from The Century Dictionary.
- noun One of several short planks or scantlings used at the heading of a tunnel through soft material. They are driven parallel with the axis of the tunnel into the material to be excavated, near the top of the heading and in advance of the lining or permanent timber, and serve to support the roof of the heading until the entire face of the heading can be excavated and the lining or permanent timbering can be brought up and completed. See
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This was done by the usual post, cap, and poling-board method.