Geology Down The Yukon Guide

Eagle Bluff, viewed from a canoe on the Yukon River

NPS Photo by Josh Spice

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.

Bluffs before Sixmile Bend.

NPS Photo by Josh Spice

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

NPS Photo by Josh Spice

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.

Sixmile Bend in fall colors

NPS Photo by Josh Spice

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.
Bluff with vertical layers

NPS Photo by Josh Spice

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.
High bluffs near the Seventymile River

NPS Photo by Josh Spice

Barren cliffs downstream from the mouth of the Seventymile River expose sandstone, mudstone, conglomerate, and coal (Cretaceous and Tertiary).

A view of the Ogilvie Mountains from the Yukon River

NPS Photo by Josh Spice

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

NPS Photo by Josh Spice

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.


Last updated: October 31, 2018

Contact the Park

Mailing Address:

101 Dunkel St
Suite 110

Fairbanks, AK 99701



Contact Us