Pleistocene glacial deposits are identified easily; however, the existence of pre-Pleistocene glacial deposits has been challenged for several reasons. Recognition of earlier widespread glacial deposits is associated by many workers with continental drift, a process that complicates paleomagnetic and paleoclimatic reconstruction. In addition, pre-Pleistocene glacial deposits can be interpreted environmentally in several ways. Alleged tillites containing poorly sorted clasts, striated rock fragments, and rock flour are similar to deposits formed by subaerial and subaqueous mass movements. Few criteria alone are decisive; therefore many environmental criteria must be sought.

Among the more important physical characteristics of glacial deposits are ultramicroscopic markings on the surfaces of quartz sand grains; massive, nonsorted debris with abundant rock flour, silt, sand, and blocks; striated stones; deflection and penetration of laminae by stones; stone shape; presence of erratics; extraordinarily large boulders; ice-molded structures; striated and polished pavements; and thickness and extent of stratigraphic units.

The more important chemical criteria include comparisons of chemical and mineralogical composition of clasts and matrix, differing mineralogy of the stones, and oxygen-isotope ratios of fossil shell material.

Biological criteria include fossil invertebrates and vertebrates capable of existing in cold climates; and the identification of fossil floras which may be characteristic of cold climates.

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