The distribution of Pliocene and Pleistocene volcanic and sedimentary rocks in the Williamson River basin, Klamath County, Oregon suggests a boundary trending northeast from near Collier Memorial State Park to near Sugarpine Mountain. Pliocene rocks include 3.7 to 4.6 Ma trachybasalt to trachydacite flows and pyroclastic flows interbedded with calc-alkaline andesite, ~3 Ma HAOT flows (locally pillowed) and tuff cones, and ~2.0 Ma basaltic andesite stratovolcano (Soloman Butte) and slightly older flow field. The Pliocene section is composed of volcanic rocks of wide chemical variety. After 3.7 Ma erosion of uplifted fault blocks produced excellent examples of inverted topography (Wildhorse Ridge). Starting at approximately 1.6 Ma thin high alumina basalt flows were erupted locally onto the erosion surface cut into the Pliocene section.

Subsidence in areas northwest of the boundary accommodated deposition of fine to coarse-grained sedimentary rocks that interfinger with high alumina basalt flows. Cinder deposits are present in the vent areas of small shield volcanoes. Water wells indicate the section is greater than 170 m thick. The Pleistocene section contains relatively abundant sedimentary rocks and basalt flows of narrow chemical range. Near the Cascades, basaltic andesite and trachyandesite flows overlie and may be interlayered with sedimentary rocks.

The northeast-trending boundary between rocks of Pliocene and Pleistocene age is cut by north- and north-northwest-striking normal faults within the Cascade neotectonic zone. The easternmost fault, the Wildhorse Ridge-Military Crossing fault zone, cuts middle Holocene pyroclastic fall deposits from the eruption of Mount Mazama.

The stratigraphic framework and faults control the flow paths of surface and groundwater within the basin. The Pliocene section is associated with low water yields while the Pleistocene section is host to large production wells.