It has been well proven that the Missoula Floods greatly impacted central Washington and there are many examples of the magnitude of the floods, its reach, and dynamics. However, there is still debate on the number, size and timing and dating of these events. What is also less well known are some of the casualties of the floods. At least one feature is helping to tell the story of some of the last floods to grace the landscape.
In 2015, a mammoth was discovered at Tonnemaker Hill Farm along the north edge of the Frenchman Hills in Grant County, Washington. Luke Tonnemaker was plowing up an alfalfa field when he made the discovery. Intensive study of the site was led by geologist George Last, paleontologist Bax Barton, local soil scientist and geologist Mark Amara, Gary Kleinknecht, education director, from the MCBONES Research Foundation, a volunteer excavation crew from MCBONES and the Tonnemaker family. The researchers coordinated with farm co-owners, Luke and his father, Kole Tonnemaker, and their wives Amanda and Sonia respectively, to professionally excavate, stabilize retrieved mammoth bones, identify as many bones as possible, and recreate the geology and history of the site.
Mammoth remains are not that uncommon in Washington with several hundred animals discovered in the state, though fewer than 100 of those discoveries are located in eastern Washington. What is significant about this mammoth site is that it is one of the few finds that has received this much intensive study. Over 130 bones and bone fragments from a single Columbia mammoth were recovered with about 69 specimens identified. The animal was estimated to be between 25-30 years of age based on characteristics of its dentition, and it even lived with a damaged rib which had healed.
George Last prepared diagram
The presence of three distinct volcanic ash (tephra) layers and soil analyses suggests that at least four different Missoula flood event episodes are represented in the sediments beneath the mammoth bone bed. Dating of the site revealed that the animal post-dated identified Mount St Helens tephra eruptions that deposited ash in distinct horizons below the bones in water borne deposits. Dates associated with ash samples identified at WSU confirmed two of the layers correspond to Mount St. Helens S series while the topmost tephra shows some compositional variability but had similarities to the J or S series. Tephra ages ranged roughly from 14,000-16.000 years ago. Further dating through Utah State University was provided by optically stimulated luminescence (OSL) samples which gave dates ranging from 16-19,000 years ago. Since the bones are above the highest tephra and OSL sampling sites, the find is definitely younger than 16,000 years ago and was on the edge of one of last Pleistocene Missoula floods slack water lakes to cover this part of Washington. These conclusions were corroborated by analyzing the stratigraphy of the site and describing the soil which confirmed environments of deposition.
Since the bones were above the aforementioned tephra layers and OSL sampling locations, and are at the interface of apparent windblown silts and Missoula floods slackwater deposits, it is still unclear how and when the animal died. However, the co-location of the mammoth remains with apparent ice-rafted erratic cobbles and boulders in fine-grained Missoula flood sediments, supports interpretation that the mammoth could have been carried in and left behind by a late breaking Missoula flood. Alternatively, it could have become mired in the mud along the shoreline and/or was killed by predators or even died in a subsequent drier period. Still the intensive studies to date have shown that this is an area with unique geologic history, one that is still unfolding.
The Tonnemaker family has the recovered bones on display at their farm store and annually, including 2026, hold tours of the mammoth site during the Othello Sandhill Crane Festival held in March each year. There are a variety of papers, poster presentations and articles published or defended at meetings held by the Northwest Scientific Association and Geological Society of America, and a final report of findings between 2016-2024, with summaries highlighted, is available for viewing at the farm.
Article by Mark Amara, Geologist, IAFI Lower Grand Coulee Chapter