Photo by Greg Burke
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Looking like the baked layers of a prehistoric cake, the colorful lakebed sediments and rhyolite of Chalk Basin provide an awesome backdrop to the Owyhee Wild and Scenic River. The black basalt contrasts with lighter-colored lake sediment.
The geologic history of Chalk Basin reaches back more than 16 million years, when caldera-forming eruptions covered much of the area in great rhyolite flows up to 800 feet deep. So much material erupted that a great depression, called the Oregon-Idaho Graben, formed. Eruptions of rhyolites and basalts continued to occur within the graben for several million years. These volcanic deposits, combined with sediment deposition within the multiple lake basins of the graben, make up Chalk Basin.
Chalk Basin lies within the Owyhee Canyon Wilderness Study Area, an area that the BLM in the late 1970s recommended be excluded from future development in order to protect its primeval and natural character.