For those who dont know about karst environments it is the surface clues that tip cavers off as to what lies beneath.
The area in which I live (Southern Ontario) is comprised of a sedimentary plain that is overlain by a blanket of glacial till. Mid way along the northern shore of Lake Ontario a lobe of metamorphic and igneous rock juts down toward the town of Kingston. This formation is known to geographers as the “Canadian Shield”. It stands as the only exception to the otherwise limestone and dolostone terrain in the southern reaches of the province.
Caves most generally form in limestone, dolostone or marble environments because of the chemical reactions that take place and the abrasive properties of running water with a “scouring bed load”.
This blanket of earth in the south thins as it approaches the Canadian shield. It is all along that axis between the two rock types that local cavers have made their most impressive finds. It would also appear that some caves have formed more rapidly than would otherwise be expected. Marcus Buck, a respected local cave authority explains this in saying that the water running off the shield onto the sedimentary rock is quite acidic and this has served to increase its erosive properties.
Though the province’s caves appear to be concentrated in a line along the join between the Shield and the sedimentary rock I suspect that their apparent concentration is a result of both their greater visibility where the soil thins and the suitability of the environment. The local water is relatively acidic and the surface rock strata (The Bobcaygeon Formation) seems exceptionally well suited to tunnel development.
Sinkholes are increasingly probed by the caving community. They generally signal a point at which surface water sinks into an underground drainage system. This is where adventurers know that explorable tunnels might be found. Collapsed cavities beneath the surface might also create surface depressions. As these dips are usually hard to cultivate farmers tend to leave them alone. Sinks will appear as clumps of bush in the middle of a plowed field or as places where a valley meanders up into a dead end. A farmer near Rockwood told me how they had lost livestock down a sink and how he as a child had pitched rocks down the shaft. It appeared to “just keep swallowing the field”.
A few years ago a dig near Shelburne revealled a massive shaft. As the cavers progressed downward they found numerous bones. One big black rib was identified as belonging to an ice age caribou.
The Shelburne sinkhole fill had seemed to be a mixture of surface clay and large glacially deposited boulders. There is the theory that as soon of the land was cleared the ensuing erosion had plugged an otherwise quite accessible tunnel system. At the bottom of the sink diggers worked thigh deep in a freezing pool. Powered by a generator on the surface the cavers had operated huge mud sucking nozzles that drew the sludge up fire hoses to a spill site in a nearby field. The largest boulders were hauled away by a van powered winch.