Cobble of conglomerate cut by a fault, and healed by a quartz vein, Rillito River, Tucson, Arizona

&amp;amp;amp;amp;lt;p&amp;amp;amp;amp;gt;This cobble is full of history. &amp;amp;amp;amp;amp;nbsp;The rock itself reflects sediment deposited in a braid-fan deposit, produced by intermittent streams flowing in a semi-arid environment. &amp;amp;amp;amp;amp;nbsp;Times of higher flow are preserved in the granule conglomerate. &amp;amp;amp;amp;amp;nbsp;The black laminae are heavy sands, including magnetite. &amp;amp;amp;amp;amp;nbsp;If this were not enough, the rock had been faulted, as is evident by the 1 cm offset of the layers. &amp;amp;amp;amp;amp;nbsp;The fault surface was later dilated slightly and filled by quartz, now in the form of the quartz vein. &amp;amp;amp;amp;amp;nbsp;Then, at some stage the rock layer made it to the surface through uplift and erosion. &amp;amp;amp;amp;amp;nbsp;Erosion continued and ultimately dislodged a chunk that made it into a stream bed. &amp;amp;amp;amp;amp;nbsp;The cobble then was transported downstream. &amp;amp;amp;amp;amp;nbsp;As it was transported during times of floods, the chunk became rounded to a cobble. &amp;amp;amp;amp;amp;nbsp;&amp;amp;amp;amp;lt;/p&amp;amp;amp;amp;gt;

Whereabouts

A former undergraduate student gave me this in the 1990s sometime. I’m 'blanking' on who gave it to me. She found it in the Rillilto River in Tucson while hiking. I like the range of history that is bound up in this rock: so many different chapters are preserved.

Age & Formation

The granule conglomerate probably came from Miocene fanglomerate, ~25- 20 Ma. Faulting and vein-filling is probably no older than ~18 Ma.