The drowning of a valley may be due to a rise of sea level or to a combination of this rise and actual subsidence of the land. The other essential feature is a slow rate of sedimentation so that the newly created drowned valley is not filled with sediments as quickly as it is drowned.
There have been many changes in sea level during the last million years caused by the glacial periods, when much water was landlocked as ice. During the intervening non-glacial periods the ice melted and much of the landlocked water was returned to the sea, causing tremendous coastal inundations. Twenty thousand years ago the level of the sea was about 300 ft below the present one. Ten thousand years ago it was still 100 ft lower, rising to about the present level by 5,000 years ago. It was this rapid rise that produced the many sheltered and beautiful harbours throughout the world.
Drowned valleys are commonly irregular in outline owing to tributaries of the main valley being also drowned. This is particularly noticeable in the Marlborough Sounds and Bay of Islands. Others include the harbours of Akaroa, Kaipara, Lyttelton, Manukau, Otago, Parengarenga, Porirua, Raglan, Waitemata, and Whangarei; and Whanganui Inlet, near Cape Farewell. Other harbours that owe their form partly to drowning include Kawhia, Ohiwa, Port Nicholson, and Tauranga.
by James Cecil Schofield, M.SC., New Zealand Geological Survey, Otahuhu.