SLC History and Geomorphology

The Saint Léonard Cavern (SLC) is situated in Montréal’s St. Léonard borough, in the district’s Parc Pie-XII where members of Québec’s Speleological Society study, tour, and maintain the site. The site’s utility can be attributed to its functioning as a study site for natural and climate sciences among other things, and the SLC provides a proxy record for climatic events over an interesting period on the geologic time scale. In short, this site can tell us a lot about the relatively recent climatic events and formation/alteration of Earth’s crust in this region, as far back as the most recent major glaciations (ice age events). The cavernous formation in Saint Léonard is a result of glaciotectonic activity, according the Québec Speleological Society, which is to say a fracture in a section of rock in Earth’s crust carved out by the movement of massive glaciers. Following recent closures of the site due to perceived risk, parts of the SLC have recently been reopened to the public and as study sites for various groups, thanks to maintenance by a working collaboration between the Speleological Society and the St. Léonard borough. Until quite recently, a section of the SLC with a length of 35 meters was comfortably navigable with ceiling heights of up to two meters, and cavern widths of as much as three meters. A map of this section provided by the Speleological Society can be viewed in Image 1.

The rock slab in which the SLC was carved is limestone, and dates back to the Ordovician Period, which covers a period of the Paleozoic Era from ~485 million years ago to ~444 million years ago. A formation of this age dramatically predates the existence of dinosaurs. Interestingly, fossils found at this site offer a glimpse at what life on Earth may have looked like in such distant times. The fracturing of this limestone slab by massive ice sheets of the continent-covering variety, is what ultimately created this cavity in Earth’s crust. An example of the fossils that can be found in the undisturbed sections of limestone strata in the SLC is depicted in Image 2.

IMAGE 2. Fossils in the limestone stratum of the SLC. SOURCE: Photograph by Bishop’s University ESG 362 student/caveman Andrew Howarth.


The age of the SLC appears to be approximately 15,000 years, which dates back to an interesting period on the geologic time scale for humans, in particular. Around the time that the SLC was formed, humans were making their first sizeable migrations from Eurasia to the North American continent in a climate that had been, and continued to be relatively unstable. Both events—the formation of the SLC, and human migrations into North America—are related directly to the formation and fluctuation of ice sheets (inland glaciers) on the North American Plate. The massive ice sheets that formed during cooler climatic periods carved out a variety of formations in Earth’s crust via their shifting and movement, a unique example of which is the SLC. A physical consequence of the accumulation of Earth’s water on land in solid form was the simultaneous lowering of sea levels by over 100 meters for long periods of time between 20,000 and 10,000 years ago. The result of sea level lowering on this scale is dramatic extension of coastlines, and exposure of vast new landmasses such as the Bering Land Bridge, which connected modern Alaska at its West coast to modern Siberia at its East coast during the late Pleistocene Epoch. It was across this Bering Land Bridge that the first large human migrations into North America took place.

The first discovery of the SLC by non-indigenous inhabitants of the region occurred in 1812, when a farmer who owned the land now occupied by Parc Pie-XII and residential space discovered the cavern opening in his field. The SLC is rumoured to have functioned as a dwelling for aboriginal people, and even as a cache for patriots and munitions in the 1837 Rebellion, though no physical evidence of past use is present in the modern SLC. Some notable dates and periods stretching from the first discovery of the SLC by non-indigenous people of the region, to the recent discovery of new sections in the SLC are outlined below, in Table 1.