Given that the Niagara Escarpment is, by definition, a cliff you’ll see surprisingly few fences and safety barriers as you travel along it hike. The Bruce Trail Conservancy has (rightly) left nature alone and trusts that visitors are savvy enough to know a drop off when they see one and generally keep to the trail. Once place that you will see a fence is near the Queenston Quarry, approximately midway on the trail between Queenston Heights and Fireman’s Park. These fences have nothing to do with the steep Escarpment though, they exist to protect you from a very man-made hazard.
Beyond these fences you’ll find five large unmarked circular holes that drop into the ground. Like buried silos they drop down into the side of the Escarpment, each lined with limestone bricks mined from the Queenston Quarry. These are kilns. Ovens used treat the raw materials and produce natural cement.
The kilns date back to 1882 and were part of an operation by the Issac Usher & Son company. They mined a natural layer of rock cement, a mixture of lime and clay, from the hills next to the main limestone mining operation at the Queenston Quarry. The caverns they hollowed out are now known to locals as the Queenston Caves. While the caves do expose the mud floor of an ancient sea they’re far from naturally occurring structures.
Natural cement has fallen out of fashion. The standard used in construction these days we call Portland cement. Natural cement has something of a terroir to it. It reflects the properties of the land from which it was made. As such it was unreliable as a building material. Portland cement has more stable properties, sets faster, and performs uniformly regardless of where it was produced.
If you climb down the Escarpment next to the kilns the substructure reveals itself to be a large stone building with fireplaces at the base of each cylinder. The mixture of clay and lime mined from the Queenston Caves would have been brought down here to the kilns, fired, and later ground into powder and barrelled.