom line, which may be taken as the outside edge of 

 the plateau, follows the general trend of the coast line ; but off the 

 eastern side of the Island there are several remarkable deep and 

 narrow notches, the two most notable of which are off Trincomalee 

 (Koddiyar Bay) and five miles north of Batticaloa respectively, 

 where the point of the deep water approaches to less than a mile off 

 the shore in the first instance and to within two miles in the other. 



I am miable to speak definitely of the rest of the plateau, except 

 of that part which occurs within the limits ri the " Sealark's " 

 survey during the last two years, as represented in fig. I., and in 

 that area but one notch occurs, namely, off Panadure, where the 

 100-fathom line apiDroaches the coast to within about nine miles. 



In fig. I. it will be seen that the plateau shoals progressively 

 from the south towards the north. The average depth at the 

 southern part is about 36 fathoms, and this continues to abreast of 

 Barberyn lighthouse, at which point a gradual shoaling begins, 

 until, just northward of Colombo, the plateau is covered by only 

 20 fathoms of water. 



L 7(2)08 



