Named for Ellemeham Draw, feature 2 mi north of good exposures in bluffs northeast of Wannacut Lake, sec.1 T39N R26E Loomis 15' quad, Okanogan Co, WA, which is designated type locality. Comprises: basal member of greenstone and tuffaceous siltstone, middle member of monolithologic greenstone conglomerate, and upper conglomeratic member of heterogeneous fragments of metamorphic rock. Approximately 900 m thick; it ranges widely in thickness as do component members. Lowermost member unconformable on Kobau Formation at Shankers Bend, 1.5 mi north of Ellemeham Draw; at contact relative age documented by thin conglomerate containing quartzite pebbles and cobbles of Kobau. Upper contact with early Eocene clastic rocks is unconformable. Because the Ellemeham was deposited on rocks that were metamorphosed, probably in Late Triassic, is itself metamorphosed, and is unconformably overlain by unmetamorphosed rocks of early Eocene age, it is considered to be of Jurassic or Cretaceous age.

Source: GNU records (USGS DDS-6; Menlo GNULEX).

Age modified to Late Triassic or Jurassic based on unit probably being younger than Late Triassic Loomis pluton, and clearly older than K-Ar biotite age of 157 Ma from hornfels developed by crosscutting dike of alkalic intrusive rock. (Age surprising inasmuch as locality well within influence of Late Cretaceous and early Tertiary regional thermal event.)

Source: GNU records (USGS DDS-6; Menlo GNULEX).