Penecontemporaneous weathering of the Old Red Sandstone of the Midland Valley of Scotland

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Abstract
Synopsis: Evidence from thin sections, surface textures of separated grains, and from stratigraphic variations in the mineralogy of heavy-mineral concentrates, suggests an in situ origin for the red colour of Old Red Sandstone sediments. Haematite has been formed diagenetically from the break-down products of mafic minerals and increases as other heavy-minerals decrease in the older parts of the succession. Alteration was related to conditions of free circulation, over a relatively short period, of oxidizing ground waters in a warm climate. Two intervals of such weathering are recognized, one during the Lower Old Red Sandstone, the other during the Upper. The contribution of post-Devonian weathering seems to have been insignificant.