The Meaford–Dundas Formation in southern Ontario is a medium gray shale with good fissility and resistant interbeds of gray fossiliferous limestones and siltstones. The hard layers are up to 20 cm in thickness and comprise 10 to 20% of the formation. The shale layers vary in thickness from 50 cm to 2 m.The clay minerals are principally illite, iron-chlorite, and small amounts of vermiculite and mixed-layer types. The carbonate content seems constant across the area at about 4 to 5% of the formation, except for the southwestern area where the carbonate increases to 20 or 25%. This increase is chiefly in dolomite content, a feature which reflects such factors as original conditions of deposition and possibly diagenesis subsequent to burial. The quartz content in the shale beds, and especially in the hard interbeds, increases towards the north to an average of 35 to 40% compared with 10 to 15% in the south. Framboids (aggregates of pyrite grains in spheroidal clusters) are a striking feature of the shale beds of the Meaford–Dundas Formation in the Meaford area.Fabric studies by means of X-ray diffraction patterns and scanning electron photomicrographs reveal, in most cases, high parallelism of clay platelets in the bedding planes, resulting in the good fissility of the shale.The principal source rock areas are the Appalachian orogen in the east (Taconic Mountains), which probably supplied most of the clay minerals and some quartz, and the Canadian Shield in the north, which provided the basin of sedimentation in the south with heavy minerals and additional quartz.