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Geology Down The Yukon Guide
Eagle Bluff, the rust colored cliff dominating the skyline northwest of town, is composed mainly of greenstone. The rusty color results from oxidation of iron in the greenstone. Thick veins of quartz cut through the bluff.
Folded and faulted layers of shale, sandstone, mudstone, and conglomerate are exposed on the left side of the river between Eagle Bluff and Sixmile Bend. This is the Nation River Formation (Devonian).
Calico Bluff welcomes the river traveler to Yukon-Charley Rivers National Preserve. Rhythmically layered limestone and shale constitute the Calico Bluff Formation (Mississippian and Pennsylvanian). The spectacular folds and faults are evidence of the compressional forces in the earth. The yellow-green layers in the center of the bluff are black shale that has been coated with sulfur, a typically abundant substance in organic shale. Ford Lake Shale is visible once again on the downriver side of Calico Bluff.
High above the next bend is a jagged light gray outcrop known as the Limestone Hogback. It is composed of Funnel Creek Limestone (Cambrian). This resistant, siliceous limestone typically forms massive cliffs.
Upstream from the mouth of the Seventymile River, a striking bluff displays vertically oriented rocks. This formation is the Step Conglomerate (Permian), which is composed of mudstone, shale, sandstone, and conglomerate.
Barren cliffs downstream from the mouth of the Seventymile River expose sandstone, mudstone, conglomerate, and coal (Cretaceous and Tertiary).
Looking toward the Tatonduk River drainage on a clear day, you see a panoramic view of the Ogilvie Mountains. These mountains are composed almost entirely of limestone and dolomite (Early Paleozoic).
Montauk Bluff is a part of the Nation River Formation. On this bluff and the adjacent ridge are exposures of chert-pebble conglomerate, shale, and fine, olive-gray sandstone. This formation is found extensively throughout this section of the preserve.