Both Dan and I were initially baffeled by the appearance of the underground cavity, we had arrived already having heard that it was a cave we were to explore. The breakdown at the entrance was reminiscent of many local caves – there is a great deal of such collapsed debris around Rockwood Caves and other more obscure tunnels. The apparently undulating wear marks on the roof were somewhat speleogenic in appearance and the bleached-white splatter of speleothems further fueled our suspicions. Wooden beams and bits of old equipment soon served to tell us otherwise.
I was now thoroughly puzzled by the roof, what were these unusual little hillocks. They were perfectly smooth, like mounds in a sandy beach. They were undoubtably a natural feature.
I am left with the thought that the roof probably marked the boundary between a layer of useful building sandstone and some less desirable deposit. The width and height suggest that the tunnels had been carved from a relatively thin layer, and in places, likely from the natural tendency of the rock to part along a weaker plane, the rock has split from the roof along some natural bedding plane. In these circumstances the underside of the overlying strata would now lie exposed to those in the mine beneath. The undulating mounds are the inverse of that surface.
In the book, “Ontario Rocks; Three Billion Years of Environmental Change” Nick Eyles, a professor at University of Toronto writes about the unusual sedimentary surfaces that one can find in the rock. The pattern that was lain down by the water hundreds of millions of years ago and eventually preserved as rock was dependant upon the regularity of the currrent, the speed of the flow and the size of the sediment.
In fast flowing water, the sedimentary deposits will generally be lain down in paper thin sheets. Those conditions reveal themselves to us today in layers that flake from a flat rock surface like peeling paint. You can see this kind of sedimentary structure atop great-green sliding slabs of sandstone at Church’s Falls near Belfountain. This flaking veneer occurs where the sediment was lain down in water that was flowing beyond 80 cm/sec. It is referred to as “a parting lineation”.
Sinuous crested dunes formed in coarser sediment in slightly slower water and ripples as can be so profusely seen along the banks of the Maitland in the Devonian Age rock formed where the sand was finer and moving at less than 60 cm/sec. The Whirlpool Formation Sandstone was deposited around 420 million years ago in shallow seas.
Not having really had the understanding of what I was looking at while I was in the mine I am left to now decide what kind of conditions caused that crazy, bumpy roof as a sea-bottom surface. One possibility might be the phenomenon known as “Hummocky cross stratification”. It is a lumpy surface that develops when there are large storms in shallow seas. The sediment is thrown in every direction across the sea floor by orbital currents.
Feel free to offer an opinion on the roof as seen in the accompanying photo, I might be totally off track.
FOR MORE INFORMATION ON THIS AMAZING SANDSTONE MINE THE CAVERS AMONGST YOU WILL BE TREATED TO AN ARTICLE ON THE EXPLORATION BY DAN. SEE THE UPCOMING ISSUE OF THE TORONTO CAVER.
REFERENCE, Eyles, N. “Ontario Rocks: Three Billion Years on Environmental Change”. Markham: Fitzhenry and Whiteside, 2002.
You might also wish to explore the rock for yourself. My book, “Rockwatching; adventures above and below Ontario” details the geological history of that fascinating time and in the Belfountain chapter you can follow a guided geological tour of the Credit Valley and one of the better Whirlpool Sandstone exposures. See “Rockwatching” beneath the flapping bat in the sidebar to the right.