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HISTORY  The area was still unsettled by Europeans when, on December 3, 1800, it was viewed by Lieutenant James Grant, outward bound from England aboard the 60 ton brig H.M.S. Lady Nelson. He noted in particular two prominent mountain peaks, all of which, in accordance with the fashion of the time, he named after notable Englishmen: Cape Northumberland and Cape Banks; Mount Gambier and Mount Schank. However, it was not until the middle of the century that intrepid pioneers began to venture into this unknown territory. Since then the development of the region has been spectacular. The first of these pioneers was Stephen George Henty ( 1811 - 1872 ) who, with a small party, travelled overland from Portland in 1839, returning in 1841 to take up land. Other pastoralists soon followed his example. These early pastoralists were later joined by a significant farming community and later still by an impressive timber industry. Mount Gambier and district, with its famous lakes, caves and forests, continue to attract tourists in great numbers. BLUE LAKE  The Blue Lake at Mount Gambier in the South East of South Australia is a unique attraction. It has formed in the vent of a recent volcano, on the side of which the City of Mount Gambier has been built. The city relies on the lake as its source of potable water. Water in the lake is exceptionally clear, the quality is very good, but it is hard due to filtration through limestone which underlies the district. This hardness can be easily removed by house hold water softeners.The Blue Lake is an exposure of the regional groundwater system, with the water replenished by infiltration of local rainfall and stormwater together with lateral groundwater flow. GEOLOGICAL HISTORY The Gambier Limestone which underlies much of the South-East of South Australia, was depostited 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. In relatively recent times, a volcanic eruption blew a hole through the limestone, and formed a structure known as a maar at Mount Gambier. This consists of a rim made of ejected basaltic material resting directly on Gambier Limestone. The Mount Gambier volcanic comlex is one of approximately 20 eruptive sites in the South-East.The Blue Lake has formed in the largest of the volcanic craters. It is an exposure of the water table which occurs naturally within the Gambier Limestone; the limestone is exposed as white strata in the sides of the crater. Investigation of the sediments from the lake bottom indicates that it formed about 28,000 years ago. COLOUR CHANGE The colour of the Blue Lake changes dramatically from grey to a vivid blue over a few days in late November each year. Then, after summer, the colour gradually faded until the grey winter appearance is resumed.Numerous theories have been put forward to explain the colour change, but is now considered to be a three-phase process: * temperature stratification occurs in the lake over the summer months, with a layer of warmer water overlaying the cooler water. *precipitation of calcium carbonate in the warmer part of the lake during the summer months, which removes any water-borne impurities. *the absorption of all but the blue wavelengths of light by the warm water in summer. THE GAMBIER LIMESTONE UNCONFINED AQUIFER The Gambier Limestone forms an unconfined aquifer in the Mount Gambier area. The ground water is not confined by an overlying impervious layer of rock or sediment and therefore remains unpressurised. Caves, known locally as sinkholes, and smaller cavities havedevoloped extensively within the Gambier Limestone due to solution activity by percolating rainfall and groundwater. Some of the larger sinkholes are below the watertable and are filled with water. Several have become popular with scuba divers, although potentially dangerous.  VALLEY LAKE In addittion to the Blue Lake, there are three other lakes formed in the crater of the extinct volcano. Brownes Lake, Leg of Mutton Lake ( both of which are now virtually dry ) and the Valley Lake. The Valley Lake is popular with boating enthusiasts and the parks and gardens adjacent to the lake are popular for family outings. 
Picinini Ponds sinkhole
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