Complex sentence: The tunnel under Gateshead, was bored through sandstone and excavated coal seams; old coal mine workings, some of which dated to the middle ages had to be filled in before the tunnelling began.
Split into simpler sentences: The tunnel under Gateshead, was bored through sandstone and excavated coal seams. Old coal mine workings, some of which dated to the middle ages had to be filled in before the tunnelling began.