Sheath folded quartz vein, Santa Catalina Mountains, Tucson, Arizona

&lt;p&gt;This appears to be part of a sheath fold. &amp;nbsp;The rock itself is a set of quartz-rich layers. &amp;nbsp;Sheath folds form under conditions of heavy-duty simple shear (non-coaxial deformation). &amp;nbsp;First there develops a strongly asymmetrical fold oriented perpendicular to the shortening direction. &amp;nbsp;But with continued shearing the hinge of the fold can flare out in the direction of tectonic transport, into a form that resembles a wind sock. &amp;nbsp;The fold assumes the form of a cylinder, closed at one end. &amp;nbsp;Cross sections through such folds look like ‘eyes.’&amp;nbsp;&lt;/p&gt;

Whereabouts

A neighbor came over to our house one morning and handed this to me (in ~1994). She found it while looking for golf balls along a desert course. It obviously had been transported downslope from the Santa Catalina Mountains. She thought it might be a fossil bone. I ‘pronounced’ it a sheath fold, and hope that I am right. It is not often that you can hold a sheath fold, and most of its puzzling geometry, in your hand.

Age & Formation

Unknown.