FOLDS RESULTING FROM VERTICALLY 

 ACTING FORCES^ 



W. I. ROBINSON 



Michigan Geological Survey, Lansing, Michigan 



Because the most striking examples of folded rocks are to be 

 seen in mountainous regions, folds are usually thought of as the 

 visible results of compressional forces which act more or less parallel 

 with the surface of the earth, the typical force of this kind being the 

 compression effect of differential contraction of the lithosphere. 

 But with the rapidly accumulating knowledge of more stable areas 

 of the present land surface and especially from the increasing mass 

 of data from deep well records, the importance of another kind of 

 fold has come to be recognized. Although it seldom affects the 

 present topography directly, this kind of fold is of great economic 

 value because of its influence on the distribution and accumulation 

 of gas, oil, and brines. Some of these folds are actually the direct 

 result of a force acting vertically, others which also owe their 

 origin to a vertical force seem to be rather the effect produced by 

 components of this force acting more or less parallel with the beds. 

 Most of the folds due to vertical forces occupy circular or oval 

 areas, either singly or as groups. This shape or arrangement 

 indicates an apical area which has been directly affected by an 

 up-thrust or a down-throw. 



At least five types may be recognized: domes or quaquaversal 

 folds, radial linear folds, concentric terrace folds, linear terrace 

 folds and monoclinal folds related to deep-seated faulting. Of 

 these only one class, the domes or quaquaversal folds, is typically 

 due to a force acting upward. As one would expect from the 

 theoretical conception of the earth as a failing structure, most of 

 the forces acted downward. 



' Published with the consent of the State Geologist. 



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