The buried Ep-Archean and Ep-Algonkian erosion surfaces of the Grand Canyon are outstanding examples of well-preserved peneplains. The maximum relief of the Ep-Archean surface is less than 50 feet, and monadnocks rise a maximum of 800 feet above the level of the Ep-Algonkian surface, about 95 per cent of which is relatively flat.
Residual regolith on these surfaces shows that they are of subaerial origin and that chemical weathering was dominant in the closing stages of the erosion cycle. The ultimate product of extended weathering consists of quartz, muscovite, clay, and hydrated iron oxides. In some places a residual enrichment of iron appears certain. The most maturely weathered materials contain an average of 88 per cent insoluble residue. Moderately fresh potash feldspar on the surfaces and in the overlying beds is attributed to removal of the more maturely weathered material by the encroaching seas.
Various lines of evidence are cited to show that climatic conditions toward the close of the Ep-Archean and Ep-Algonkian intervals were relatively humid. The most noticeable effects of marine processes are the reworking of the weathered mantle and erosion of monadnocks. A spectacular slide breccia resulting from such erosion is described. A calculation based on the best available evidence indicates approximately 100 million years as the length of the Ep-Algonkian interval, and the Ep-Archean interval may have been longer.