The Oxford Dolomite of southeastern Ontario is considered to be equivalent to the Ogdensburg Dolomite of northern New York State. The Oxford Formation is redefined in terms of its lithology, distribution, thickness, sedimentary environment, and age.A series of representative sections can be correlated with wells drilled in southeastern Ontario. The formation rests disconformably upon the March Formation (Tremadocian) and is overlain disconformably by Middle Ordovician (Chazyan) sediments. The lower contact of the formation is widely exposed, but the upper contact is known only from boreholes.Lithologies are not distinctive; a lower division in which quartz sand is abundant and an upper division in which it is rare or absent can be recognized. The total thickness of the formation is not known; however, a minimum thickness of 605 ft (184.5 m) is present in a well near Williamsburg, Ontario. Conodont faunas indicate that the age of the Oxford is Lower Arenig and the disconformity at the base of the formation marks the base of the Arenig series in the Ottawa – St. Lawrence Lowlands.