Chert blocks
Chert blocks
early Middle Jurassic to Late Triassic
Large chert blocks and lenses in sheared argillite matrix. Probably olistoliths. Around the North Fork American River, chert blocks have yielded Late Triassic (Norian), Early Jurassic to Late Triassic (Sinemurian(?) to Norian), and Early Jurassic (Toarcian to Pleinsbachian) radiolarians.