27. Surface Expression of Subsurface Structures

Photo 49: Fold structure at Abraham Lake, Alberta. 2011-05-18

Fold Structures

When rocks are under a lot of stress, due to plate movement or otherwise, they can respond by bending or breaking (see post on Unit 26 for faulting). Folds are bended earth-layers or strata. All rock bends, but sedimentary rock strata bends easier than for example igneous granite. Faulting and folding often occurs together. Photo 49 displays a clear cut through of a syncline, the downfold. The upfold, or hill, of a fold is called anticline. Over time, these fold structures erode; the crests of the anticline become flatter and the valleys of the syncline fill in with new sediment. In a natural situation, the upper soil layer in a syncline is therefore often younger than that of the anticline.