Dressed Sandstone Step Tabling and Skewputt Terminal Corbels in Scottish Crow-Stepped Gables

The monumental character of Scottish Baronial architecture is inseparable from the unique geology of Scottish sandstones. Unlike soft, crumbly sandstones found in continental lowlands, Scottish building stones were formed under immense geological pressures during the Devonian and Carboniferous periods. The result is a family of dense, quartz-rich freestones celebrated across the world for their extraordinary compressive strength and resistance to mechanical weathering.

Chief among these is Craigleith sandstone, quarried on the outskirts of Edinburgh, which provided the stone for Edinburgh Castle, the Palace of Holyroodhouse, and the New Town. Composed of over 95% pure silica grains bonded by a dense siliceous matrix, Craigleith is virtually impervious to atmospheric acid and frost. Across the north in Aberdeenshire and Moray, masons worked with Permian sandstones such as Clashach and Devonian Old Red Sandstones, whose natural iron oxide cements impart a warm, golden-buff to deep-terracotta hue that glows against dark Scottish storm clouds.

In traditional Scottish masonry, the first and most inviolable rule taught to an apprentice mason is bedding orientation. Sedimentary sandstones are formed in horizontal layers (laminae). When building a vertical rubble wall, stones can occasionally tolerate varied orientations, but for exposed coping stones and corbiesteps, incorrect bedding is fatal.

Corbiesteps must strictly be laid on their natural bed—meaning the original geological sedimentation planes lie horizontal, parallel to the ground. If an inexperienced mason cuts a step "face-bedded" (with laminae standing vertically parallel to the outer face), rainwater penetrates the micro-cleavages between sand layers. During Scottish winter freezes, water expands by 9% upon turning to ice, exerting thousands of pounds of hydraulic pressure that exfoliates the face of the stone in ragged, crumbling sheets within a single decade. Natural-bedded stones, by contrast, present only their edge grain to the weather, withstanding hundreds of years of scouring gales without loss of detail.

A crow-stepped gable represents a series of flat horizontal surfaces exposed to torrential rainfall. If a step tread were dressed dead flat, rainwater would pool upon the stone. Over time, standing water saturates the stone and leaches downward into mortar bed joints, initiating internal wall decay. To eliminate this hazard, Scottish masons carved a weathered wash onto the top tread of every step.

This wash is a continuous planar slope—typically angled at a pitch of 1:12 to 1:15—that slants outward and downward toward the roof slate. Gravitational drainage accelerates water runoff, clearing the tread within seconds of a rainfall event. Furthermore, masons cut a drip throat (*throat groove*) beneath the projecting nose of each step. This semi-circular channel creates an abrupt 90-degree break in the stone plane. When surface water flows over the edge, it cannot overcome gravity to track backward along the underside of the stone; instead, it is forced to detach and drop cleanly onto the slate below.

At the base of the stepped gable rake sits the masterwork of the mason’s chisel: the skewputt. Functioning as a combined retaining block, springline anchor, and cantilever corbel, the skewputt must withstand both the vertical dead load of masonry above and the downward sliding thrust of the descending corbiesteps.

Carving a skewputt begins with a monolithic block often exceeding 150 kilograms. The mason carves a stepped seating notch on its upper surface to cradle the first descending corbiestep. The outer projecting face was then detailed according to the patron’s heraldic status: Renaissance scrolled volutes (*roll corbels*) were favored on seventeenth-century burgh tenements, while highland baronial castles featured deeply undercut family coats of arms, thistle blossoms, or grotesque human masks whose open mouths served as decorative waterspouts. Embedded deeply into the heavy wallhead rubble, the skewputt locks the entire stepped gable in perpetual structural equilibrium.