Tag Archives: sedimentary rock

Geo-pic of the week: Rock Beds

Bluff above Buffalo River edited

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 clip_image001

Geo-pic of the week: Siliceous Oolite

Silicified oolite

Ooids are tiny grains that are typically composed of CaCO3 either as calcite or aragonite.  They precipitate from seawater in concentric bands around a nucleus (for instance a fragment of rock or fossil) in turbulent shallow conditions. 

Once ooids form, they can accumulate and be cemented to form a sedimentary type of limestone called oolite.  The above picture is a magnified and tumbled piece of oolitic chert collected fromgravel on Crowley’s Ridge in northeast Arkansas.  The difference between this and typical oolite is that it came into contact with silica(SiO2)-rich ground water after it formed.  The SiO2 then replaced the CaCO3 the rock was initially composed of.   The polished surface provides an ideal view of the internal structure of the spherical ooids.