James Schoenwetter

“ Crossdating is thus of vital importance and is constantly undertaken. The archaeologist tries to employ both absolute and relative clocks to find out the age of a site. Stratigraphy yields a series of relative dates for artifacts within the site; geologic-climatic and pollen dating yield a series of dates for types of sediment and samples collected in association with those artifacts; tree ring dates yield the absolute age of the site which allows the pollen, stratigraphic and geologic-climatic dates to be comprehended in terms of absolute age. ”

“ Another widely used absolute clock is that based upon the orderly decomposition of carbon. This is the radiocarbon or C-14 method of dating. Molecules of various substances are made up of atoms. We know now that not all atoms of a substance are precisely the same. We speak of isotopes of an atom. To clarify this let us think of the atoms of carbon as being made up of a mass of ping-pong balls. Some atoms will have more ping-pong balls than others, but all will have enough and in the proper order to be carbon atoms. ”

“ Plant and animal fossils are clues to ancient climates, since living organisms have a tendency to live in climates to which they are best adapted. If we find the fossil bones of a giraffe or an ostrich in the Sahara Desert, we can conclude that at some time in the past the Sahara was not a desert but a veldt, since giraffes and ostriches live in the veldt today. If stratigraphy allows us to relate artifacts to those fossils, we can maintain that the makers of those artifacts lived at the time the veldt existed in what is now the Sahara area. ”

“ A clock of the second type yields what is called a relative date, placing an event as before or after another event, but does not tell us exactly how far they are apart in time nor how long ago they occurred. ”

“ If rocks are constantly wearing down, it follows that the surface of the ground is constantly building up. Thus the surface we walk on is a younger, higher surface than that which our ancestors walked on. When we dig below the surface, those things we find which are at higher levels are younger than those which we find at lower levels. The deeper we dig, the older things get. ”

“ Animal fossils are relatively rare, and fossil remains of plants which can be seen with the naked eye are even rarer. But microscopic pollen grains are not particularly rare in the stratigraphic sequences of sediments, and are directly associated with artifacts in many kinds of sediment. The pollen of most plants is protected by a tough coat like that on many seeds. Millions of pollen grains are 13 produced by local vegetation each year, and a percentage of them are buried in the yearly accumulation of sediment. ”

“ Now all living things contain carbon atoms, and some of those atoms are C-14 isotopes. The amount of C-14 isotopes in a living organism quickly reaches a stable percentage after which there is no increase or decrease while the organism is alive. After it dies, the C-14 supply is not replenished, and with the passing of 5,730 plus or minus 40 years, it has half the number of C-14 isotopes it had when alive. ”

“ Certain trees whose water requirements are high live near streams or other places where their roots can tap a constant supply of water. Such trees, referred to as complacent, have annual rings which are all of about the same width. Other sensitive trees live in places where they must depend almost wholly on rainfall for their water supply, as on the slopes of hills or in the clefts of rocks. ”

“ Through dating those climates, the artifacts with which the pollen sample was associated are dated.
Some archaeological sites do not allow dating by any of these methods. The farmer plowing a field may pick up an arrowhead and wonder how old it is. Can the archaeologist date the arrowhead? 14 The answer is a qualified yes. What the archaeologist will try to do is give an educated guess as to the age of the arrowhead. He might, for example, know that arrowheads of that type are found at a site which is dated about A.D. 1000 by various methods. ”

“ The chemical properties of certain minerals change when the minerals are heated to high temperatures. If they are heated again they will glow, and the amount of time they glow upon reheating depends on the amount of time that has elapsed since they were heated originally. ”

“ During the winter, when the glacier freezes or the density of the water in the lake increases because of the cooler temperature, less particles are deposited, and those which are deposited are usually of a characteristic color or texture. ”

“ Work is now in progress to determine how the field has varied through time and how successfully one can date materials such as pottery or hearths which were exposed to high temperatures in the past by the difference between the present magnetic field and that trapped in the object. ”

“ At present we have a tree ring calendar for certain species of trees extending back to about 100 B.C. This is called a master tree ring chronology. A log from an archaeological site may contain only one or two signatures and be, in itself, a floating chronology, but by comparison with the master chronology one can determine the cutting date. Having a series of cutting dates for the construction timbers in an archaeological site yields the probable dates at which the site was built and occupied. ”

“ In any given area, especially in arid and semiarid regions, some years have more rainfall than others. The sensitive trees will produce wider rings during years when there is more rainfall and narrower rings in years when there is less. ”