The present arrangement of the drainage of the St. Maurice Valley in the province of Quebec has little relation to the preglacial drainage. The modern St. Maurice River occupies parts of the valleys of several preglacial or interglacial streams and now flows up old valleys in two cases, and there are other examples of reversed stream courses. The pre-glacial drainage had a rectilinear pattern due to the geological structure, with the major streams flowing from northwest to southeast or from northeast to southwest. The upper parts of the St. Maurice, the Vermilion, and the Mattawin rivers have been turned from their former valleys by deposits of glacial drift and now flow northeasterly in parallel courses. Three lines of extensive glacial deposits indicate three halts of the ice sheet.