The two types of (anticlinal) structure seen in the Great Artesian Basin are described as follows. (1) Marginal type - formed where a relatively thin blanket of Mesozoic sediments is draped over ancient topographic highs and has undergone differential compaction, e.g. as at the Oodnadatta Anticline which overlies a ?Palaeozoic buried ridge, asv proved by mapping and boreholes. Oodnadatta penetrated the Tambo and Roma formations underlain by a 300 thick sandstone aquifer, and revealed rich Cretaceous marine faunas. Drape folds such as the Oodnadatta Anticline subdued features, with limb dips less than 3 degrees. Similar structures occur along the margins of the Peake and Denison Ranges, and NE of Marree. (2) Central type - has an epeirogenic origin in thick sediments (8,000 or more). Tendency to occur in en-echelon arrangements, producing pronounced elongated domes (e.g. as at Innamincka, Curalle, Betoota) or more complex double anticlines (Nappamilkie - Haddon and Grey Range structures). Such structures have limb dips between 3 to 20 degrees, and commonly average 5 to 8 degrees. Individual closures about 400 to at least 600'. Evidence from the GAB is that central folding began in Upper Cretaceous, reaching a climax in the early or middle Tertiary, It cause can be explained by transcurrent movements in basement.