An early Eocene date for the origin of the Golden Valley Formation in North Dakota is confirmed by the discovery of fossil fish, amphibians, reptiles, birds, and mammals. Riparian and aquatic animals and plants that were associated in this early Tertiary biofacies indicate warm, humid, swampy lowlands with subtropical forests bordering sluggish streams. Montmorillonite in the formation may represent ash from volcanoes to the west.

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