armored mud ball

An armored mud ball is the compact lithic record of an ephemeral journey, whereby a mud glob gathers a crust of stones that together metamorphose into rock. Like the caddis larva forming an exoskeleton of sand—soft center, hard shell—the armored mud ball represents the preservation of a traveling ball of mud or clay, coated with sand, pebbles, or larger stones. Presumably, the pebbly coating occurred when the original mud ball was dandied by waves across a beach, or rolled along the bed of a stream, then was buried in protective strata and hardened over geologic time. A similar effect occurs with armored ice balls, where a snowball rolling off a glacial scarp gathers a rind of ice particles that protect the softer interior. Prized as oddities by geologists, armored mud balls have been found, among other places, in Japan, the South Dakota Badlands, and the Valley of the Connecticut—which holds the best-known stream-formed exemplars.

Kim Stafford