While paddling around Jordan Harbour, we noticed many exposed sandy bluffs along its shore. I was curious about why and how these came to be formed. So, when I got home I did a little googling,gathered up some information and theories, then thought let's make it into an earthcache! Posted coords will bring you to one of the sandy bluffs, and can only be accessed by watercraft, any access by land would be trespassing.
To start with, I found this geological survey which said that analysis of this area showed Lake Iroquois stratified sands. (Stratified means many layered);
Glacial Lake Iroquois existed at the end of the last Ice Age, approximately 13,000 years ago. The lake was bigger than present day Lake Ontario, it’s shoreline in this area extended to approximately present day King Street. The level of Lake Iroquois was approximately 30 m (~100 ft) above the present level of Lake Ontario, and it was formed because downstream, the retreating glacier had blocked the waterways that were draining the lake. When the ice dam finally broke open, the water levels eventually decreased to what we see today.
Another factor that may also play a part in the changing Lake Iroquois shoreline is Post-glacial rebound. This is the rise of land masses after the glaciers melted. The crust of the earth may feel solid to us, but far below that, the outer core of our planet is actually viscous in nature, and with ice that was up to 3km thick putting its weight on the crust of our earth, the crust became depressed under that load, causing the viscous core to displace. As the glaciers melted, the weight was released and the crust rebounded. The initial uplift was almost immediate, but even today, the residual uplift continues slowly due to the slow viscous flow equalizing in the outer core of our planet.
We see the sandy bluffs around the shoreline of Jordan Harbour because they have been exposed by the erosional forces of the waterway you are on.That erosion lets us see into the past. We know that these sands have been placed here at the time of the ancient lake, but how did they form? One possible explanation is that they are evidence of Lake Iroquois Beach Ridges.
A beach ridge is a ridge running parallel to a shoreline, is generally straight, and is commonly composed of sand and sediment brought in by waves. Beach ridges are formed near the upper reach of the wave movement. More than one ridge can be formed, the height depends on water level, tides and wave height. A fall in water level, or an uplift of land, isolates a beach ridge from the body of water that created it. When the water level drops, the ridge often remains, and as water levels continue to drop, a series of beach ridges can form. As time goes on, vegetation appears, grows and takes hold to stabilize the beach ridge. These geological formations are evidence of the shoreline of glacial lakes.
Questions to answer. Please email or message me the answers.
1) On your paddle over, have a look around. How many exposed sandy bluffs can you see? Do any of them on opposite shores look like they may have been connected prior to water erosion?
2) Estimate the height of the exposed bluff at GZ. This will give you an idea of how high the water was when this was Lake Iroquois.
3) Do you think that these sandy bluffs are beach ridges? Why or why not?
4) a picture of you/your gps/paddle at GZ would be appreciated.
References:
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/P0764//p0764.pdf
https://opentextbc.ca/geology/chapter/17-3-landforms-of-coastal-deposition/
https://www.giantsrib.ca/shaping-the-escarpment/
https://en.wikipedia.org/wiki/Glacial_Lake_Iroquois
https://www.researchgate.net/publication/238621614_Reconstructing_paleo_lake_levels_from_relict_shorelines_along_the_Upper_Great_Lakes
https://en.wikipedia.org/wiki/Post-glacial_rebound
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/beach-ridge