Geo-pic of the week: Fracture-Fill at Shine-Eye

The photo above shows a vertical dark rock in the center of flat-lying white rock. The dark rock is a sandstone deposit, probably Mississippian-aged, and the white rock is Silurian-aged limestone. If one were to follow the sandstone dike upward, it would lead to a sandstone bed sitting on top of the limestone. Since the limestone was deposited first, we can infer that it was exposed to weathering. The limestone was solutioned and deep fractures or cracks formed. Afterwards, sand was deposited in the area, filled the fractures in the limestone, and eventually lithified into sandstone. There are several of these sandstone-filled fractures present along the Buffalo National River in Silurian-aged limestone. The one pictured above is located at Shine-Eye.

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