The geology of the Thunder Bay area exhibits a sequence of flat-lying Proterozoic Aphebian sedimentary rocks essentially free from tectonic deformation. These unconformably overly the Archean metamorphic rocks.
Within the Proterozoic sequence lies a pronounced, thin, stratiform lapilli tuff. The tuff comprises crystal fragments, pumice fragments as well as accretionary lapilli and lapilli of other origins.
There has been a strong post-depositional compaction. This was recognized from shape (strain) analysis of the accretionary lapilli and from the shape and orientation of the magnetic susceptibility ellipsoid. A preliminary paleomagnetic investigation indicates a paleolatitude of the time of sedimentation (or diagenesis) of about 66°.