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 produced by local vegetation each year, and a percentage of them are buried in the yearly accumulation of sediment. Once they are buried, the tough outer covering is preserved.
When pollen grains are extracted from their sediment matrix, the different types of plants which grew in the area at the time the sediment was laid down can be recognized from the distinctive characteristics of their pollen grains. The pollen analyst has the job of interpreting this information to discover the nature of the vegetation patterns in the past and the climates associated with them. 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? 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. By correlation, he could apply that date.
If the arrowhead is not of a type which has been reliably dated, the archaeologist may rely upon the method of dating known as seriation. This method depends on the second assumption basic to the principle of stratigraphy, that similar objects tend to be about the same age. The farmer’s arrowhead may not be exactly like any which has ever been discovered, but it probably will be more like some known ones than others. Observing the general style of the arrowhead and the way in which it is made, the archaeologist can make a pretty good estimate of when it was made.
Let us assume that the archaeologist goes into an unexplored area where no absolute or relative dating techniques are available. There he finds a number of sites with potsherds of types new to archaeology. Can he date these sites? Again he uses seriation, but this time he inverts the logical proposition. If things which look alike tend to be of the same age, things which do not look alike should tend to be of different ages.
His first task would be to separate all the different types of potsherds. Taking a specimen of each type, he lays them out in a row. If there is any difference in the sites through time, the styles of pottery will change correspondingly. But there will be some styles which change slowly and some which will influence others. For example, if we were to seriate the style of automobile rear fenders for the period 1956 to 1964, we would observe that they began to sprout taller and taller fins; then the size of the fins became reduced more and more. Some of the style aspects of pre-1956 rear fenders went along with the development of fins and others dropped out. Some of the style aspects which were developed with the fins were retained after the fins decreased in size.
Seriation of the design styles on the potsherds will result in a series of developments in style which are probably in chronological order. Broad straight line designs may give way to mixed broad and narrow straight lines, then to narrow straight lines, then to narrow wavy lines. The archaeologist could then maintain that sites which have potsherds with broad straight lines are separate in time from those with sherds having narrow wavy lines. He won’t know which is the older, because the sequence could work either way. Stratigraphy or the cultural similarity between other artifacts at the sites will probably resolve this problem.
The reader will now have realized that many of the clocks used by archaeologists are interrelated. Relative dating clocks such as stratigraphy, pollen dating and geologic-climatic dating are utilized together where possible, and all are dependent upon the principle of stratigraphy. 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. Radiocarbon dates act as a check on the tree ring dates and, if they agree, lend support to the pollen and geologic-climatic dates. The pollen and geologic-climatic dates from the site are compared with similar dates from other sites as additional checks. Since the clocks used by the archaeologist tick at different rates of speed, and since not all of them are dating the same thing, the archaeologist usually ends with a series of dates for any given site.
New geochronological techniques are always being invented and perfected. Here is a list of some that are expected to become available in the next few years: