Geo-pic of the week: Phantom Quartz

Phantom quartz refers to quartz crystals that show the outline of smaller quartz crystals inside of them.  They are thought to form when there are at least two periods of growth.  The phantom crystal is visible either because it differs slightly in composition or because it was coated with some material prior to the second round of crystal growth. 

In the crystals above, all of which were collected near Mt. Ida, Arkansas, partial dissolution of the phantom crystals prior to the second round of growth left the phantoms with a ragged appearance.

Geo-pic of the week: Rock Beds

Why do rocks have beds?  Are rock beds where geologists sleep?  Sometimes, but that’s not the point of this article.  The picture above, taken on the Goat Trail at Big Bluff, overlooking the Buffalo National River, is a great example of a sedimentary rock composed of many individual beds (layers).  The reason that rocks are bedded is due to either gaps in deposition or abrupt changes in the grain size of sediment being deposited in an environment.

Here’s an example;  when a storm causes a river to flood its valley, the water deposits sediment as the flood recedes.  Typically, there’s a period of non-deposition before the next flood event deposits a new layer of sediment over that one. This time between floods allows weathering to alter the character of the first flood deposit.  That weathered surface will eventually differentiate the flood deposits into distinct beds of rock. 

Bedding can also form as a result of flowing water gaining or losing velocity.  The size of sediment that water carries (and eventually deposits) is directly related to flow rate.   A sudden change in flow rate creates bedding distinguished by differences in grain size.

Everyone in the photo above was eventually air-lifted to safety… Just kidding!  They’re still up there