ABSTRACT

The highly controversial Stonewall formation is sometimes considered Ordovician and at other times Silurian in age. It appears as a transitional zone as evidenced by its gradational lithologic nature and fossil content with underlying and overlying beds.

Isopachs above and below the base of the Stonewall suggest that a difference in sedimentation exists between the time of deposition of the upper Stony Mountain and a unit including the lower Interlake formation and an upper zone of the Stonewall formation. As might be expected the basal Stonewall unit reflects Ordovician topography with subsequently higher units masking any fur ther relation to Ordovician deposition. Ordovician sediments could possibly be picked as high as this basal Stonewall unit but because of a minor unconformity sometimes found at the base of this unit, the upper Stony Mountain is postulated as the top of the Ordovician.