The Oak Ridges Moraine lies about 25 km inland from the north shore of Lake Ontario and parallels the lake for about 160 km. The width varies from less than 1.6 to 14.5 km, and the maximum relief is about 150 m. Data from boreholes indicate that the maximum thickness of sediment in the moraine is about 215 m. Previous writers considered that the moraine was formed in several stages between lobes of the Laurentide glacier during the Wisconsinan or earlier.At the western end of the moraine up to 50 m of glaciofluvial outwash has been exposed. These upper sediments were deposited during and after the Port Bruce Stadial of the Late Wisconsinan Substage (< 15 000 years BP). In the central part of the moraine, in Uxbridge Township, the facies associations consist primarily of longitudinal gravel bars and well defined trough sets of dune origin, typical of the proximal portions of outwash plains. The finer grained distal facies occur to the west and northwest, and are characterized by transverse sand bars and sequences of ripple beds. The silt and clay fractions of the outwash were deposited mainly in proglacial lakes ponded against the north flank of the moraine south of Newmarket, and in King and Vaughan townships.