Clay Balls in the West San Joaquin Valley, California

Abstract

Genetically, the term “armored clay balls” is preferable to the frequently used term “armored mud balls.” Such balls, crowned with pebbles and other debris, are common in Holocene alluvium of dry areas, and are formed by erosion of chunks of cohesive clay. In the study area, clay balls are common in the west side piedmont alluvium of “interfans” (coalescing fans of short, minor, ephemeral creeks), which were deposited mostly as “mudflows.” Armored clay balls are usually absent in the alluvium of major fans, with the exception of being found in the apexes of two major fans. In spring 1980, over 300 balls were also noted in a channel on the major Panoche fan, some 12 air km from its apex. These balls, however, originated locally, from the erosion of a small farm earth dike. Most armored clay balls have a homogeneous inner core and a more heterogeneous, ½ to 3 cm thick outer shell, formed by en route aggregation of pebbles, and perhaps some clay, around an original core that has rolled over a clayey bottom. Both contemporaneous and fossil clay balls occur in groups representing a single event, such as a landslide. After deposition and upon drying, many balls crack and disintegrate. Three groups of closely packed fossil armored balls have been found during the study: one at a depth of some 10 m in a clay layer exposed in Tumey Gulch, a second at the apex of Panoche fan, and a third in a drill hole core at a depth of 7 m. All studied armored clay balls have armoring on both hemispheres. Secondary removal of armoring on the lower hemisphere by receding water was, however, reported in the literature to have occurred in clay balls formed in short gulches with running water. Hence, the distribution of armoring on balls in piedmont alluvium may be used for determination of character of transportation, which may be by either relatively pure, slowly receding waterflow or by dense mud. These data may be of importance to regional engineering geologic studies for highway construction or other similar projects.