Large-scale superficial structures in the Northampton Ironstone Field

Abstract

Summary Detailed mapping reveals many structures, unrelated to the slight regional tilting, folding and faulting, that are determined not by deep-seated movements but by disturbances that are demonstrably of superficial origin. These structures include cambers, gulls, dip-and-fault structure and valley bulges. As a result of these movements dissected Inferior and Great Oolite strata are lowered vertically to the extent of 100 feet or more, so that they swathe the hill tops and valley sides. The causal processes of this camber structure include sub-surface erosion and valleyward outflow of the underlying Lias. Gulls are widened joints in the camber filled with material from above. They usually* trend parallel to the strike of the cambered strata and may attain a width of 40 yards. Step-faulting of similar trend is commonly associated with advanced cambering. The throw of each fault in this " dip-and-fault " structure is compensated for by steep downslope dips (up to 40°) in the inter-fault blocks. Valley bulges comprise a variety of upward displacements of the strata that are confined to the valleys and are due to differential loading of the incompetent Lias. Their origin and mode of development are discussed in relation to the physiography of the area and the fundamental part played by the Lias clay. The economic significance of the structures in the exploitation of ironstone and in water supply are briefly considered and reference is made to analogous structures in other areas.