This is the famous Tatlock marble quarry owned by giant Swiss mineral company OMYA. The quarry is open to the public, can be accessed on from Highway 511 about 12 km north of Lanark then east on county road 9 (@ 2.4 km to site). It produces calcium carbonate derived from natural marble (or limestone) rock formation. This is the crumbly rock that OMYA is gradually stripping away for use as filler in things like paint, plastics, paper and toothpaste.
There's some interesting geology here. The limestone was formed by the skeletal fragments of marine organisms populating a vast inland sea. Over time ancient geological processes metamorphosed limestone through heat and pressure into marble. Striations and darker bands clearly show how the original limestone layers were gently folded and thrust upward as they transformed into marble over time.
Lanark County is basically distinguished by limestone bedrock in the east and south and Precambrian Shield in the west and north (Mt. Pakenham to Lanark Highlands to Muskoka and beyond). The line between the two regions is visible all the way from Champlain Lookout in Gatineau Park.