Large-Scale Superficial Structures in the Northampton Ironstone Field

Abstract

In the discussion of this paper, Mr. F. G. H. Blyth suggests interpretation of the observed structural details by the methods of soil mechanics. In the writer's opinion, the prospects for the success of such an attempt are no better than those of an attempt to explain the tectonics of the Alps on the basis of the current concepts of the strength of materials. Both rocks and clay have a double personality. One of them represents the behaviour of these materials under laboratory conditions. It reflects the standard or technical strength. The other one represents the behaviour under small stress of long duration. It reflects what is sometimes referred to as the fundamental strength, which indicates the smallest stress which does not produce any flow. There is no known relation between these two types of strength. High standard strength may be associated with very low fundamental strength and vice versa. A stress in excess of the standard strength produces fractures which cut across the internal boundaries in composite bodies, whereas a stress above the fundamental strength of the major constituents of composite bodies leads to intricate patterns whose details reveal every local change of the mechanical properties of the material. Minor constituents such as thin, rigid layers whose fundamental strength is greater than the average stress gradually disintegrate, due to local stress concentrations, and the fragments are slowly dragged into a flow pattern. Intense deformation of this type leads to the clay pebble-beds mentioned on p. 11 of the paper.