Word: layering
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...Natural History, found stone tools dating back 2.6 million years in the Afar region of Ethiopia. Two years later their team made an even more dramatic discovery. Not far from their first find, they uncovered the fossilized remnants of a 20-year-old female Australopithecus lying in a layer of sediment 3 million years old. Unlike most other fossils of early man ?a tooth here, a bone fragment there, occasionally a portion of a skull?this one comprised a good part of the skeleton...
...into its chemical components and simply disappear. Few fossils that are formed survive; most are destroyed by the continuing erosion of wind and water. Fewer still are discovered. Though hominid fossils may exist elsewhere, they are found most abundantly-and frequently-in eastern Africa. There, geological processes have exposed layer upon layer of sediments that skilled anthropologists can mine for fossils...
...other direction, the Haymarket stop near the North End is worth getting off for. At the market vendors offer fruit and vegetables from open wooden carts on the street, which is always covered with at least a day's layer of garbage. Good deals are available there, but watch out for the hucksters who will slip you a bag of rotten peaches from behind their stands in place of the perfect peaches enticing you out front. It is wise to go in the evening, when the sellers want to go home and will therefore sell whatever they have left...
OZONE DESTRUCTION. The layer of ozone, a highly reactive form of oxygen, in the stratosphere absorbs much of the ultraviolet radiation from the sun, protecting humans and other organisms from an overdose of these solar rays. But the layer is fragile, susceptible to a number of chemicals that convert ozone back into ordinary oxygen. According to the U.N. report, the widespread use of fluorocarbons as propellants for aerosol spray deodorants and other products may have already depleted the ozone layer by 1%, and could cut it by as much as 10% by the year 2050 unless the use of fluorocarbons...
...accelerating use of nitrogen fertilizers, which are converted by bacteria into nitrogen oxides that eventually rise into the stratosphere, could result in the destruction of as much as another 15% of the ozone layer. Most disastrous would be a large-scale nuclear war, which would blast enough nitrogen oxides into the atmosphere to reduce the ozone layer by as much as 70% for a period of from five to ten years. Even a slight increase in ultraviolet rays seems to cause a higher incidence of skin cancer, and a significant depletion of the ozone layer could cause far-reaching damage...