Geologic units in Walla Walla county, Washington

Additional scientific data in this geographic area

Quaternary nonmarine deposits (Pleistocene) at surface, covers 40 % of this area

Periglacial eolian deposits. Buff to light-brown, massive, homogenous, unconsolidated loessial silt; some water-laid material locally. Probably early Pleistocene.

Glaciolacustrine deposits (Pleistocene) at surface, covers 32 % of this area

Fine-grained sand and silt, well-stratified, with some gravel, clay, and diatomaceous earth. Contains clastic dikes in Walla Walla area.

Miocene volcanic rocks (Middle Miocene) at surface, covers 11 % of this area

Dark-gray to black, dense aphanitic basalt flows; commonly columnar jointed, less commonly irregularly and platy jointed; some flows vesicular, grading to scoriaceous; includes minor pillow lava, palagonite beds, and interbedded soil profiles and sedimentary beds; contains diatomite beds locally. Maximum thickness in south-central Washington may be in excess of 10,000 feet; much thinner in western Washington, where flows are mostly associated with marine sedimentary rocks. Includes acidic and intermediate volcanic rocks in northern Cascade Mountains.

Younger glacial drift (Pleistocene) at surface, covers 9 % of this area

Advance and recessional outwash, stratified drift, and associated deposits. Primarily silt, sand, and gravel with some clay. Includes alluvium locally and scabland deposits of eastern Washington.

Alluvium (Holocene) at surface, covers 7 % of this area

Mostly unconsolidated silt, sand, and gravel valley fill with some clay; includes low-level terrace, marsh, peat, artificial fill, and glacial deposits locally.