From
"Notes on the Geology of the Andaman Islands" by R.D. Oldham.
Rec. Geol. Surv. India, Pt.3 1885 135-145
Page 143 Click on figures for enlarged versions
"Since the publication of Kurz's Report on the vegetation of the Andamans, it has been an accepted fact that the Andaman Islands are, and have been, during recent times, undergoing subsidence. It was difficult to conceive how this could be the case, for the Arracan coast to the north and the Nicobar Islands to the south, between which the Andamans form the connecting link, are both fringed by raised beaches which show that they have recently been elevated, but the observations recorded by Mr. Kurz were so unanswerable that they were allowed to override the argument from analogy. Mr. Kurz's conclusions were based principally on the fact that he found the stumps of trees, belonging to species which only grow above high-water mark and beyond the reach of salt-water, in the mangrove swamps and on the sea shore, while, as corroborative evidence, he adduces the facts that according to the report of the Andaman Committee the sea had encroached some 40 or 50 feet since the first settlement on Chatham Island , Port Cornwallis, and that "Lieutenant Jameson of Chatham Island has informed me that a similar encroachment of the sea is taking place at that Island in Port Blair". As regards the latter point, there is no evidence that the "encroachment" of the sea at Port Cornwallis was due to subsidence, and as far as can be judged by the lithograph in the report of the Andaman Committee, and the woodcut in Dr. Mouat's book, both taken from a photograph, I should be inclined to look upon it as a case of encroachment by erosion of the sea shore and not by subsidence. The evidence of the trees is, however, almost certainly conclusive, for the only explanation possible, apart from an outward set of the soil towards the sea, such as is know to take place under certain circumstances, is that the land is sinking, and I can myself produce an observation which supports this conclusion. The large bay on the north-east coast of Havelock Island is for the most part fringed with low lying land, next to the beach this rises some 4 or 5 feet above high-water mark, but in many places behind this it sinks to form a hollow, and then rises again to the same level as the outer ridge, or rather higher. The whole of this low land is covered with forest, but wherever there is one of the hollows just mentioned, there the forest trees are all dead, and the soil is often moist with sea-water; the soil of these low lying patches must have once been dry, like that where the forest is still growing, and the uniformity in size of the dead and the living trees shows that in all probability several generations have lived and died on rise and in hollow alike, until, as the land gradually subsided, the sea-water rose in the soil of the hollows and the trees succumbed to its fatal influence.
We may therefore take it as proved that the Andamans are at the present day gradually sinking, but there is ample evidence in the raised beaches that fringe the shores of the Andamans, that in the immediate past elevation has exceeded subsidence. These raised beaches have not, so far as I know, been mentioned by any previous observer, but are conspicuous enough, especially on the islands of the Archipelago. At Port Blair itself there is a small terrace at the north-eastern corner of Chatham Island; it has been enlarged artificially to form a site for the bungalow of the officer in charge of the island, but appears to be in part a raised terrace of marine erosion. A similar terrace may be seen north of the harbour where the road from North Point to North Corbyn's Cove runs in places on a terrace separating the steep hill side from the sea shore. In a small cove east of Perij, on the southern side of Shoal Bay, there is a narrow terrace, clothed with forest, from whose inner limit the hill rises in a bare, almost vertical face of rock which could not have been formed, in this position at least, by any other means than marine denudation. Along the east coast of the South Andaman this raised beach can be seen forming a terrace from a few yards to over half a mile in width in almost every bay. Apart from the theory that it is raised sea beach, the only one supposition on which this terrace could be accounted for is, that it is due to a gradual encroachment of the land on the sea, either through the action of vegetation in catching the wind-blown sand, and so raising the surface above extreme sea-level, or by the action of the waves which during storms might throw up sand and shingle far above the level which the sea would otherwise reach; the first supposition is inconsistent with the fact that shells and fragments of coral of considerable size may be found lying about on the forest-clad surface of the terraces in positions where they cannot be accounted for by either human or crustacean agency, while the facts that the forest trees are as large and old on the extreme edge of the terrace as further inland, and that the seaward margin often cuts into a miniature cliff of 3 or 4 feet high, is everywhere fringed with falling and fallen trees and tangles of roots from which the soil has been washed away, conclusively prove that so far from there being any extension of the land surface, it is being encroached upon by the destructive action of the waves.
The terrace is well-developed in the Andaman Archipelago, especially on the west coast of Havelock Island, where the hills rise with precipitous face out of the forest, being now separated from the sea by a stretch of dry land; but at one place, where the sea has cut away this terrace and formed a low cliff, a bank of coral and shingle, evidently of littoral origin, was seen resting on soft calcareous sandstone, well above the reach of even the highest tides.
The beach I have described is everywhere low, and not more than 6 to 8 feet above mean sea-level, but there are indications of a terrace of marine erosion, corresponding to that seen on the Car-Nicobar and elsewhere at about 30 Ð 40 feet above the sea; both Outram and Lawrence Islands, and a large part of the other islands of the Archipelago, are low and flat-topped in outline, so that it would seem that they were plateaux Ð old plains of marine denudation.
There is not wanting evidence that the depression of the island, which is going on at the present time, has but recently commenced, for the kitchen-middens of the Andamans are in positions where a very slight subsidence would submerge them, and the time that they represent may be understood by the section of one which I examined near Port Mouat; it was 12 feet in thickness in the centre, and in this there was a bed 1foot 6 inches thick of vegetable mould, with shells scattered through it, marking a period when generations of shrubs and plants must have lived and died while the midden was abandoned, or only occasionally visited. This was doubtless started on a rock rising among the mangroves and gradually extended on to the mud; and it is a noteworthy fact that the surface of the mud under the shells does not appreciably differ from the general level of the mud outside. It shows that at a time when probably not one-sixteenth of the present bulk of the midden had accumulated, the level of the mangrove swamps was very nearly what it is now. Had the soil surrounding the rock on which the midden was started been well clear of the influence of the tides, it would certainly not have supported a growth of mangrove, and so far as my experience has shown me, would consequently have been of a very different character to what is actually found, while, had the surface of the mangrove swamp stood much lower than what it now is, the mud would certainly have risen above what was the base of the kitchen-midden in its earlier stages; in other words the surface of the mangrove swamp was then very nearly at the highest level it could reach, and as this is limited by the height to which the tide rises, it shows that during the time represented by the formation of this midden Ð a period which must be measured by centuries, if not by tens of centuries Ð the land has not appreciably altered its level relative to the sea.
A
west-east cross-section of West-East section through Andaman Island by Tipper. G.H. (Geology of the Andaman Islands with refernces to the Nicobars, Mem. Geol Surv India
35, 1911, Fig3) presented to support the existence anticlinal folding. In a section entitled "Recent changes of level in the
Andamans, Tipper found evidence for uplift and subsidence in the recent
past-possibly due to differnet phases of the earthquake cycle- that confused
him sufficiently to reserve an opinion of present trends.
Excerpt page 18 "During my stay in the Andamans I saw only one place which seemed to give any evidence on this subject. On the north shore of Stewart Sound, there is a small area on which the trees are dying due to apparently salt from sea water. It seems comparable to a similar place seen by Oldham."
On
page 14 Tipper relates that Colonel Alcock recognised the interior of South
Sentinel island as a corals "raised a few feet and continuous with the
corals surrounding it" and remarks theat North Sentinel was also said to
be composed of coral.
SRTM imagery provide a digital elevation map of North Sentinel Island that supporting the presence of marine terraces. Early reports suggest that the the island has been raised by crustal movements accompanyiong the 26 Dec 2004 earthquake.