Larch Mountain is the remnant of an ancient shield volcano, with broad slopes covering tens of square kilometers. It is currently the tallest peak in the Boring Lava Field, a volcanic field active during the Plio-Pleistocene time frame. Active between 1.8 and 1.4 million years ago, the volcano is composed mainly of basalts, although the summit at Sherrard Point is composed mainly of iron-rich andesite. Larch Mountain's basalt is tough to distinguish from the surrounding Columbia River Basalt, although the Columbia River Basalt is slightly lighter in color and less brittle. Sherrard Point is the eroded remains of the original volcanic plug.