Prior to the late Silurian, there were only inorganic, shallow, microbial protosoils. Soil formation (pedogenesis) properly began in the Silurian as chemical weathering of high ground resulted in minerals and clays being deposited in basins. Early organic soils containing plant material have been found in the Early Devonian Rhynie Chert, near Aberdeen, where fossils of early vascular plants have also been found. The development of deeper and stronger plant rooting systems on Archaeopteris, Lepidendropsis/Protostigmaria), and Rhacophyton in the Late Devonian resulted in deeper soils containing more organic material by the process of pedoturbation.
Clays from the smectite group are found in Devonian palaeosols. These clays take up water and swell in wet conditions and so shrink when they dry out because of evaporation. This results in vertical fissures which allow fluids to penetrate the soil. With each hydration / evaporation cycle, the concentration of calcium and magnesium carbonate minerals increases and these saturated mineral solutions react with the soil altering the aluminium based clays to either calcretes or dolocretes. This alteration is post deposition as can be seen by the limited penetration down from the surface. Older
calcretes tend to have been altered to dolocrete.