A. Introductory. (See Plate 8.)
Akaroa Harbour has long been regarded as a typical caldera, which has been drowned either by lowering of the land or by the rise of the sea. Its floor gradually deepens from mud-flats at its head to 18 fathoms at the entrance, a figure which gives the minimum amount of drowning to have taken place. In the upper portion spurs stretch down the inner slopes with rough centripetal orientation, once having divided stream-valleys, and still dividing them in their upper portions, while the lower parts of the spurs now divide the bays which fringe that stretch of the shore of the harbour. The most striking of these spurs is that which ends in Onawe Peninsula. Taking the bays in clockwise direction round the harbour they are as follows:—Tikao Bay, Le Petit Carenage or Brough's Bay, French Farm, Barry's Bay, Duvauchelles and Head of the Bay, Robinson's Bay, Takamatua or German Bay, Lushington Bay, and lastly French Bay on the shore of which the township of Akaroa stands (see Plate 8). The entrance to the harbour is flanked by bold, vertical cliffs, which rise on the western entrance to a height of about 500 feet; on the eastern entrance they are lower.
The harbour presents some resemblance to that of Lyttelton, differing chiefly in so far as it is of later date and therefore has experienced a less prolonged erosion by stream and sea. While the crater-ring of Akaroa is broken down in only one place, viz., the harbour entrance, that of Lyttelton is broken in two, there being no counterpart in Akaroa of the low ridge near Gebbies Pass, where the earliest rocks of the area are exposed as a result of the removal by erosion of the covering of the later andesites and basalts of the Lyttelton system. In both cases the formation of the caldera may be attributed chiefly to the erosion by streams converging on a great central hollow, originally an explosion crater of moderate size, and in both cases there is something of a break in the profile of the inner slopes of the hollow. In the case of Akaroa this lies at an elevation of from 300 to 600 feet above the sea, and is most definite
in the stretch from Tikao Bay to Le Petit Carenage or Brough's Bay, and between French Farm and Barry's Bay (see plates 10, 11, 12), photos 1, 4, and 5) where the level surface suggests a shore platform. In other parts of the periphery of the harbour where the break occurs, such as between Takamatua and Akaroa (see Plate 11, photo 3), the beds do not lie flat, but there is unfortunately no clear exposure, either in natural or artificial sections, of the contact of the rock at this break in profile with the rocks forming the steeper slopes above and having the quaquaversal outward dip characteristic of a volcanic cone. In the case of Lyttelton the break just referred to is associated with the presence of volcanics and sedimentaries which form a substratum of much earlier date than the upper part of the cone, and this suggested a closer examination of the lower slopes in the case of Akaroa in order to see if there is any analogy in structure and geological history to that of Lyttelton. The basal beds of the latter were dealt with by the present author in his account of the “Geology of Gebbies Pass and Neighbourhood” (1936), and some reference was made to the possible structure of the Akaroa volcano in an earlier paper entitled “The Intrusive Rocks of Banks Peninsula” (1923). When the latter was written the author had not the advantage of a knowledge of the complexity of the early geological history of Lyttelton revealed by a subsequent and more detailed examination of the surrounding area.
Perhaps the most important locality in the Akaroa area is Onawe Peninsula, therefore a full account of its special features which bear on the question at issue will be given first.