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.

Stiff sedimentary clays: geological origins and engineering properties

“…The two associated phenomena of cambering and valley bulging were originally described in detail by Hollingworth et al (1944), who encountered these features in the Jurassic strata of Northamptonshire, UK. In brief, camber involves limestones, usually horizontally bedded, that appear as anticlines on hill tops and valley sides, the anticlinal distortion resulting in a complicated series of relatively minor faults in the limestone ('dip-and-fault structure'), while in the valley floor the underlying strata, usually clays, are thrust up as a 'valley bulge'.…”

Engineering geomorphological mapping as a technique to elucidate areas of superficial structures; with examples from the Bath area of the south Cotswolds

“…Following his involvement in the first full appreciation of the nature, extent, and significance of superficial structures (Hollingworth et al 1944), it was apparent to Dr Kellaway when remapping of Sheet No. 265 was begun that similar structures would probably occur in the Bath area.…”

“…The first full descriotion of superficial structures was made by Hollingworth et al (1944). Taking most of their examples from the Jurassic rocks of the Northamptonshire area, these authors described cambers, gulls, dip and fault structures, valley bulges, and slips.…”

The elevation of the Lower Greensand ridge, south-east England

“…This supposition is indicated by the dashed line on Figure 5, suggesting that the steepening of the dip of the structural surface as the outcrop is approached from the south-east ( Figure AA) is counteracted by a decrease of dip further north-west. Whether this is indicative of a monoclinal tendency, perhaps reinforced by some cambering near the eroded edge of the formation (Hollingworth, Taylor & Kellaway, 1944), one can only speculate in the absence of more information on the lower contact, but it was decided to use both the best-fit quadratic (X 4 ) and cubic (X 3 ) upper-contact elevations in the subsequent analysis.…”