The basalt at Trimble Knob (and other igneous dikes in the area) was originally thought to be of Paleozoic age by relative age dating using cross-cutting relationships. In 1993, Southworth and others give a date of 35.0 ± 0.5 Ma for basalt of Trimble Knob In 2012, Bulas and others dated the eruption to 48.86±.37 million years ago. Both dates place the eruption during the Eocene epoch. The basalt intrudes the Devonian Millboro Shale.