You’re not around geologists very long before you hear the word “formation” mentioned. For that matter, almost anybody out enjoying a landscape with rocks in it is apt to use the word themselves, as in “there were the coolest rock formations out by Diablo Canyon!” Geologists cringe just a little when they hear that – not that you’d notice – because to them the word “formation” has a very definite meaning.

This is not a formation. It is, however, "Camel Rock"

This is not a formation. It is, however, “Camel Rock”

A formation is a group of sedimentary strata, volcanic beds, or igneous intrusions with upper and lower boundaries that can be easily traced and mapped across the countryside.

The word ‘mapped’ is critical in this definition. The first step geologists make in their attempt to understand the natural history of a region is to construct a geologic map, showing the relations between the different rocks there. In order to do this, he or she has to make some distinctions among the various kinds of rocks, subdividing the outcrops into “meaningful units” that are large enough to plot on the map and distinctive enough that other geologists can agree on their selection.

Typically the distribution of formations is shown by using different colors or patterns:

A simple geological map of New Mexico

A simple geological map of New Mexico

Because of the way they are chosen, formations have lithologic significance, consisting of a single rock type, or a cluster of closely associated rock types. Formations are the basic Rock Unit of stratigraphy.

This is reflected in the formal names given to formations, based on their definition at a type section, at a specific geographical location, where other geologists can inspect the choice. For example, the Mancos Shale, named after a town in Colorado, or the Redwall Limestone, named after the famous cliff in the Grand Canyon, are cases where the formation is basically one rock type. In cases where the formation is chosen to be a mix of associated rock types (still distinctive enough to trace and map!) the geographic name prefixes the word “Formation”. An example is the Galisteo Formation, named after a village in New Mexico.

In sedimentary formations, the strata within a formation tend to be more or less parallel, or conformable, with one another.

The Galisteo Formation, showing a mix of strata

The Galisteo Formation, showing a distinctive mix of conformable strata

Because of the way they are chosen, formations also have a genetic significance. Each one records a time of fairly uniform environmental or depositional conditions, different from adjoining formations. Understanding this is vital in our attempt to work out how conditions changed with time.

Finally, formations have a built-in time significance. By virtue of the principle of superposition – younger layers must rest on older layers – formations can be put into a relative time order, from oldest to youngest. In the case of igneous intrusions, the principle of cross-cutting relationships serves the same purpose. If an intrusion cuts through another body of rock, it must be younger in age than the rock it intrudes.

That’s a lot of significance packed into one common word! “Formation” truly is a useful concept in geology.

And yet, as geologists – especially petroleum geologists – soon discover, the formation is not the perfect stratigraphic bookmark we tend to think it is.

Among sediments, depositional environments can exist simultaneously, side by side, in a given area. Think of shrimp boats dragging their nets through the mud while swimmers frolic on clear sandy beaches at the shore. These environments – one accumulating mud, the other sand – migrate with time. A barrier beach may slowly build out over a muddy marine shelf, which, millennia later, will show up in the geologic record as sandstone over shale. We’d very likely define two formations in our mapping – say, the Point Lookout Sandstone over the Mancos Shale – and consider that the sandstone is everywhere older than the shale. But we would be wrong. The two formations actually interfinger and there are places where the sandstone here is the same age as the shale there.

Such formations are called diachronous – “passing through time”. Most sedimentary formations are diachronous to some extent. Now this may sound like the sort of hair splitting only a stratigrapher could enjoy – but understanding these sorts of relationships can be critical in defining potential petroleum traps, sources, and seals in an oil-bearing basin. An entire branch of stratigraphy called sequence stratigraphy has developed among oil companies (rooted in the insight of perceptive geologists long before, I must emphasize) in order to establish accurate time lines within and across formations, repackaging the strata into a different kind of “meaningful unit” called a sequence.

But these are subtleties we can let rest for now. You have to start somewhere, in every science, and the notion of a formation – properly used –  is one of the first stepping stones in geology. So bite your tongue next time you hike in Zion National Park or wander through Carlsbad Caverns. A geologist might be listening.


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