A translucent cross-sectional slice through a fossil colonial rugosan coral, roughly triangular in outline, showing seven complete corallites along with three partials. The pale tan colour of the preserved structures contrasts sharply with the black background. Each corallite displays many radiating septa, with numerous short zigzag cross-arms (carinae). A white scale bar beneath the section is 3 cm long. One of many such thin sections in the Invertebrate Palaeontology collections of the Royal Ontario Museum. It was made during an extensive 1950's study of Devonian fossil corals of the Hudson Bay Lowlands in Ontario.

Something #fossiliferous for #ThinSectionThursday: traverse slice thru a #Devonian (~ 393 MYA) colonial rugosan #coral. Astreoid/thamnasteroid growth w individual corallites showing zigzag carinate septa. Billingsastraea sp. cf. B. verneuili, Kwataboahegan Fm, Mattagami River, #Ontario 🇨🇦 Scale=3 cm

An aerial view looking north across Houston Point, a low headland, and towards the dark blue open waters of Hudson Bay. The headland is nearly flat and almost devoid of vegetation. The tide is near high stand, flooding a broad expanse of rock-strewn shallows to the east, with a series of arcuate beach ridges framing the headland. Towards the centre of the headland low patches of pale rock are exposed, marking the remnants of ancient reef structures that have been almost completely removed by glaciation.

Over Houston Point, looking almost vertically down through an open helicopter window at a rocky exposure (seen at low tide) that is made up of roughly concentric ridges in an irregular bull's eye pattern. This is a "Philip structure" which marks the sagging of strata beneath the overlying load of a reef body which has subsequently been removed. The whole structure is about 150 metres across.

A lower level oblique view at Houston Point, looking across a Philip structure at ebbing tide with the open waters of Hudson Bay in the background. Irregularly concentric ridges are seen in the foreground, marking the edges of beds dipping gently towards the centre, where a portion of the deeply eroded reef core still remains.

A close-up view of the exposed Attawapiskat Formation at Houston Point. The white limestone is essentially a coquina made up of the undeformed shells of a large brachiopod -- Pentameroides septentrionalis. These formed extensive banks flanking the large reefs which have been removed by glacial action. Fragments of tabulate corals eroded from reefs are also present. The head of the geological hammer in the lower part of the image is about 18 cm long.

My last #Strataday featured exhumed 3D #Silurian (~435 MYA) Attawapiskat Fm reefs in n. #Ontario - this week we see the same unit in very different context. On Akimiski Is. in #HudsonBay the reef structures have been glacially scoured leaving only remnants - but incredibly #fossiliferous! #Nunavut 🇨🇦