Banks Peninsula is situated in about the middle of the east coast of the South Island on the margin of the Canterbury Plains. It is approximately 450 sq. miles in area and its highest point is Herbert Peak, 3,014 ft. It comprises two extinct volcanoes which were active less than half a million years ago. Their craters have subsequently been enlarged to many times their original size by stream erosion; they were then invaded by the sea during the postglacial world-wide rise in sea level beginning about 15,000 years ago. They now form the harbours of Lyttelton and Akaroa. Originally Banks Peninsula was an island, but it became tied to the Canterbury Plains at some late stage in geological history when the growing alluvial plain reached its base.
Akaroa is the larger volcano and probably reached a height of over 4,500 ft. Estimates of its original height are uncertain, since they cannot be based solely on the projection of existing lava slopes upwards for the reason that volcanoes of this type often have domed profiles with a falling off in gradient towards their tops. The flows consist mostly of basalt and andesite, and ash beds are not very common. Thick hard lava flows, suitable for quarry sites, are surprisingly hard to find. Dykes, vertical walls of volcanic rock pushed up fissures from below, are common and form prominent ridges in places. In the case of Lyttelton Volcano their radical arrangement centres on Quail Island, which, therefore, is thought to be near the site of the original vent. The original vent for Akaroa Volcano is considered to be near Onawe Peninsula. Because the volcanoes are situated sufficiently close to one another there is a zone of intermingling of their lava flows several miles wide, hence the impossibility of saying from which volcano the various flows originated. At a fairly late stage in the geological history of these volcanoes, when the topography was similar to that of the present day, lava was erupted from near Herbert Peak and flowed down the southern slopes of Lyttelton Harbour to form the prominent spur on which Diamond Harbour now stands. The lower slopes of the peninsula are mantled with a yellow, wind-blown silt, called loess, which was blown by the norwesters during the most recent ice advances from the wide beds of rivers such as the Waimakariri. An alternative theory is that the loess originated from sand banks out to sea. During the lowered sea level periods of the ice ages these became land and the silt was then blown inland by the prevailing easterlies. Moa bones, as well as gizzard stones of these and smaller birds, are common in the loess, which is now used quite extensively for brick and pipe manufacture.
Banks Peninsula has a somewhat more salubrious climate than Canterbury Plains, with a higher rainfall and fewer frosts, particularly on the lower slopes. Snow is common on the highest slopes during the winter months and often lies for several weeks on the tops, although the thickness is not great.