Geologists use seismites, in combination with other evidence, to better understand the earthquake history of an area. If age and distribution of seismically-generated features can be determined, then recurrence interval and seismic hazard risk can be assessed.
Along the Credit River Valley, these strange shaped masses of sandstone called “pillows” are exposed in the shales of the Georgian Bay Formation. Ontario currently is located mid-continent but over the last 3 Billion years, this area has lain over an active plate margin where eastern North America was colliding with other continents.
Geologists have recorded a number of ancient earthquakes that are preserved in these deformed rocks both along the Credit River and along the Niagara Escarpment. Scientists are using these ancient earthquake faults to understand modern earthquakes.
440 Million years ago, North America became part of a supercontinent (Pangea) and what is now Mississauga lay under the Iapetus Ocean. Africa and South America collided with eastern North America continent during the Taconic Orogeny resulting in the formation of a subduction zone. Large andesitic volcanoes formed the Taconic Mountains off the coastline and left behind deposits of ash which scientists have chronicled in Ontario and Michigan. As the eastern North American continent slipped into the subduction zone, the Taconic Mountains were pushed ashore and the large Appalachian basin formed between the Canadian Shield and the Taconic Mountains.
As this area lay under the waters of the Appalachian basin, violent earthquakes shook the seafloor causing sand to sink into the soft mud and form these rock pillows in the soft shale layers.
Hydraulic Erosion
Hydraulic action is erosion that occurs when the motion of water against a rock surface produces mechanical weathering. Most generally, it is the ability of moving water (flowing or waves) to dislodge and transport rock particles
Logging requirements
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You will need to park at the listed parking coordinates and wade across the shallow credit river to see the pillows and make your observations or if you are tubing down the credit river to find the Credit River series, you will be able to stop at this location and make your calculations from the river. DO NOT ATTEMPT THIS EARTHCACHE DURING TIMES OF HIGH WATER LEVELS OR HEAVY RAINFALL!
1. At the posted coordinates you will be able to see the exposed seismites or “pillows” along a section of rock face on the Credit River? How many exposed “pillows” can you count in the rock face at GZ.?
2. Are all of the pillows formed within the one sandstone layer or are there more than one layer of “pillows”?
3. There is a large “pillow” that has broken off from the rock wall and is lying at the edge of the river. This rock is rounded on the bottom and flat on top. Estimate the volume of this stone (measure the length x width x depth in metres). Multiply this number by 2323 ( kg/cu.m of solid sandstone) to get the approximate weight in kg of this rock.
4. Look at the exposed “pillows” either lying on the ground or the portion remaining embedded in the exposed wall. Does the pillow look like it is one solid rock or can you see different layers, similar to rings of a tree? If you do see layers, are they the same colour, diameter and texture?
5. Touch and feel one example of a stone “pillow” and in your own words describe the feel of the rock. Is it rough , grainy or smooth?
6. Do the “pillows” that have fallen off the rock face show evidence of hydraulic erosion. For example, has the action of the water broken or pitted the sandstone or have these sections retained their “pillow” shape for the most part. If there is any weathering, describe in your own words your observations.
7. Optional: Photo’s are appreciated however no spoiler pictures please.
References:
ONTARIO ROCKS three billion years of environmental change - Nick Eyles
Road Rocks ONTARIO: Over 250 Geological Wonders to Discover
https://www.britannica.com/science/Taconic-orogeny
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