Ill ORDOVICIAN STRATIGRAPHY AT CLIEEDEN CAVES, NEAR MANDURAMA, N.S.W. By N. C. Stevens. (With Plates iii-iv and 4 Text-figures.) [Read 2Sth May, 19 52.] Synopsis. The Cliefden Caves Limestone, which contains abundant shelly fossils of Middle Ordovician age, conformably overlies basic volcanic rocks, and is overlain by siliceous limestones and tuffs with Upper Ordovician graptolites, trilobites and brachiopods. The structure of the area is discussed, and notes on facies and facies change are appended. Introduction. Cliefden Caves are situated 11 miles north of Woodstock and 12 miles west-north-west of Mandurama. Previous literature on the geology of the area is mainly concerned with the limestone, which was the first limestone discovered in Australia (Oxley, 1820). Caves in the limestone were reported upon by Wilkinson (1892) and Trickett (1908). Carne and Jones (1919) mapped most of the main limestone bed, and called it the Belubula Limestone Belt. Until recently the limestone was assumed to be Silurian, largely due to a list of fossils collected by Trickett (1908). North of the Belubula River, Booker (1950) recorded Silurian fossils from a limestone which he assumed to be continuous with the limestone at Cliefden Caves. However, detailed mapping and further fossil collections have shown that the main limestone bed and associated strata are of Ordovician age. A preliminary note (Stevens, 19506) on this occurrence was the first record of fossiliferous Ordovician limestone in New South Wales. The nearest locality from which Ordovician fossils have been recorded is Junction Reefs, five miles to the east (Hall, 1900; Pittman, 1900), but the exact locality is in doubt. Stratigraphy. A generalized sequence is given in Table 1 and Text-fig. 2, the data being drawn from excellent sections along the Belubula River and tributary creeks. Table l. Formation. Lithology. Age. Upper Ordovician. 4. Angullong Tuff. Andesitic tuffs, tuffaceous shales, calcareous^ tuffs, small limestone beds. > 3. Malongulli For-Laminated impure limestones, limestone, lime-| mation. stone breccias, tuffs and shales. J 2. Cliefden Caves Massive and shaly limestones. S .,.,,, Limestone. I f ddl . e 1. WalU Andesite. Andesites, basalts, spilites, tuffs and breccias. ) Ordovician. 1. The Walli Andesite is the name given to the volcanic rocks which are the basal formation of the sequence. They are well developed between Walli and Limestone Creek, extending north to the edge of The Large Flat, and south beyond Woodstock. The width of outcrop is about four miles, but as the base is not visible and the structure is uncertain, the thickness cannot be estimated. The formation was regarded as Silurian, equivalent to the Cargo Andesite (Stevens, 1948, 1950a), before the Cliefden Caves area was examined.