This report summarizes the outcropping rock types and structure present on the Vennachar and Borda 1:50,000 mapsheet areas on the western end of Kangaroo Island. The sheets cover most of Flinders Chase, which is a flora and fauna reserve. The oldest rocks identified are metamorphosed quartz-biotite sandstones and quartzites of the Lower Cambrian Inman Hill Formation of the Kanmantoo Group, which have been folded into open and close, south-west trending macroscopic anticlines and synclines. These metasediments have been intruded by a biotite/granite and tourmaline-bearing pegmatites of probable lower Palaeozoic age. A marine calcarenite limestone of probable late Pliocene age occurs at the Kelly Hill Caves, east of the mapped area. A period of late Tertiary peneplanation and deep weathering has resulted in a layer of laterite (ferruginous pisolites) being developed on the Inman Hill Formation, possibly at the end of the Pliocene or early Pleistocene. A pink oolitic ?fossiliferous limestone on the mapped area may be older than the ferruginous pisolite, or may be an indurated version of the Bridgewater Formation which has been coloured by iron oxides. During the middle Pleistocene, low sea level(s) resulted in coarse calcareous sands being blown inland and reaching thicknesses of up to 400 feet. This consolidated calcareous aeolianite (the Bridgewater Formation) now forms spectacular coastal cliffs. Late Pleistocene estuarine and swamp deposits containing Diprotodon remains occur at Rocky River Station. Calcareous beach sand from the present and recent past has been blown inland from the southern coast, to form extensive very high dunes which are now fixed by vegetation. Flinders Chase is covered by very thick mallee scrub which prevented access to areas away from the tracks - recommendations are made for the modification of vehicles to be used in future mapping programmes in the area.