Valleys of various types have been found in the Gulf of California. Of these the most impressive is the series of deeply incised canyons that extend down the slopes around the lower end of Baja California. Crystalline rocks together with sedimentary formations of Miocene and Pliocene age have been dredged from the steep walls of these canyons. The canyons head near the shore and wind outward with many dendritic tributaries terminating at depths from about 900 to 1,300 fathoms. Narrow vertical to overhanging walled gorges at the head of some of these canyons, very different from local land canyons, are probably being excavated at the present time by the sand flows and other gravity-induced movements which have been observed along their steep courses. Sand with underlying g avel and rock fragments found in cores and box samples have been taken from the canyon axes over a wide range of depths. These indicate that a scouring action is working on the canyon floor. Despite these evidences of present-day marine erosion it seems likely that the pattern of the valleys was established by old, deeply submerged stream valleys that have been considerably modified by marine processes, especially as they sank below sea level.

Some valleys of the Gulf floor are the result of tectonism and it is possible that such movements may have helped form the canyons. In one place a canyon appears to have been cut off by faulting, and in another place a narrow gorge cuts through what appears to be a fault scarp. Other valleys owe their origin entirely to slumping and other types of mass movements on the steep slopes. This last group, like the tectonic valleys, is very distinct in character from the eroded canyons.

A problem of considerable interest is the explanation of the deeply weathered granitic rocks found on the walls of several of the canyons.