By looking at other outcrops in the area, our geologist is able to draw a geological map which records how the rocks are related to each other in the field.
From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.
They would all have fitted nicely into the field relationships that he had observed and his interpretation of them.
The field relationships, as they are called, are of primary importance and all radiometric dates are evaluated against them.
For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.
His geological cross-section may look something like Figure 2.
Clearly, Sedimentary Rocks A were deposited and deformed before the Volcanic Dyke intruded them.
A date determined for bone associated with an artifact (Caulapan sample M-B-6, see below) agrees with a radiocarbon date for fossil mollusks in the same bed and indicates man's presence more than 20 000 years ago.
However, some of these bone dates exceed 200 000 years.
In an attempt to date stone artifacts of Early Man excavated from several sites at the Valsequillo Reservoir, a few kilometers south of Puebla, Mexico, Szabo applied the uranium-series method on bone samples known to be either from the same geologic formation as the sites or in direct association with the artifacts.
The geologic context of the bones was studied by Malde, and the archaeological sites were excavated by Irwin-Williams.
Or he may suggest that the result was due to a characteristic of the lava—that the dyke had inherited an old ‘age’. 200.4 ± 3.2 million years) implies that the calculated date of 200.4 million years is accurate to plus or minus 3.2 million years.
In other words, the age should lie between 197.2 million years and 203.6 million years.
Because such dates for man in North America conflict with all prior archaeological evidence here and abroad, we are confronted by a dilemna — either to defend the dates against an onslaught of archaeological thought, or to abandon the uranium method in this application as being so much wasted effort.