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From 1870 onwards, the industry changed from a large number of small owners to a few, relatively large groups, owned mainly by the ironmasters. These included Stanier’s Silverdale, Apedale and Knutton (Racecourse) Works; Earl Granville’s Shelton Works; Robert Heath’s Norton and Kidsgrove Works; Kinnersley’s Clough Hall Works; Duke of Sutherland 's Florence, Kemball, Stafford coal and iron work, and Williamson’s Chatterley-Whitfield Works; all owning a large number of collieries mining both ironstone and coal.  In 1872 the Coal Mines Act required the employment of qualified and certified mine managers. Due to these factors pits were being sunk much deeper and were becoming more extensive.