The argillite slate is a thinly laminated, gray, fine-grained siltstone, composed of clastic (transported) rock fragments. The geologic history of the Tillery Formation began 550 million years ago in the Cambrian Period, approximately 330 million years before dinosaurs. Rock fragments of volcanic ash origin were deposited in a water environment (sedimentation) and later solidified into solid rock (lithification). Consequent burial and tectonic pressure then changed (metamorphosed) the rock into argillite slate.

Along with the deposition of the volcanic ash was an occasional ash (debris) flow or gravitational mud-type flow into the same deposition basin. Additional layers, consisting of volcanic tuff with high calcite concentrations, formed within the system. Subsequent millions of years of geologic forces caused the alternating layers of material to fold and fault, causing disorder to the once ordered, layered system. Along with this disorder came diabase dike rock intrusion of Triassic-Jurassic age (about 180-220 million years ago), which caused additional rock structures of vertical emplacement that further complicated the system.

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