A thrum is a localized, small-scale displacement or faulting along a seam that throws the bed slightly out of alignment. While not as massive as a major fault, it requires miners to adjust their cutting angle or drill pattern to relocate the continuation of the pay zone. It often presents as a sudden step up or down in the rock face.
Heard on the job
We hit a thrum on the right rib, so the coal drops about two feet lower than the rest of the face.
Insider tip
Minor thrums can often be mined through with minimal delay, but they weaken the immediate roof structure.