The stratigraphic column.

a kind of geologic Dewey Decimal System for filing strata, young on top of old.

The adjacent diagram shows the stratigraphic column and indicates how the rocks of Connecticut fit into it. Fossils are the column's heart and soul; for many years they provided the only means by which a rock could be dated and placed in the column. Today, geologists often can supplement fossils with radioactive dating, the dating of rocks by measuring the decay of radioactive elements in a mineral. Since radioactive decay proceeds at a steady rate (for example, about one ten-billionth of uranium decays into lead every year), the amount of decay indicates the age of a rock in years. This technique was worked out in 1905 by Bertram Borden Boltwood, a Connecticut chemist, and finally provided geology with an absolute means of dating rocks and calculating the age of the Earth.` Fossils, on the other hand, only place a rock in its relative position in the column, indicating which strata are older or younger but saying nothing about absolute ages.

The names of the broadest subdivisions of the column - Archeozoic, Proterozoic, Paleozoic, Mesozoic, and Cenozoic - reflect the fossils characteristic of each level. The suffix zoic comes from the Greek word "zoe," meaning life. The prefixes indicate how recent that life is: Archeozoic, "archaic life"; Proterozoic, "early life"; Paleozoic, "older life;" Mesozoic, "middle life"; and Cenozoic, "new life. " Most of the smaller subdivisions of time on the right side of the column are named for areas of the world where strata containing fossils of that age were first recognized. (For example, the Cambrian is named for Cambria, the Latin name for Wales, and the Jurassic for the Jura Mountains of Switzerland and France.) Another commonly used term is Precambrian (meaning "before the Cambrian"), which refers to all rocks older than the Paleozoic.

The stratigraphic column has been worked out almost entirely on the basis of radioactive dating, fossils, and layering in sedimentary rocks. Layering is also an important characteristic of most metamorphic rocks.

*A Paleozoic pegmatite from Glastonbury, Connecticut was one of the first rocks Boltwood dated.

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