The peaks of the Grey Rocks outlier are pillow flows and breccias resting unconformably on a section of sediments and volcanics that stand vertically and terminate downward disconformably over a low-angle detachment marking the top of Trinity serpentinite basement. Adjoining to the north are four units dominated by a thick argillite and shale section. Beneath are a volcanic and sediment section, and a unit of silt and sand with ash flow breccia and thin flows of intermediate composition. The latter unit rests depositionally upon Trinity serpentinite. The lowest unit is a sedimentary serpentine melange. The Grey Rocks and the thick argillite-shale unit had been correlated with the Copley greenstone and Bragdon formation, leading to the conclusion that the outlier is a detached remnant of the Paleozoic Redding Terrane. However, the Copley section varies stratigraphically, unlike the relatively uniform Grey Rocks, and a French group has determined that they contrast geochemically. The Bragdon is largely a well-bedded shale characterized, in part, by chert pebble conglomerates and a few marine fosils. None of these are found in the sediments of the outlier.

The Grey Rocks and the volcanic-sediment block to the north are not related stratigraphically, but together, they occupy a single isolated outlier. How do the Grey Rocks ride on a low-angle detachment, whereas the northern block is in part in depositional contact with the Trinity basement of both? A low-angle detachment in serpentinite beneath part of the northern block has features indicating that it is a continuation of the shear zone beneath the Grey Rocks, suggesting that the northern part of the outlier includes a slice of its original basement and that the north and south blocks were carried on a single detachment. This relationship, together with strong lithologic differences between the outlier and the Redding section, suggests that the former is not related to the latter. Most importantly, a complex tectonic history for the outlier is recorded by extensive unconformable and disconformable contacts. This is in contrast with the straightforward stratigraphic development of the Redding section.

The Trinity Terrane is surrounded to the west by a region of younger accreted terranes. If the source of the outlier fragment remains beneath these, it may never be recognized.