Experimental investigation into the mechanism of folding

The investigation of geological structures due to folding led de Sitter to form an opinion on the mechanical problems involved (Bibl. 7). His principal contention is that in simple cases the relative movements of particles with respect to eachother during deformation leading to a fold, have been purely concentric. During such concentric deformation all layers maintain their original thickness and length over the whole profile. All differential movements of neighbouring particles are parallel and therefore concentric. Towards the core of the anticlines and deeper down in the crust the geometrical relations, as construction will show, necessitate a deviation from this principle and either thrusting or plastic thickening of the rocks must take place. The concentric habit for larger units will therefore be partly lost. The most satisfactory treatment of this problem would be a mathematical analysis of the principal normal stresses in a fold, applied to measured properties of the rocks involved. We are still far removed from this ideal solution. De Sitter therefore requested Kuenen to co-operate in an experimental investigation that was intended to elucidate some of the mechanical features of folding and to test the convictions won from the study of folds in nature. The actual experiments were carried out by Kuenen in his laboratory at Groningen. By frequent intercourse, however, de Sitter took an active part in guiding the research. This report embodies the more relevant results. Theoretical considerations have been reduced to a minimum in order to give, as far as possible, an unbiased exposition of the experimental data. Before entering on a description of the experiments some terms and conceptions that will find frequent use must first be discussed. By stress and strain are meant respectively: force per unit surface and deformation in terms of relative displacement.