The Gambier Limestone, which underlies much of the South-East of South Australia, was deposited when the sea covered a large part of the southern Australian landscape more than 15 million years ago. It is rich in the fossil remains of small marine animals and shells, and even contains rare shark's teeth. The limestone can be broadly divided into an upper bryozoal limestone and a lower dolomite unit.
In relatively recent times, a series of volcanic eruptions blew holes through the limestone, and formed structures known as maars' at Mount Gambler. These consist of a rim made of ejected basaltic material resting directly on Gambler Limestone. The Mount Gambler volcanic complex is one of about 17 eruptive sites in the South-East (Fig 1).
The Blue Lake has formed in the largest of the volcanic craters (Fig 2). It is an exposure of the water-table which occurs naturally within the Gambler Limestone which is exposed as white strata in the sides of the crater. Recent work by the CSIRO involving dating of sediments from the lake bottom indicates that it formed about 29 000 years ago (Leoney et al., 19951, whilst Robertson et al. (1996) and other workers have determined that the volcano may have formed as recently as 4500 to 5000 years ago.