A pit prop or mine prop (British and American usage, respectively) is a length of lumber used to prop up the roofs of tunnels in coal mines.
Canada traditionally supplied pit props to the British market. As coal mining declined in importance and metal supports were used, the term became infrequently used.
Though it was merely a log cut to a particular length, it was classified as a finished product and so got around the extra Canadian tariffs on the export of raw lumber.
Because of the large quantities exported, it is probable many ended up in British pulp mills.