ROCK FOLDS DUE TO WEATHERING 721 



raphy. Lightly dipping sandstones are usually marked by sloping 

 surfaces, and, in this particular case, the ridge has an altitude of 60 

 or 80 feet above the surrounding plain. It hardly seems possible 

 that in surface beds having a thickness of about 3 feet the superincum- 

 bent load of material which lies on the hill-slope above the point 

 where the fold was formed has been sufficient to overcome friction 

 and cause the rock to creep down the slope. 



The only other explanation that seems at all plausible is that of 

 expansion due to weathering. The process ordinarily called weather- 

 ing is doubtless complex, consisting of chemical and mechanical 

 changes in the body of the rock itself, most of which result in 

 increased volume, but probably the most important element is the 

 opening of joints and cleavage fissures. In this field jointing is 

 highly developed. In unweathered sections the joints are scarcely 

 visible, showing simply as incipient cracks in the rocks, but when 

 exposed at the surface the joints are very abundant and almost 

 always open. 



The opening of joints is probably largely due to changes in tem- 

 perature which result in the expansion and contraction of the rock 

 itself, as well as of any rock particles which may have fallen into 

 the cracks; to the freezing of water in the joints, and to the expansive 

 force of roots. Although the amount of opening on each joint is 

 small, the aggregate of hundreds or thousands of joints would tend 

 to set up stresses parallel to the bedding, and in course of time these 

 stresses would reach the point of rupture of the beds involved, and 

 a fault or fold would be produced. 



The phenomenon is most interesting to the students of structural 

 geology, and it affords a measure of the amount of expansion these 

 beds have undergone since the present surface was formed. 



