material, heaped one on another. Like a great stack of pancakes, the first and oldest layers that came off the griddle are at the bottom of the pile and the last and youngest are at the top. Layering is plainly visible in all road cuts blasted through Connecticut's brownstone, every layer a vast flapjack extending under the surface of the land. Each layer is called a stratum, two or more are strata, and the whole phenomenon is known as stratification (from the Latin for "stretched out").

At any place in the world, large gaps in time can occur between adjacent layers of rock; for example, the Great Unconformity at Roaring Brook represents a gap of about 200 million years. It is as if the galactic short-order chef was a bit careless in stacking his sedimentary flapjacks on the terrestrial platter; some of the flapjacks were laid down off-center and don't completely cover those underneath, allowing the pancakes above and below to touch. Also, erosion has eaten portions of many of the flapjacks while the chef had his back turned. As a result, a fork-full taken out of the stack anywhere in the world would miss many of the pancakes.

One of the most useful contributions of early geology was the stratigraphic column, a method of depicting the entire stratigraphic stack. By combining sections of the rock record from all over the world, the stratigraphic column represents the entire history of the Earth with no gaps. A sketch of the stratigraphic column hangs on every geologist's office wall. It is to the geologist what the periodic table of elements is to the chemist.

Because the sedimentary stack is usually obscured by soil, vegetation, and recent sediment dumped by glaciers or rivers, it is often difficult to know where a small exposure of just a few strata fits in the stratigraphic column. Are the strata in question somewhere near the bottom and therefore relatively old, or are they younger and near the top? Around the beginning of the 19th century, geologists recognized that sedimentary rocks contained a built-in code - fossils - that made it possible to place isolated rock strata in their proper place in the sedimentary stack. The constant evolution in the life forms occupying this world has left a natural code of fossils in the rocks that is different for every time period. Each level contains characteristic fossils, providing

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