A drowned valley and a penal colony

Why the First Fleet stopped here, and the country it was declared to be.

What you should be able to do after this: Explain how Sydney Harbour's geology made the site, and how the legal fiction of terra nullius shaped Australia until 1992.

0 of 4 · about 9 min

Deep time

A river valley that the sea came into

Sydney Harbour is a ria: a river valley drowned when sea level rose after the last glacial maximum. That is why it has the shape it does — deep water reaching far inland, a branching outline with many sheltered coves, and steep sandstone sides.

For a sailing ship this is close to ideal: deep enough to enter, sheltered enough to anchor, and large enough for a fleet. The First Fleet arrived at Botany Bay in January 1788, found it inadequate, and moved a few kilometres north to this harbour within days.

On the map

Sydney Harbour Sydney

Check your understanding

What kind of feature is Sydney Harbour?

Answer the check to continue.

Sources for this chapter

UNESCO · Sydney Opera HouseUNESCO World Heritage Centre · checked 2026-09-07

Every source in this quest

UNESCO · Sydney Opera HouseUNESCO World Heritage Centre · intergovernmental · checked 2026-09-07
Australian Bureau of StatisticsAustralian Bureau of Statistics · statistical · checked 2026-09-07

Written by Globe Quest editorial and last reviewed 2026-09-07. Every claim above is traceable to one of these sources; where accounts genuinely differ, the chapter says so rather than choosing for you.