A variety of rock types subjected to abrasion durability tests in a tumbling mill yielded variable attrition rates. An abrasion durability scale derived from experiments with monolithologic pebble populations was subdivided into four groups: weakly durable marble and schist; moderately durable basalt, granodiorite, gneiss and gabbro; durable obsidian, metasandstone and metabreccia; and ultradurable silicic rhyolite, quartzite, and chert. In experiments with polylithologic pebble populations the relative durabilities remained the same but attrition rates of less durable rock types increased whereas rates of more durable types decreased. The longer the tumbling time (or travel distance), the greater the concentration of high durability clasts. Further, the presence of relatively low durability clasts increased the distance of transport necessary to achieve a given attritional loss for high durability clasts. Upper Cretaceous and Eocene conglomerates of the San Diego area are understood more clearly using these experiment-derived principles. Conglomerates of both ages were derived from the east but the Cretaceous clast populations are comprised of Peninsular Ranges detritus whereas Eocene clast populations are nearly 3/4 silicic rhyolite of exotic origin. Cretaceous conglomerates contain rock types that reflect outcrops in the nearby Peninsular Ranges; clasts vary from weakly durable to durable. The mixed durability clast suite, together with representation of all major Peninsular Ranges bedrock exposures, indicates short-distance fluvial transport. Eocene conglomerates are dominated by ultradurable rhyolite clasts that do not occur in bedrock outcrops in the Peninsular Ranges. Evidently the rhyolite clasts were introduced to a major, west-flowing fluvial system possibly originating in the modern-day area of north-central Sonora. The attrition rates of the ultradurable rhyolite clasts were decreased at the expense of the less durable, but more locally derived, Peninsular Ranges clast suite to produce conglomerates in the San Diego area that are overwhelmingly dominated by exotic rhyolite clasts, although the rhyolite source rocks probably constituted only a small fraction of the total source terrane.

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