I was fascinated by the arch on the railway bridge at Staveley. The railway is not perpendicular to the road. If it were, the bricks would run parallel to the stones at the bottom. But the bricks must run perpendicular to the curvature of the arch. Arches transmit compression along the curvature. Bricks are excellent in compression but if they were not perpendicular to the curvature, there would be a sideways component of force on the mortar - a shear force. Mortar would not be strong enough to resist such a tearing force and the bridge would collapse.
