498 



R. T. CHAMBERLIN AND F. P. SEEPARD 



The pressure was thus appHed at the lower edge of the gently inclined 

 strata and consequently at a level somewhat below the resulting 

 fold. This arrangement seemingly should favor the development 

 of underfolds. In some cases it did, but in two experiments where 

 an underfold began to develop, an over thrust formed on the steeper 

 limb before the folding became very sharp, and this quickly became 

 the dominant structure with further compression. This order of 

 events seemed very significant, suggesting a strong tendency to 

 overthrust rather than underthrust whenever the opportunity 

 allows. In two other experiments where everything was arranged 



Fig. 4. — Successive stages in the development of overfolds. Top figure after 6 

 inches of shortening; bottom figure same after 8 inches of shortening. Fold a formed 

 first. Fold h, which started as an underthrust, has changed into a pronounced over- 

 thrust. Note the thinning of the under limbs. With still further compression flexure c 

 developed into a strong overfold. 



to develop underfolding, overturned folds resulted. On the other 

 hand, when the blocks were placed in the opposite positions, so that 

 the layers sloped downward from the pressure block to the resistance 

 block and the active force was applied somewhat above the fold, no 

 trouble was experienced in developing overfolds. 



It was found also that the nature of the materials influenced to 

 some extent the direction of yielding. When the materials were 

 relatively competent and brittle (sand, clay, and plaster), and the 

 mass was deformed by faulting rather than folding, overthrust and 

 underthrust faults resulted in more or less equal proportions. 

 Wedge faults developed in some instances. But in the plastic 

 models it was found almost impossible to produce underthrust 



