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Outcrops versus Exposures, an Essay

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Basalt outcrops in southern Oregon

Photo (c) Andrew Alden, licensed to About.com (fair use policy)

The terms geologists use to describe bedrock that's available for the hammer are two: exposures and outcrops. Exposure covers all cases, whereas outcrop is used for an exposure that is natural. The faces sculpted on Mount Rushmore are exposures, but Mount Rushmore itself is an outcrop. (Many geologists would argue that I'm wrong, and they are not wrong.) The subtler shades of meaning of these two words reflect their deepest roots.

Rock Outcrops

The first people to call themselves geologists, about 200 years ago, visited mines and talked to lots of miners. In England, the miners used the words "cropping" or "cropping out" to describe rocks showing themselves above the ground or mineral seams unearthed in a mine. These are ancient words: the verb crop goes back to Old English and beyond; it means to grow or swell. (Farmers changed it first into a verb to describe the growth of fruit and grain, then into a noun to denote the fruit or grain itself, then into a verb again for the act of harvesting the crop.) Today we still use the archaic form of the verb in to crop up, meaning to emerge, and to crop out, in speaking of rocks. To the miners, an active process of growth and emergence, even a vital force, was implicit in their word "outcrop."

The early geologists, who wrote for polite audiences, made a point of noting that "cropping out" and "outcrops" were miners' slang, not educated English. Miners have always been superstitious people with magical beliefs, and the notion of rocks growing was a clear sign that they saw the underground as an active, living place. Geologists were bent on avoiding all taint of the supernatural, even in their figurative language.

But the terminology stuck, and as geology became popular in the mid-1800s "outcrop" soon entered everyday language as a noun and, inevitably, a verb derived from it (along with "outcropping," a noun derived from that derived verb). Careful users of geological terminology retain "crop out" as the verb and "outcrop" as the noun deriving from it: we say, "Rocks crop out in outcrops." But even the professional literature has many instances of "outcrop" used as a verb, and "outcropping" has a place today when the point is to be decidedly casual.

Rock Exposures

"Exposure" is a noun based on the verb expose, to reveal or uncover, which has its origin in Latin and came to us through French. Its root meaning in Latin is to bring forth. We still feel this sense when we speak of a "rock exposure" in a roadcut or quarry face or building foundation, where the bedrock is actively brought forth by human activity.

We have a strong sense as geologists that bedrock forms deep underground. Thus wherever bedrock appears at the Earth's surface, something must have removed an overburden to reveal it. The rock just lay there the whole time. Whether it was erosion or bulldozers that did the removing, a passive process of unroofing or exhumation is implicit in the word "exposure."

Niceties and Ironies

Whether a body of rock looks like it grew out of the ground (outcrop) or was uncovered (exposure) would seem to make no difference—and many geologists make no distinction—but I think the two terms have subtle connotations. Outcrops are natural, but exposures need not be. An outcrop should have a rounded, organic look while an exposure should be more chiseled. An outcrop should protrude whereas an exposure may be flat or concave. An outcrop offers itself; an exposure grudges being opened to inspection. Exposures reveal petrology; outcrops show personality.

But the miners in their centuries of observation and lore intuited something true: ore veins and granite dikes are clearly invaders of the older rocks they occupy. These things did rise and swell upward from below; their shape implies their process—they do grow. "Cropping" was just the right word. Geologists recognized this too, but unlike the miners they came to understand that the activity happened and ended an unimaginably long time ago. The miners' beliefs in subterranean actions and agents—their imps and pixies and tricksters—arise naturally from human psychology in the underground setting.

And we also have a large class of rocks—lavas—that do in fact "grow" on the Earth's surface. Lava emerges from the Earth and lies there naked, shaped by its own energies. Are lavas outcrops or exposures? The geologist calls them neither, preferring the more specific words "flow," "bed," "pillow." If pressed, the geologist might well choose "exposure" as the more neutral term. Lava formations don't have the look of something coming out from under the soil; instead, soil gradually grows upon them.

So perhaps there is a case to make that outcrops refer only to formerly buried bedrock (which would imply that lava is not "bedrock"). As erosion exposes and gently sculpts the rocks, their details emerge on their skin: variations in hardness and texture, fractures and joints, weathering pits and resistant strata. The outcrops take on character. The irony is that the body of rock that looks most organic and "alive" is in fact the most passive.

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