Concretions

I recently picked up this specimen on a hike to the Weiser Ridge area in Nevada on 03/29/2012. Though its shape vaguely reminded me a little of the state of Idaho, it was the concretions that really caught my eye. A concretion is a hard, compact mass of sedimentary rock formed by the precipitation of mineral cement within the spaces between the sediment grains. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'. Concretions form within layers of sedimentary strata that have already been deposited. They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock. This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum. Thus, as in the image above, over hundreds and sometimes thousands of years, the effects of wind and rain erode the softer materials found in the base rock, leaving these harder minerals as ‘bumps’ on the surface.  The image below was taken on 11/10/2011 while hiking the Muddy Mountains Wilderness Area. This area has many sandstone rocks filled with red concretions that were created as the result of iron oxide or 'rust' growing within the rock. It is unclear why they begin to form, however, once the process starts, the iron continues to concentrate there. Iron spots, being harder than the sandstone, form a 'bump' as the softer exposed sandstone wears away.