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The Marble Mountains, in California’s Mojave Desert, contain extensive exposures of Cambrian sedimentary rocks. The Cambrian is the oldest unit of the Paleozoic era, extending from about 540-490 million years ago, and is famous as the time when macroscopic fossils with “hard parts” first appeared.
The most characteristic fossils of the Cambrian are trilobites, creatures distantly related to insects and crustaceans. Typically they consist of a mineralized head part (the "cephalon"), with segmented middle and tail parts. Because of its size and mechanical resistance, the cephalon tends to be the most easily fossilized part.
Trilobites are abundant in the Cambrian rocks, particularly in a greenish-rusty colored shale traditionally called the Latham Shale. In fact, much of the northern part of the range is included in the Trilobite Wilderness Area.
The most convenient area to search for trilobite fossils, however, is at the southern tip of the mountain range, where there is a de-facto quarry where the public is welcome to dig for fossils. (Within reason, of course; don’t bring a backhoe!)
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Know Before You Go
From old US 66 (California State Route 66, the National Trails Highway), turn south onto Cadiz Road from the hamlet of Chambless, about 11.4 miles east of Amboy. Cadiz Road here is paved; follow it about 4.2 miles to a junction (at about 34.52071 N, 115.49596 W) with a dirt road coming in from the left. Turn left here. (If you cross the railroad crossing you've gone about 0.2 mile too far.) Go about 700 feet to an intersection with a well-defined dirt road coming from the left (about 34.52957 N, 115.49344 W). Turn left here and go about 0.56 mile to where a road comes in from the right (about 34.52793 N, 115.48923 W). Turn right here.
After about 1200 feet, follow the shallow bend to the left. About a mile ahead in the drainage in front you will see a gray limestone bluff (located about 34.53633 N, 115.47647 W). The shale cropping out below this limestone offers the best prospects for finding fossils. You can keep driving toward the limestone on the road, but vehicles without four-wheel-drive and sufficient clearance will have to park at some point. You can then walk from there.
The trilobite fossils will lie flat on the bedding planes (layering) in the shale. Split the shale along the layers; trilobites will appear as impressions on the surface of the layers (see photos). A mason's hammer, which has a broad horizontal chisel opposite the hammer face in place of the "claw" of an ordinary hammer, will be useful. Some small chisels will also be handy.
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Published
May 23, 2025
