Folds

Folds occur when sedimentary beds (layers) of rock are compressed by tectonic forces. When compressed roughly equally on both sides, the beds can form a syncline (U or V shaped) and/or an anticline (arch shaped). All of the following images are cross-sectional views of beds.

This is a wide, gently deformed syncline.

These are narrow, moderately compressed anticline and associated syncline folds. At the top of this cliff, the folds become less symmetrical and "migrate"sideways. In addition, other folds can develop: notice the addition of another anticline on the upper left side of the image.

This is an overturned fold, which means that the compression was asymmetric (stronger on one side versus the other), which resulted in an asymmetric (partly "flopped over") anticline. The red dashed lines in the image above indicate where beds were present before removal by means of erosion.


These very compressed beds occur in core area of the previously mentioned overturned fold. Notice that beds in the center of this image are initially vertical, become horizontal, and go back to being vertical again. This is a rare sight for field geologists. 



This is a very small (about 10 feet in height) "kink fold," in the immediate area of the where the previous image was taken. These small "kink folds" have the same overall pattern (except in miniature) as the large overturned fold in the area.

This image was taken about 1.5 miles northward of where the previous images of the overturned fold were taken. The thin beds in the core show strong convolutions.



Farther to the north, "spaghetti" bedding occurs in the core of the overturned fold. This type of intensely deformed bedding occurs when the relatively "soft" mudstone beds "flow" like tooth-paste because of intense pressure.

This image shows a cross-section of a smaller overturned fold. There is also a small reverse fault (this kind of fault will be discussed in my next post), indicated by the red arrow. The angulation (= an angular unconformity) between the white bed and the underlying cavernous brownish bed, both in the lower left-hand corner of the image, was caused by channelized erosion of the brownish bed before the white bed was deposited.