Australia: The Land Where Time Began

A biography of the Australian continent 

2,300 Ma

Arnhem Land is in the tropical zone, about 200 km to the east from Darwin, in the Northern Territory.

Arnhem Land is based on the an ancient sandstone plateau composed of sand that was deposited from 2400 to 1400 million years ago. The plateau is not high, rising from about 180 m at its southern end to about 400-450 m in the north. The plateau, though not high, is prominent because it has low land on all sides. To the north is a narrow coastal plain from 2 to 30 km wide, and in a few places, there is no coastal plain, the edge of the plateau, the cliffs, falling to the sea. To the south are the arid desert plains. To the east, plains, up to 80 km wide, continue all the way to the Gulf of Carpentaria. On the western margin is the 1145 km long Arnhem Land Escarpment, extending from Murgenella in the north, to just south of Katherine Gorge.

A number of rivers have their source on this plateau, the East Alligator River and South Alligator River, the Roper River, the Katherine River (Katherine Gorge), the Liverpool River and the Daly River. Some spectacular gorges with many waterfalls have been formed where these rivers have cut down along faults and joints in the sandstone. The area is made up of some very rugged country and grasslands on the river flats. The climate is controlled by the northwest monsoon that dumps as much as 1600 mm of rain on Arnhem Land over 5 months of the summer Wet Season, then no rain at all during the winter Dry Season, until the following summer.

The sandstone rocks are still bedded horizontally, though they have been extremely metamorphosed, making them very hard. Long straight crevices have developed, a  result of periods of faulting. Millions of years of weathering and erosion have produced a very broken, stony landscape, most of which is flat-topped, but with a number of hills and low ranges. There are many cliffs and craggy outcrops.

The temperatures don't vary much in Arnhem land, averaging 34^oC in summer and 30^oC in winter. In about November, severe electrical storms are the beginning of the Wet, and by December, the Wet is earning its name, with torrential downpours every day. The incessant heat becomes very humid and the the Wet is at its peak in January, the wettest month of the year. The rivers become roaring torrents and spread across the plains. By May or June, the rain has stopped and the south-east trade winds bring dry air to the area.

1. Helen Grasswill & Reg Morrison, Australia, a Timeless Grandeur, Lansdowne, 1981