[1] The formation is exposed in outcrops along the Strait of Juan de Fuca on the Olympic Peninsula in Washington state, USA.

[1] The Hoko River Formation consists of sediments deposited on the inner and middle slopes of a deep marine fan system.

[1][2] It is composed primarily of siltstones and some sandstones exposed under and to the south of the main Makah Formation outcrops along the Strait of Juan De Fuca.

[2] Of note is that the samples tested for both the Makah and Hoko River Formations showed a slight counterclockwise tectonic rotation.

M. feldmanni was first described from the formation by Torrey Nyborg, Ross Berglund, and James Goedert in 2003 as the oldest member of the genus Euphylax.