The slate itself was exploited at least as early as 1860; the largest quarry was Dandderwen quarry (known as Whitland Abbey Slate quarry after the name of the company which exploited it). There is some suggestion that the Gilfach quarry may have been worked as early as the 16th century. It is also claimed that the slate used on the roof of the Houses of Parliament, when rebuilt in the 1830s, originated from the Gilfach quarry. Much of the slate industry went into decline after the 1890s, and by the 1930s most were closed due to competition from cheaper sources. Gilfach quarry on the eastern side of the Eastern Cleddau was still in operation until 1987.