Open-pit mining
After mining finishes, the mine area may undergo land rehabilitation. Waste dumps are contoured to flatten them out, to further stabilise them. If the ore contains sulfides it is usually covered with a layer of clay to prevent access of rain and oxygen from the air, which can oxidise the sulfides to produce sulfuric acid, a phenomenon known as acid mine drainage. This is then generally covered with soil, and vegetation is planted to help consolidate the material. Eventually this layer will erode, but it is generally hoped that the rate of leaching or acid will be slowed by the cover such that the environment can handle the load of acid and associated heavy metals. There are no long term studies on the success of these covers due to the relatively short time in which large scale open pit mining has existed. It may take hundreds to thousands of years for some waste dumps to become "acid neutral" and stop leaching to the environment. The dumps are usually fenced off to prevent livestock denuding them of vegetation. The open pit is then surrounded with a fence, to prevent access, and it generally eventually fills up with ground water. In arid areas it may not fill due to deep groundwater levels. Instead of returning the land to its former natural state, it may also be reused, converting it into recreational parks or even residential/mixed communities.
Parc Slate Quarry
Moses Kellow, who was born on 5 June 1862 at Delabole, Cornwall, close to one of the largest slate quarries in Britain. His parents moved to Caernarfon some three years later, and besides his school education, his father trained him in engineering and geology. By the age of 20, around 1882, he was managing Parc Quarry. Slate in the Blaenau Ffestiniog region is generally found in veins which slope downwards at an acute angle from the horizontal, resulting in the quarries being underground once the initial surface rock has been removed. The veins are separated by layers of harder rock, often granite or chert, which are called "hards". The slate vein worked at Parc Quarry was about thick, and sloped downwards at about 42 degrees. Unusually, the pillaring line, one of the planes in which rock can be split, was almost horizontal rather than vertical, and the foot joints did not run in a suitable direction to detach slate blocks from the floor of a chamber. Additionally, the rock above the vein was not hard enough to support the roof of large chambers. Because of this, Kellow's father, William, decided in 1880 to work the quarry from the bottom upwards, rather than adopting the more normal practice of working from the top downwards.