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I've not come up against clear explanations when trying to think about a situation where an ice sheet is advancing and comes up against a cliff. What happens next? How does the ice sheet cross the cliff? Does the base get stuck?

Does it create some landforms into the original cliff-face? The forcing that would apply should be very strong (and perhaps it will continue to apply over a very long time period). Also, how does the flow of the glacier happen in these locations? Will the cliff-face be exposed to liquid water flow throughout the period that the ice sheet has covered it, and won't this cause very rapid erosion of the cliff-surface?

From a geologic perspective, this would have been happening across the Baltic Clint though most of these cliffs are in the range of 15–25 meters—in other words, relatively small for an ice sheet to cross. However, there are also some exposed (presently above-water) parts which are slightly higher.

I've put a small map (20m DEM; generally one front of cliffs by the modern coastline and then occasional ones further inland) below which illustrates where this would have been happening during the last glaciation period based on the example of Tallinn, Estonia, though by no means is this the only place where such a situation would have occurred.

enter image description here

Of course, the DEM has further problems, including that ~20k years of isostatic uplift has raised these a fair bit since so the actual obstacle that the ice sheets met should have been smaller (perhaps was: are the remnant limestone mounds the part that didn't get washed away?).

I also find the sentence (from the above Wikipedia page) weird:

However it is not known to which degree the Baltic Klint originated in postglacial time or if it evolved from cliff-like forms sculpted by the Fennoscandian Ice Sheet. In Gotland 20th century cliff retreat rates have been estimated at 0.15 to 0.78 cm/year.

I don't see how the ice sheet's advance/retreat could have caused such a cliff if the limestone plateau did not exist there beforehand.

I'm not expecting an answer based on this location specifically, but rather perhaps there are some imaged cliff-faces in Greenland/Antarctica where this behaviour has been recorded presently? I tried some searches and found this which seems relevant but I wasn't able to access the full text.

gktscrk
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  • Seems obvious that the glacier would come to the foot of the cliff, pile up until it over-topped it, then the ice above the cliff would just keep going. Or am I missing something? – jamesqf Jun 15 '20 at 04:37
  • glaciers transport rocks often from hundreds of kilometers away,where rocks and limestone meet something has to give and in this case it is the limestone.ice behaves the same way as water and where the flow is fastest most erosion will happen. – trond hansen Jun 15 '20 at 07:16
  • It is imo too problematic to judge landscape features from just a DEM. Though the area clearly was formed by glacier flow. But the feature may also be the outcome od the glacier flow. A glacier has depositonal as well as erosional features. For a well formed answer I'd try to find a proper publication over the area ... –  Jun 15 '20 at 09:34
  • Well, a glacier of 2-3km in thickness easily flows over such landscape features. It may be an erosional ground feature of glacier flow, but it may halso have (or being in the course of doing so right now) later. Some random read I found, hth: https://www.jstor.org/stable/26482028?seq=1 –  Jun 15 '20 at 10:16
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    @gktscrk: I am just guessing here, but I would think the glacier would sort of plane off the top of the cliff. After all, glaciers are quite powerful eroders of the landscape, e.g Yosemite valley, numerous fijords, the Finger Lakes, &c. – jamesqf Jun 16 '20 at 05:04
  • @jamesqf quite powerful is an understatement,the norwegian fjords are carved out by glaciers,more than 1000 meters of granite rock is eroded away by the ice.the continental shelf is the material that was moved by the ice during multiple ice ages. – trond hansen Jun 16 '20 at 05:59
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    I think, you need to specify some things. Are you interested in the behaviour of a typical alpine glacier, or rather and Ice Age ice sheet? Also, are you interested in how the glacier "behaves", or how it could create such features? – Erik Jun 16 '20 at 06:20
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    @Erik: Thanks, I've tried to make the above more specific as well as copy the content of my comments into the question. I was thinking of an ice sheet and not a glacier. – gktscrk Jun 16 '20 at 10:59
  • Hmm, it's is getting a bit chaotic. I think you should lean back and do some basic research and decide if you're after understanding ice sheet dynamics (big scale) or glacier flow (smaller scale). The features a glacier leaves behind are well published and a lot can be found on the internet on geo-related sites. In this case, though, it may be that the feature in question is not a glacial feature but "just" a holocene (actually taking place) erosional structure. –  Jun 16 '20 at 11:44
  • Glaciers are driven by back pressure they can move uphill as long as there is enough weight in the head. Keep in mind the forces on the cliff are huge so it is unlikely to stay a cliff for long. – John Jun 17 '20 at 15:34
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    You might look at the Niagara escarpment as a similar example: https://en.wikipedia.org/wiki/Niagara_Escarpment – jamesqf Jun 20 '20 at 17:24
  • @jamesqf: Thanks, that's a great example! Pity that article doesn't mention what the last glaciation did to it, but, nevertheless, the general discussion was very good! – gktscrk Jun 22 '20 at 04:33

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This depends on a few things.

If the cliff is small but widespread or the glacier is a huge, the glacier flows uphill until the cliff is gone.

If the cliff is large the glacier flows around it, is diverted by it, or piles up behind it depending on the land around it.

Glaciers are pushed along by back pressure, like toothpaste out of a tube. As long as there is enough accumulation the toe of a glacier can be driven uphill, but like anything else glaciers take the path of least resistance so if it can be diverted by the cliff it will.

Note that the forces on the cliff will be huge, and exactly the forces rock is weak against, so the cliff is over topped it will be stripped away very quickly.

enter image description here

enter image description here

John
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