Observing rock layers exercises your mind

No, the term "relative age dating" does not refer to a socially taboo subject, just a series of logical observations, called principles. They were developed long ago to assess which rock units are older, in a relative sense, than others.

To understand one of these, the Principle of Superposition, consider dealing cards (not from the bottom of the deck; in the Old West that could lead to serious consequences). The first card dealt is laid down before the next card is placed atop it, hence the first card is "older." Look at the east side of the Rogue Valley. The oldest rock layers low on the slope were laid down first then succeeded by increasingly younger rock layers toward the skyline.

Now consider an irate card player who draws a cleaver and chops a losing hand (cards, not an appendage) in half. The split occurs after the cards are dealt, so the split is younger. That illustrates the Principle of Cross-Cutting Relations. That principle is represented in nature by a fault that breaks the rocks apart: the fault occurred after the rocks were deposited. The card dealer, miffed that his only deck has been mangled, glues the split cards back together. The glue represents magma (molten rock) injected into the fault (magma, like water, exploits weaknesses in rocks). This succession can be seen on the Mount Ashland access road just before the saddle where a dirt road drops toward the Colestin Valley. Layered rocks (including white, hardened, volcanic ash, called "tuff," containing carbonized leaf fragments) are cut by a near-vertical fault that was later intruded by a dark dike (the now-cooled magma).

Another tool is the Principle of Inclusions. A thirsty card player asks for whiskey to pour over a lonely ice cube already in his glass. The ice cube is older than the whiskey. Dark rock inclusions of older rock (ice cubes) in younger, lighter-colored magma (whiskey) represents pieces of older, partially digested rock included in the magma. Such features can be seen in the granite along Interstate 5 toward the Siskiyou Summit or along the Mount Ashland access road.

Before the advent of radiometric age dating, geologists used these principles to develop a relativistic geologic time scale. Exercise your powers of observation using these principles to determine which rocks in a hillside or road cut are older (careful of trucks lest you end up as a Peterbilt hood ornament). You're getting much smarter, aren't you? Well, at least you're getting outside.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.


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