Wednesday, July 16, 2014

Sedimentary Geology Basics

I was speaking with an acquaintance a couple of days ago, and he mentioned that all three of his children were following his footsteps, after the completion of their university degrees, into the field of geology.  Two had already graduated and entered the field, and the third was about to.  It occurred to me that children of geologists have a tremendous advantage entering the field, since they learn a lot of the fundamentals just from the way their parents talk about the rocks around them as they are growing up.  Because of their experience and training, there are certain assumptions that geologists make when looking at rocks.  It is easy to forget that other people don't immediately make these assumptions.  This picture is of a spring that I walk by on my way to work each morning.  I've tried to capture, in a simple graphic, several of the absolute basics of sedimentary geology.  Sedimentary rocks are layered, and the layers have names.  These layers are usually more-or-less horizontal, at least to start with.  Unless they've been tossed around somehow, the oldest ones are on the bottom, and younger ones are on the top. Layers in rock are generally called strata (plural stratum).  The study of rock layers is 'stratigraphy.'  Minor layers in the rock are called bedding planes, and may represent changes in tides, storms, or longer term events.  These don't necessarily have names.  Larger groups of these minor layers are named as 'members,' and groups of members make up 'formations.'  Members and formations are formal 'lithostratigraphic units,' and these represent distinctive, recognizable groups of rocks that are roughly the same age and typically share something significant in common in terms of their composition and appearance.  The concept that rocks are grouped and that the groups have names is absolutely key if a person wants to learn about the rocks in their area.  Knowing the name of a rock unit allows you to research everything that has been learned and published about it through the cooperative work of generations of scientists.  Here is a link to an explanation of lithostratigraphic rock units, published by the International Commission on Stratigraphy:  and here is a link to a wikipedia page discussing the concept of a geological 'formation,' the dominant unit in stratigraphic study:

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