The landfill/quarry we visited was originally a limestone quarry; once they had the hole in the ground they needed to fill it with something so why not trash (and why not get paid to fill it).
The limestone bedrock is blasted daily to create some massive boulders. The boulders are then loaded on some equally massive dumptrucks. There are scarce few minutes between trucks, so a lot of rocks are being moved.
The trucks then dump their load into a large building where the rocks are crushed. Our guide made us stop the bus to watch the process. While watching a dumptruck unloading might seem mundane, the enormous size of the truck and its boulder load did seem to captivate the students.
Once the rocks are crushed, the resulting sediment is sorted by size (sand, pebbles and gravel, I think) and piled up. The piles are massive. I’ve been wanting a good picture that shows the angle of repose; I got several.
The pebbles and gravel are used for road construction and provide a matrix for concrete.
Since limestone dissolves fairly easily in rainwater, the sand-sized and smaller particles (< 2mm diameter) aren't used for construction -- hard, insoluble quartz sand is preferred.
Limestone: calcium carbonate (CaCO3)
However, the limestone sediment piles sit out in the open and some the finer grains (silt sized particularly), and any dissolve calcium carbonate, get washed into the nearby ponds, which turn a beautiful, bright, milky green.
Finally, in addition to the limestone sediment piles, there is also one enormous pile of broken up concrete. One of the things that stuck with the students was that fact that you can recycle concrete.