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ARCHAEOLOGICAL EVIDENCE OF TECTONIC ACTIVITY (AND TSUNAMIS?) IN SRI LANKA
 
 
 
by S.U. Deraniyagala
Former Director-General of Archaeology
 
Investigations conducted by the Archaeological Department under the direction of the writer in 1969-72 revealed traces of prehistoric human activity within coastal alluvia. These were gravels, incorporating stone tools, which had been deposited at levels corresponding with the then existing sea levels. The gravels with artifacts are today at considerable heights above the present sea level: 15 metres at Bundala-Patirajawela dated to 125,000 years ago and 8 metres at Bundala-Wellegangoda dated to 80,000 years before the present. Occurrences of ancient coastal gravels inland around Mankulam and on Aruakallu at 60-80 metres and at Minihagalkanda in Yala at 40 metres have yet to be scientifically dated. This phenomenon of uplifted shores has been attributed principally to pressures generated by tectonic factors within the last one million years or less as indicated by the presence of man-made tools within the gravels.
 

Elevated coastal gravels (solid red) and ancient coastal dunes (stippled red)

The above-mentioned findings were presented in publications of the universities of Cornell and Harvard in the U.S.A. and the Archaeological Department (Deraniyagala 1976, 1988, 1992:101-104,606,608). They were contrary to the prevailing view that Sri Lanka is situated in a stable tectonic environment.

However, an article by Professor C.B. Dissanayake on South Asian plate tectonics, that appeared in the Daily News of 31st December 2004, geologically corroborated the geo-archaeological evidence, which had been secured some thirty-five years ago, that Sri Lanka is indeed in a region of potential tectonic instability. He states that studies by J. Cochrane at the Lamont-Doherty Earth Observatory indicate the splitting of the Indo-Australian plate about 300 km off the southeast coast of Sri Lanka (where Minihagalkanda is located). The present writer (1988; 1992:103) had also proposed tectonic instability along the Laccadive-Chagos line, approximately 300 km off the west coast of India. According to Professor C. Synolakis, recent investigations have indeed revealed that it is splitting (Daily Mirror of 17th January 2005). Moreover, at least some of the ancient coastal gravels mentioned above suspiciously resemble residues from past tsunamis: note the high-level unsorted gravels and boulders over an undulating landscape associated with deep amphitheatres carved out by the sea at Minihagalkanda (ref. Deraniyagala 1992:84-6).

Minihagalkanda, Yala: Miocene limestone (white/brown) overlain by coastal gravels at ca. 40 metres above present sea level, capped by ancient coastal dunes (red).

In view of these indications, it is a matter of urgencythat the rates of coastal uplift in Sri Lanka be investigated with state-of-the-art scientific methods. The locations for sampling have already been mapped out by the Archaeological Department and a scientist, Dr Mohan Abeyratne of the Central Cultural Fund, trained at the Australian National University in radiometrically dating the sediments. What is needed at present is the equipment for ‘optically stimulated luminescence’ testing. A project proposal has apparently already been submitted by Dr Abeyratne to the Atomic Energy Authority and an estimate prepared for procuring the equipment. It is now necessary to implement an enhanced version of it; and, since the dating techniques are being regularly upgraded, a back-up from international scientific institutions and consultants would probably be needed. This project could constitute an important component of the proposed Indian Ocean-rim earthquake monitoring programme. Since the said project is of direct relevance to risk assessment in developing the coastal regions of Sri Lanka, its rapid implementation should be a national priority.

Ussangoda, Ambalantota: ancient coastal gravels at ca. 15 metres
above present sea level

Bibliography
Deraniyagala, S.U.

1976 – The geomorphology and sedimentology of three sedimentary formations containing a Mesolithic industry in the lowlands of the Dry Zone of Sri Lanka (Ceylon): 1972. In Kennedy & Possehl (1976: 11-27).

1976 – Ecological backgrounds of South Asian prehistory. Cornell Univ.: South Asia Program.

1988 – The prehistory of Sri Lanka: an ecological perspective, 1st ed. Harvard Univ., Anthropology Dept.; Ann Arbor, Michigan: University Microfilms.

1992 – The prehistory of Sri Lanka: an ecological perspective, 2nd ed. Colombo: Archaeological Dept.
Kennedy K.A.R. & G.L. Possehl


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