Nearly complete joint face, with full plumose pattern, Tucson Mountains, AZ

&amp;amp;lt;p&amp;amp;gt;This joint surface that cuts across bedding is essentially complete; i.e., we are looking onto an entire single fracture event. &amp;amp;amp;nbsp;The origin of the fracture is evident, for the plumose hackles converge to the point of origin. &amp;amp;amp;nbsp;The fracture propagated from that origin primarily in one direction along the median line. &amp;amp;amp;nbsp;The hackles flare to the edges of the fracture surface, meeting another fracture (at right angles to the first) where the energy dissipated. &amp;amp;amp;nbsp;&amp;amp;lt;/p&amp;amp;gt;

Whereabouts

I collected this in the mid-1970s in the Tucson Mountains. It is not common to find a single small rock that has a complete joint surface, and with such a remarkable display of plumose features.

Age & Formation

Siliceous sedimentary rock within the Amole Arkose Formation of Lower Cretaceous age, ~120 Ma.