If one pays a visit to the town of Cross Lake, Manitoba, preferably on an educational purposes trip, the locals will take you to their "Gordian Knot" situation and ask you to explain to them how this sink-shaped depression could have formed.
This is a glacial landscape with smooth polished rock surfaces, yet on this particular surface there is also what it looks like a later excavation on a flat surface. Please, note that a river is right behind the photographer.
What I told my audience at the time was that a boulder was brought in at high tide and got stuck on this platform for a long time and the continuous water movement created this additional depression.
Maybe someone has another explanation.
It is called a 'glacial mill' or 'glacier mill' continuously cut and polished by glacial water. Your photos show the bottom of the pothole.
Hello Christos and Guenter,
Thank you both for your contribution. I never thought of that explanation and I am glad to know. It is worth erecting a monument on site with the explanation and graphics. Your answer would please a lot of people of native Canada who find it hard to accept that their country was recently glaciated.
Thanks again,
Michael
Possibly where there once was a concretion that was removed, leaving a round depression.
Dear Mr. Issigonis,
it is a subglacial process where the glacier melts and fragments of different grain size incorporated into the ice (englacial) get released so that in some of the podholes or whirlpools a slurry of water and rock fragments turn into an abrasive mix. It is a fluvio-(sub)glacial process similar to that giving rise to eskers ( "railway embankments") which are the opposite expression of this form sculptured out of the basement bedrock.
Best regards
Harald G. Dill
Michael, Go to this website and scroll through the sometimes garish touristy stuff to to see examples of glacier-made potholes: http://www.tripadvisor.com/Attraction_Review-g188064-d196128-Reviews-Glacier_Garden_Gletschergarten-Lucerne.html You will see some holes with boulders in place. We visited Lucerne, Switzerland in the late 1980s and were duly impressed by the park, even without quite so much preservation. And, search online for Lucerne Glacier Park - Image Results to get a pile of photos that can be copied and attributed to make an interpretive display. You will recognize the correct ones from the Glacier Park website (can't copy/paste those). The swirling, grinding process is well known to occur under glaciers. it can also happen in more normal setting, such as at St. Vrain Park above Jamestown, Colorado. It's good to see someone having fun with natural phenomena.
Hi! Another example of both fossil and recently formed giant pots from Switzerland near Poschiavo (produced by the Palue glacier water) on video: https://www.youtube.com/watch?v=ADFYrCFw82g
I agree with those above that this is a pothole. It may be glacial, but potholes also form in modern rivers given adequate velocity and bedload to perform the abrasion. Potholes can be seen on the present-day floor of flood channels of the Potomac River at Great Falls National Park (see photo)
I agree with Kevin and with the first explanation given by Michael. Considering the small size of the pothole, its location just beside the river, and the fact that water level occasionally rises above the pothole (as evidenced in the third picture), my vote is for fluvial erosion.
Dear colleagues,
the question of whether the pothole resulted from a fluvial process only or from glacio-fluvial processes only is difficult to answer based upon a look at some photographs.
Potholes need a persistent abrasion and continuous water flow as, e.g., underneath a waterfall in the riverbed of a perennial fluvial drainage system. Episodic or periodic inundation during high floods does not carve a hole out of a hard rock in a way like that.
Fluvial-glacial process held accountable for the formation of this pothole needs a correlation of this hole in time and space with either erosional features such as subglacial channels or depositional features such as eskers or kames.
If the potholes fit into either a fluvial drainage system or the fluvial-glacial geomorphological landscape then we get a step closer to the solution.
It is more the rule than the exception to come to a point like this where going ahead will not get us closer to the truth.
Best regards
Harald G. Dill
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