The northwest-trending Ottawa-Bonnechere graben is separated from the Lake Ontario homocline, the northern border of the Allegheny synclinorium, by the block-faulted Madawaska Highlands of central Ontario. Ordovician sediments are preserved as infaulted outliers in the graben and form a plain adjoining the resurrected pre-Paleozoic Cashel peneplane, the northern belt of the homocline. The stratigraphy in the two districts is similar, and the sediments must have been continuous originally. Northeast-trending monoclines cross the Paleozoic plain of the homocline; their age is unascertained. Thick Paleozoic rocks concealed the pre-Cambrian of the southern Canadian Shield at the time of faulting of the Ottawa-Bonnechere graben, in the early Tertiary. The faults have trends corresponding to those of joints of earlier origin and have displacements of as much as 1500 feet.

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