This earthcache is not flood-friendly. It will be inaccessible in springtime and during other high-water events. Ground zero can only be safely accessed by walking down the staircase and along the beach. DO NOT ATTEMPT TO REACH THIS SPOT FROM ABOVE.
Shale is a fine-grained sedimentary rock that forms from the compaction of small mineral particles (silt and clay) that we commonly call "mud", making shale a type of “mudstone”. Unlike other mudstones, shale is both laminated and fissile. "Laminated" means the rock is made up of many thin layers ("laminae"); "fissile" means the rock easily splits into thin pieces along these layers. It usually forms in locations with slow-moving, sediment-filled water, such as a river delta.
Shale can take on various colours depending on the materials it contains. Black and grey shales usually contain at least 1-2% organic material and form in oxygen-poor locations. The darker the colour, the more organic material it contains. Other shales may be red, green, brown, or yellow, depending on the materials they contain.
Heat and pressure deep underground can transform the organic material found in darker shale formations into fossil fuels. Because shale is made of the smallest mineral particles – clay and silt – the spaces between particles (“pores”) are very small. As such, fluids like oil, natural gas, and even groundwater can permeate, but not easily pass through shale. This makes shale a source rock for many oil and natural gas deposits around the world, including the Bakken Formation beneath North Dakota and surrounding areas.
It is not uncommon for shale to contain fossils, whether of ripple marks, raindrop impact craters, or complex organisms. Fossils create weak points in the rock, so the fossils are often exposed when the rock breaks along that plane. Perhaps the most famous fossil-bearing shale deposit is known as the Burgess Shale, in British Columbia.
The Ottawa-Gatineau area is underlain by all three of the main rock groups. Sedimentary rocks, including sandstone, limestone, dolomite, and a variety of shales, are prevalent on both sides of the Ottawa River. Igneous rocks such as gneisses, schists, syenite, and granite are more common on the Québec side. Pockets of marble and amphibolite, metamorphic rocks, are also present in the region. These various bodies of rock are called formations. They are usually hidden below ground, but are occasionally visible as outcrops or along roadsides. The Rockcliffe Formation is a body of shale and sandstone that underlies various places in the region, most commonly along the banks of the Ottawa River. At Sheila McKee Park, the river has eroded its “right” bank considerably, exposing many layers of underlying shale and sandstone. The rocks in the Rockcliffe Formation formed during the Ordovician period, 440-485 million years ago.
It is not uncommon for shale and sandstone to form in the same area. As mentioned, shale is formed by the compaction of clay and silt particles; sandstone, by the compaction and cementation of larger sand particles. Along this riverbank you can find samples of both shale and sandstone layers.
Sources
- King, H.M. Shale. Retrieved August 8, 2020 from https://geology.com/rocks/shale.shtml.
- https://en.wikipedia.org/wiki/Shale
- https://burgess-shale.rom.on.ca/en/science/burgess-shale/02-geological-background.php
- Generalized bedrock geology, Ottawa-Hull, Ontario and Quebec; Harrison, J E; MacDonald, G. Geological Survey of Canada, "A" Series Map 1508A, 1979. Downloaded from https://ostrnrcan-dostrncan.canada.ca.
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To log this earthcache as found, first e-mail me the answers to the first four questions (don’t post them in your log!). You may log it as found without hearing a response from me, but e-mails with missing or poor answers may be deleted, and the cacher notified as to the reason(s) why.
Questions
1. At the posted coordinates, you will see a “wall” of shale, almost like a shallow cave, with a thick layer of sandstone cutting through it. Which contains more organic material – the shale above the sandstone layer, or below it?
2. What do you see in your surroundings that demonstrates how this shale is both laminated and fissile?
3. How high is the “ceiling” of this “cave”? Estimate the number of shale laminae visible here.
4. Mark the elevation at the water’s edge and at the top of the staircase. How far down has the Ottawa River eroded the bank to expose all these shale and sandstone layers?
5. (Optional) Post a photo of yourself on the staircase.
FTF: lostinthegarden