Conclusions and References

The current working hypothesis is that the clay was deposited in a lake that formed on top of the Grande Ronde basalt flow during a hiatus in volcanic activity. At some point, the lake dried up and a low diversity assemblage of trees colonized the exposed lakebed. The site was subsequently reflooded, and the drowned trees were entombed when the Ginkgo Flow of the Frenchman Springs basalt inundated the lake. Evidence for this includes the following:

  • Presence of a fine, silty clay layer underlying the wood-bearing basalt indicates deposition in a low energy water environment.
  • Low diversity forest with closely spaced, small diameter trees and narrow growth rings indicate colonization of disturbed/new habitat and growth in stressed conditions.
  • Modern relatives of the tree genera found in the fossil deposit are commonly found in wet soils, riparian or floodplain deposits.
  • The presence of palagonite and basalt pillows indicates that the basalt was emplaced in a water environment.

Further Work

Additional samples will be obtained to confirm the presence of Liquidambar and Gleditsia. Work is in progress to further understand the depositional environment of the clay layer, including a palynological analysis. If pollen can be successfully extracted, a study will be conducted to determine the constituents of the pollen rain. This will give a more comprehensive picture of the regional forest assemblage.


The author gratefully acknowledges the support of those who have assisted in this investigation, most notably Clyde Friend of Yakima, Washington for access to his property and the donation of wood specimens for thin section analysis. Pete Wald of the University of Washington assisted with the geological study; George Mustoe and Terry Tolan provided valuable help with data analysis and interpretation.



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