A study has been made of the red‐beds in the Narrabeen Group in the central and southern portions of the Sydney Basin and it is concluded that the sediments were deposited in a piedmont environment. Siltstone is the dominant rock type although all gradations between claystone and fine sandstone occur. Clay minerals predominate in the red‐beds of the southern sector including those exposed in the Collaroy‐Newport area whereas, north of the Hawkesbury River, the rocks are characterised by abundant quartz and significant amounts of feldspar. In the western sector the composition of the red‐beds is not appreciably different from that of the shales in the succeeding Hawkesbury Sandstone and Wianamatta Group. The Bald Hill Claystone has the features of a re‐sorted laterite but the mineralogy of the red‐beds north of the Hawkesbury River appears incompatible with such an origin. More information on the entire sedimentation within the Sydney Basin will be necessary before the problem of the origin of the red‐beds is settled.