The Lower Old Red Sandstone of Forfar and Kincardine is upwards of 14,000 feet in thickness. No indication of a base or natural top is seen in the district. The characteristic fossils of this series have all been obtained from its middle beds within a vertical range of less than 4,000 feet. There is no palæontological evidence of the age of the beds above or below that zone. It is concluded, on physical and palæontological grounds, that the fossiliferous beds of the Orcadian area are really younger than those of Forfarshire, but there is no proof that the higher part of the Forfarshire series was not contemporaneous with the Orcadian deposits.

The folding of the Lower Old Red strata into the syncline and anticline parallel to the Highland boundary fault took place subsequently to the deposition of the whole of those strata. This folding, and denudation to the extent of 8,000 feet over the anticlinal axis, were accomplished after the deposition of the Lower and before the beginning of the Upper Old Red. The surface on which the Upper Old Red rests, in Forfarshire, is broadly horizontal, but very uneven—evidently an old land-surface.