The volcanoes of southeast South Australia form the western extension of the Newer Volcanics province of Victoria, and comprise two distinct groups: a northern Pleistocene group of 15 eruption centres in the Mount Burr range, and a southern Holocene group of two isolated eruption centres at Mounts Gambier and Schank (Fig. 1). All the volcanoes are basaltic and are formed predominantly of explosively-erupted fragmental (pyroclastic) products including ash, lapilli, and scoria; lava is less common and is rarely soil-forming (Irving & Green, 1976; Sheard, 1978; 1983a, b; 1986; 1990). Mounts Gambier and Schank are aged about 4000-5000 years old and are the youngest volcanoes on the Australian mainland (Fig. 2) (Barton & McElhinny, 1980; Barbetti & Sheard, 1981; Blackburn et al., 1982; Sheard, 1990). They were erupted through consolidated calcareous sands of the Bridgewater Formation, and the resultant pyroclastic deposits may contain up to 25% of non-volcanic material, chiefly limestone fragments (Sheard, 1990).