The Glendyne Inc. slate quarry is located in Saint-Marc-du-Lac-Long in the province of Quebec, Canada. The slate deposit is of exceptional quality since it is free from pyrite and any other metallic intrusion.
The slate deposit was discovered when the transcontinental railway was built at the beginning of the XXth century. It was first operated by Europeans who had to stop production a few years later due to World War I and the Canadian government's war effort to support European allies.
In 1995, a group of local businessmen resumed operations of the quarry.