The oldest rocks you'll find on Banks Peninsula, are at the top end of Lyttleton Harbour, in a gap in the hills known as Gebbies Pass.
They are thought to be around 240 million years old. This is much older than any other rocks you will find on the peninsular. The Gebbies Pass rock outcrop is part of what geologists call ' Torlesse Terrane', which runs right along the backbone of New Zealand as our Southern Alps, Marlborough ranges, and main North Island hills.
These ancient rocks are sedimentary. They were laid down as sands, silts and muds on the sea bed, then later forced up to form dry land.
There is another category of ancient rock in Banks Peninsular, and it is also found near Gebbies Pass. It is volcanic rock and is found in the McQueens Valley and Gebbies Pass. Geologists estimate these were spewed out of volcanoes about 90 million years ago. At the time they were part of a relatively large volcanic area, but that has all since been eroded, leaving just the valley that today is farmed.
Dark flows of andesite lava occur in the western side of McQueens Valley, now a pleasant dairy farming area, and in the southern section of Gebbies Pass there are numerous outcrops of cavernously weathered rhyolite and the eroded remnants of lava domes. Unlike the later volcanic activity on Banks Peninsula, this phase included ignimbrite eruptions and must have been particularly explosive — in many respects similar to recent volcanic activity in the Taupo zone of the North Island.
Geologists have found similar volcanic rocks, of about the same age, across the other side of the Canterbury Plains at Mt Somers, the Malvern Hills and the Rakaia Gorge, and in sea drill holes out in the ocean some distance from Banks Peninsula.