Four boreholes completed in Lake Eyre North and one in Lake Eyre South have provided further information on lake sediments and the stratigraphy of the Lake Eyre Basin. Modern lake sediments constitute a thickness of less than 15 feet in the deflated lake floor. They directly overlie a dolomite sequence (the Etadunna Formation) which outcrops to the east of Lake Eyre and where it carries a characteristic mammalian fauna; it is assigned a Tertiary age but has not been precisely dated. Bores penetrated 100-150 feet of these sediments before intersecting a lacustrine sequence comprising carbonaceous sands, silts and clays to the limit of boring (983 feet); sedimentation appears to have been continuous above the Winton Formation (Cretaceous) into the lower Tertiary. General features of the lake, the constitution of the sediments and brines are discussed. The composition and content of the brines of Lake Eyre are similar to those of other South Australian inland lakes and it is considered that they all originate from oceanic sources "cyclic salt". No abnormal concentrations of potash, the halides or of minor elements were detected. Radiocarbon datings indicate that a period of 20 000 years was occupied in deflation of the lake floor to a level of 60 feet below sea level. Tilting of the present lake surface suggests that the region is a tectonically active one.