  Large shovels are often used to mine coal, which lies in thin layers near the surface. The top layers of dirt (called the 'overburden') are temporarily removed, the coal is excavated, and then the dirt is put back. This process is called 'strip mining', because it is done in huge strips along the land. In this aerial view of the process, you can see a huge excavator at work. Although it looks like a shovel, this machine is actually called a 'walking drag line', because it moves by alternately lifting and lowering two gigantic 'feet', and the bucket is dragged by cables instead of being anchored on the end of a steel arm. Here you can see the seam of coal being mined; it's a thin layer below and behind the excavator, on the left. Smaller equipment in the centre-foreground is used to remove the coal. |