Trimble Knob, located southwest of Monterey in Highland County, Virginia, is a conical hill composed of basalt, a volcanic rock, of Eocene (early Tertiary) age.

[1] Trimble Knob is an isolated conical hill in an otherwise relatively flat valley, surrounded by farmland.

The central part of the hill is composed of basalt with a diameter of approximately 150 metres (160 yd).

The basalt intrudes through the gently dipping Devonian Needmore Formation (fossiliferous shale and calcareous mudstone), and is near the axis of a syncline in the center of the valley.

Mole Hill, located in Rockingham County, is geologically similar to Trimble Knob and thought to be contemporaneous with it, along with other intrusive igneous rocks near Ugly Mountain in Pendleton County, West Virginia.