Page "Paleopedological record" Paragraph 9

Soil fossils are usually classified by USDA soil taxonomy.

With the exception of some exceedingly old soils which have a clayey, grey-green horizon that is quite unlike any present soil and clearly formed in the absence of O < sub > 2 </ sub >, most fossil soils can be classified into one of the twelve orders recognised by this system.

This is usually done by means of X-ray diffraction, which allows the various particles within the former soils to be analysed so that it can be seen to which order the soils correspond.