Tag Archives: geoscience

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

Geopic of the week: Breccia

Dolostone breccia (1) corrected

A Breccia is a rock made up of angular rock fragments mixed with finer sediment.  The one above was deposited about 450 million years ago (Ordovician) in a shallow sea in what is now the Ozark highlands, north of Mountain View, Arkansas.

Breccias can form by a variety of processes such as landslides, volcanic eruptions, storm events, cave and sinkhole collapses and others.  This one likely formed during a storm.  Turbulent ocean waves and currents washed fragments of marine organisms (dark gray) into calmer water where marine mud (light gray) had recently accumulated.  The turbulence ripped some of the mud up and mixed it with the organic material.

Breccias are just one of many clues, recorded in rocks, that help geologists understand how the earth has changed through time: a story that itself constantly evolves as new rock is unearthed and studied.

For more views of breccias click here