Problems caused by the complex but separate workings had arisen as early as the 1860s. The Welsh Slate Company's Lower Quarry mine extended directly beneath that of the Middle Quarry, its progress was limited by the speed with which the Middle Quarry progressed, because both were working the same vein of slate. But the Middle Quarry was mining more slowly than the Welsh Slate Company desired, so the latter had taken the dangerous decision to remove slate from the walls that supported the workings above. This literally undermined the Middle Quarry workings and during the 1870s underground rockfalls became an increasing concern.