Depending on the soil (they seem to have rather colorful soil there), you will see disturbances of any kind because refilling a hole will show different colors.

Depending on the soil (they seem to have rather colorful soil there), you will see disturbances of any kind because refilling a hole will show different colors. The newer the dig is, the clearer the line between undisturbed and disturbed parts, i.e. it "washes out" over time. That is how for example you can figure out locations of house posts despite the wood not being there at all, or finding out that an old grave has been partially robbed in olden times. Which also means it was not really about making sure you have old wood in old soil, but to determine the age of the wood itself by dating how long ago the sand surrounding it was exposed to sunlight (as they had to make a dig or disturb the soil at that point of time).