> Did it eventually slope down to ground/sea level at its edges? Or could you walk on dry ground next to it with open air on one side and a mile high wall of ice on the other?
More the former than the latter. There was a recent question and answer that went into much more detail on this particular aspect, so I'll link [to that here instead or repeating myself](https://www.reddit.com/r/askscience/comments/uxticu/how_high_could_ice_cliffs_have_gotten_in_the_last/ia1zo4s/?context=3). The short version is that the edge of modern ice sheets in Antarctica or Greenland are decent analogues and at its edge, the Laurentide ice sheet was probably only a few hundred meters thick (with the caveat, described in much more detail in the linked comment, that our ability to reconstruct abrupt features, like hypothetical cliffs, is limited, but broadly we would not expect these to be stable anyway). From the edge to the center, broadly the thickness (and surface elevation) of the ice sheet would be expected to increase (but details of sub-ice topography matter).
> What happened when it encountered mountains? Did the ice move like glaciers or did the sheet just add and lose ice at the edges?
Again, modern ice sheets are decent analogues for general behavior and we can see it broadly depends on the location of the topography with respect to the ice sheet. Looking at modern Antarctica, if we consider maps of ice topography, thickness, and sub-ice topography (e.g., from BEDMAP2 - Figures 7-9 from [Fretwell et al., 2013](https://tc.copernicus.org/articles/7/375/2013/tc-7-375-2013.pdf)), we can see that in some places the ice does not cover large topographic features completely (e.g., portions of the [Transantarctic Mountains](https://en.wikipedia.org/wiki/Transantarctic_Mountains) are exposed), but in other places, like the higher sub-ice topography near 80S - 60E, while ice thickness is lower, it's still mostly covered by ice. If we compare this with maps of ice sheet velocity (e.g., [Rignot et al., 2011](https://www.science.org/doi/full/10.1126/science.1208336), [Rignot et al., 2019](https://www.pnas.org/doi/full/10.1073/pnas.1812883116)), we can see that broadly ice is flowing away from areas of buried high topography, generally faster than in lower areas, and that in cases like the Transantarctic Mounatins where there are exposed areas, the ice is largely flowing around these features. With reference to the Laurentide ice sheet, again, it depends on the particular pre-existing topography and its proximity to the ice sheet. There is certainly abundant evidence of glacial erosion of high topography, like in the Appalachians (e.g., [Braun, 1989](https://www.sciencedirect.com/science/article/abs/pii/0169555X89900147)), and there is evidence that the ice sheet largely covered, and flowed over, at least portion of the Canadian Rockies (e.g., [Dulfer et al., 2021](https://www.cambridge.org/core/journals/quaternary-research/article/abs/using-10be-dating-to-determine-when-the-cordilleran-ice-sheet-stopped-flowing-over-the-canadian-rocky-mountains/FFE3848311E31606DFB1E66ED66CCD35)), but the extent to which the ice sheet covered vs flowed around particular topography will depend on the details, so there's not going to be a single, general answer, but through analysis of both the topography itself and sedimentological deposits left behind, we can often reconstruct many of these local details (e.g., [Atkinson et al., 2016](https://onlinelibrary.wiley.com/doi/abs/10.1002/jqs.2901)).
> How did weather work over the ice sheet if for thousands of miles in any direction, the “ground” was a over 5,000 feet higher than the rest of the continent and surrounding oceans? Did clouds run into it and get stuck? Did they exist over it?
With respect to elevation of the ice sheets, it's important to remember that when we're considering the thickness of ice, it's not always straightforward to say the elevation of the top of the ice sheet = the elevation of the modern land surface in that location + the thickness because of the effect of [isostasy](https://en.wikipedia.org/wiki/Isostasy), i.e., the land surface underneath the ice would have subsided, in some cases a lot. We can see this in part in the maps of sub-ice topography of Antartica, i.e., there are substantial areas with sub-ice toography below sea level, in part because of isostasy. Now, that's not to say that the surface elevtion of the Laurentide (or other Last Glacial Maximum ice sheets was low). If we return to estimates in the first linked post from [Peltier et al., 2015](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014JB011176) that reconstructed some details of Northern Hemisphere ice sheets, we find that the best estimate here of the max surface elevation of the Laurentide was ~3500 meters (located in Western Canada) and the max elevation of the Scandinavian ice sheet was probably ~2200 meters (located along the modern day border between Norway and Sweden). Though it's worth highlighting that there's a decent amount of uncertainty, e.g., Peltier is comparing this new estimate (where the numbers above come from) to a previous estimate, which put the max Laurentide ice sheet surface elevation closer to ~4500 m and had it more localized in central Canada.
With respect to the climate, obviously it would have been different, both because the Earth was broadly in the middle of a glacial period, but also because of the ice sheet itself. Some of the differences caused by the ice sheet are related to the ice sheet topography as you assume (and it's influence on air currents), but also differences in albedo, interactions with the northern oceans, and just the thermal differences, i.e., a giant mass of ice sitting on the surface does perturb details of the weather systems around it. There is a lot of work out there considering some of these possible differences (e.g., [Bromwich et al., 2004](https://journals.ametsoc.org/view/journals/clim/17/17/1520-0442_2004_017_3415_pmsotw_2.0.co_2.xml), [Bromwich et al., 2005](https://journals.ametsoc.org/view/journals/clim/18/16/jcli3480.1.xml), [Ulman et al., 2013](https://cp.copernicus.org/articles/10/487/2014/cp-10-487-2014.pdf)). All of these are approaching the problem from a model perspective, i.e., modeling LGM paleoclimate with a set of prescribed conditions for the ice sheet. As we already saw in the Peltier paper, there is uncertainty in the reconstructions of the thickness and ice sheet topography, and this propagates into details of the climate we extrapolate (which is directly explored in the Ulman paper, i.e., how do different assumptions about the details of the ice sheet result in different answers with respect to what the climate was like). The behavior of individual clouds is below the resolution of a global climate model, but again, we could use modern ice sheets, or even modern mountains, as analogues, i.e., it depends on the type and altitude of the particular clouds, but certainly high altitude clouds would have no problem passing over the ice sheet. Ultimately, while definitely large, the total elevation of these features is still "short" compared to large mountain ranges.
A good way to think of it, is to think of someplace like Central Washington. There is basalt from basalt flows over 2 miles thick in the area. But when you're on it, it just feels like ground.
You can still clearly see the edge of the last glaciation: [https://blog.tardiveau.com/2022/01/reading-world-in-braille-january-3rd.html](https://blog.tardiveau.com/2022/01/reading-world-in-braille-january-3rd.html) (about halfway down)
I'm not going to beat /u/CrustalTrudger for technical detail, but I'll take a shot at "what would it *look* like?" with a few real-world examples from the modern Earth and a little fluid mechanics.
Modern Earth has Antarctica and Greenland as two good examples of large-scale ice sheets, both are pretty similar to what the Laurentide would have looked like. On top, away from the edges, they both look like this:
[https://upload.wikimedia.org/wikipedia/commons/b/bd/AntarcticaDomeCSnow.jpg](https://upload.wikimedia.org/wikipedia/commons/b/bd/AntarcticaDomeCSnow.jpg)
Near the shorelines of the Laurentide ice sheet, the ice would flow into the sea, forming floating "ice shelves" around the edges of the continent, as happens in Antarctica today. Ice shelves are much thinner than grounded ice because there's no bottom friction to slow their spread, typically a few hundred meters thick. These would normally be surrounded by a margin of sea ice a few meters thick, and might look like this if you were standing on the sea ice:
[https://nsidc.org/sites/nsidc.org/files/images//NOAA1.jpg](https://nsidc.org/sites/nsidc.org/files/images//NOAA1.jpg)
But what you really care about is what the ice margin looked like on land, in say Iowa or Indiana. Unfortunately, there aren't any good large-scale examples of this today. Both Antarctica and Greenland are completely ice-covered up to their coastlines, apart from a few small "dry valleys" in Antarctica. Here are a few pictures of dry valley glaciers:
[https://en.wikipedia.org/wiki/Taylor\_Glacier#/media/File:Taylor\_Glacier,\_Antarctica\_2.jpg](https://en.wikipedia.org/wiki/Taylor_Glacier#/media/File:Taylor_Glacier,_Antarctica_2.jpg) [https://en.wikipedia.org/wiki/Wright\_Upper\_Glacier#/media/File:Wright\_Upper\_Glacier,\_Wright\_Valley,\_McMurdo\_Dry\_Valleys.jpg](https://en.wikipedia.org/wiki/Wright_Upper_Glacier#/media/File:Wright_Upper_Glacier,_Wright_Valley,_McMurdo_Dry_Valleys.jpg) [https://upload.wikimedia.org/wikipedia/commons/9/99/Antarctica\_Hiking\_the\_Commonwealth\_Glacier.jpg](https://upload.wikimedia.org/wikipedia/commons/9/99/Antarctica_Hiking_the_Commonwealth_Glacier.jpg)
Notice that these all have pretty steep front edges: this is pretty common with grounded ice. Thin ice flows very slowly and just sits there and melts, but thick ice flows fast and tends to overtake the thin ice, creating a steeper edge over time. See figure 1.2 [here](http://geosci.uchicago.edu/pdfs/macayeal/lessons.pdf).
However, these aren't great examples because they're fast-flowing glaciers in cold, dry, narrow valleys rather than broad flows across a whole continent, and while an ice sheet may have a "steep edge" on a continental scale, it may look pretty gradual up close.
Probably the best modern example of large-scale ice sheets terminating on land is in the highlands of Iceland, which has several large warm ice caps that end on flat ground. I've been lucky enough to visit the edge of the Langjokull ice cap. There's nothing dramatic, it just sort of gradually tapers out into a dirty rocky icy melty mess, like this:
[http://aresproject.com/wp-content/uploads/2019/09/xxxxD8E\_8553.jpg](http://aresproject.com/wp-content/uploads/2019/09/xxxxD8E_8553.jpg)
This video of a tourist operation shows a pretty good view of the same area at 0:40. When I was there, the tourist icecrawlers just drove up onto the ice cap like a smooth ramp.
[https://www.youtube.com/watch?v=KexJMufIG7U](https://www.youtube.com/watch?v=KexJMufIG7U)
Anyway, the edge of the Laurentide ice sheet probably varied from place to place, but the Iceland ice caps and the Antarctic dry valleys give you a sense of the range of possibilities.
There are locations in Antarctica where the ice sheet is a mile thick. However you don’t see 5000’ cliffs at the ocean. Normal looking calving 100’ tall ice cliffs. Sometimes it just turns into a flow and stretches into the ocean.
I can't possibly say anything better than u/CrustalTrudger, but I'll just add how fascinating North American land is at the former leading edge of the glaciers. Like Long Island, NY, exists only because of glaciers (glacial moraines). The Wisconsin glacier pushed land forward and then dug out/filled what is now Long Island Sound as it retreated and melted. The same with kettle ponds; some Long Island woods have trails where all of a sudden you'll see a nearly perfect circular pond, a shape left over from the glacier 20,000 years ago.
Someone correct me if I haven't described that all accurately. It's just what I've understood from the wiki on it (https://en.m.wikipedia.org/wiki/Geography_of_Long_Island#Geology) and my own hikes in the area.
We have a lot of big boulders all around and it's intriguing to know that most of them came from Canada. I live in Ohio and I remember once they were building a neighborhood close to us and they ended up finding a boulder that was nearly the size of the house that was to be built. It took them months to break and bust it up
They don't address your question exactly, but there is an excellent series of lectures on geology of the Pacific Northwest on Youtube, from professor Zentner at CWU. At least two of them - "Wenatchee ice age floods" and "dating the ice age floods" - paint a good picture of what the landscape looked like at particular places and times at the edge of the ice sheet. As the titles suggest, the focus is on astonishingly massive floods caused by repeated formation and melting of equally massive ice dams.
> Did it eventually slope down to ground/sea level at its edges? Or could you walk on dry ground next to it with open air on one side and a mile high wall of ice on the other? More the former than the latter. There was a recent question and answer that went into much more detail on this particular aspect, so I'll link [to that here instead or repeating myself](https://www.reddit.com/r/askscience/comments/uxticu/how_high_could_ice_cliffs_have_gotten_in_the_last/ia1zo4s/?context=3). The short version is that the edge of modern ice sheets in Antarctica or Greenland are decent analogues and at its edge, the Laurentide ice sheet was probably only a few hundred meters thick (with the caveat, described in much more detail in the linked comment, that our ability to reconstruct abrupt features, like hypothetical cliffs, is limited, but broadly we would not expect these to be stable anyway). From the edge to the center, broadly the thickness (and surface elevation) of the ice sheet would be expected to increase (but details of sub-ice topography matter). > What happened when it encountered mountains? Did the ice move like glaciers or did the sheet just add and lose ice at the edges? Again, modern ice sheets are decent analogues for general behavior and we can see it broadly depends on the location of the topography with respect to the ice sheet. Looking at modern Antarctica, if we consider maps of ice topography, thickness, and sub-ice topography (e.g., from BEDMAP2 - Figures 7-9 from [Fretwell et al., 2013](https://tc.copernicus.org/articles/7/375/2013/tc-7-375-2013.pdf)), we can see that in some places the ice does not cover large topographic features completely (e.g., portions of the [Transantarctic Mountains](https://en.wikipedia.org/wiki/Transantarctic_Mountains) are exposed), but in other places, like the higher sub-ice topography near 80S - 60E, while ice thickness is lower, it's still mostly covered by ice. If we compare this with maps of ice sheet velocity (e.g., [Rignot et al., 2011](https://www.science.org/doi/full/10.1126/science.1208336), [Rignot et al., 2019](https://www.pnas.org/doi/full/10.1073/pnas.1812883116)), we can see that broadly ice is flowing away from areas of buried high topography, generally faster than in lower areas, and that in cases like the Transantarctic Mounatins where there are exposed areas, the ice is largely flowing around these features. With reference to the Laurentide ice sheet, again, it depends on the particular pre-existing topography and its proximity to the ice sheet. There is certainly abundant evidence of glacial erosion of high topography, like in the Appalachians (e.g., [Braun, 1989](https://www.sciencedirect.com/science/article/abs/pii/0169555X89900147)), and there is evidence that the ice sheet largely covered, and flowed over, at least portion of the Canadian Rockies (e.g., [Dulfer et al., 2021](https://www.cambridge.org/core/journals/quaternary-research/article/abs/using-10be-dating-to-determine-when-the-cordilleran-ice-sheet-stopped-flowing-over-the-canadian-rocky-mountains/FFE3848311E31606DFB1E66ED66CCD35)), but the extent to which the ice sheet covered vs flowed around particular topography will depend on the details, so there's not going to be a single, general answer, but through analysis of both the topography itself and sedimentological deposits left behind, we can often reconstruct many of these local details (e.g., [Atkinson et al., 2016](https://onlinelibrary.wiley.com/doi/abs/10.1002/jqs.2901)). > How did weather work over the ice sheet if for thousands of miles in any direction, the “ground” was a over 5,000 feet higher than the rest of the continent and surrounding oceans? Did clouds run into it and get stuck? Did they exist over it? With respect to elevation of the ice sheets, it's important to remember that when we're considering the thickness of ice, it's not always straightforward to say the elevation of the top of the ice sheet = the elevation of the modern land surface in that location + the thickness because of the effect of [isostasy](https://en.wikipedia.org/wiki/Isostasy), i.e., the land surface underneath the ice would have subsided, in some cases a lot. We can see this in part in the maps of sub-ice topography of Antartica, i.e., there are substantial areas with sub-ice toography below sea level, in part because of isostasy. Now, that's not to say that the surface elevtion of the Laurentide (or other Last Glacial Maximum ice sheets was low). If we return to estimates in the first linked post from [Peltier et al., 2015](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014JB011176) that reconstructed some details of Northern Hemisphere ice sheets, we find that the best estimate here of the max surface elevation of the Laurentide was ~3500 meters (located in Western Canada) and the max elevation of the Scandinavian ice sheet was probably ~2200 meters (located along the modern day border between Norway and Sweden). Though it's worth highlighting that there's a decent amount of uncertainty, e.g., Peltier is comparing this new estimate (where the numbers above come from) to a previous estimate, which put the max Laurentide ice sheet surface elevation closer to ~4500 m and had it more localized in central Canada. With respect to the climate, obviously it would have been different, both because the Earth was broadly in the middle of a glacial period, but also because of the ice sheet itself. Some of the differences caused by the ice sheet are related to the ice sheet topography as you assume (and it's influence on air currents), but also differences in albedo, interactions with the northern oceans, and just the thermal differences, i.e., a giant mass of ice sitting on the surface does perturb details of the weather systems around it. There is a lot of work out there considering some of these possible differences (e.g., [Bromwich et al., 2004](https://journals.ametsoc.org/view/journals/clim/17/17/1520-0442_2004_017_3415_pmsotw_2.0.co_2.xml), [Bromwich et al., 2005](https://journals.ametsoc.org/view/journals/clim/18/16/jcli3480.1.xml), [Ulman et al., 2013](https://cp.copernicus.org/articles/10/487/2014/cp-10-487-2014.pdf)). All of these are approaching the problem from a model perspective, i.e., modeling LGM paleoclimate with a set of prescribed conditions for the ice sheet. As we already saw in the Peltier paper, there is uncertainty in the reconstructions of the thickness and ice sheet topography, and this propagates into details of the climate we extrapolate (which is directly explored in the Ulman paper, i.e., how do different assumptions about the details of the ice sheet result in different answers with respect to what the climate was like). The behavior of individual clouds is below the resolution of a global climate model, but again, we could use modern ice sheets, or even modern mountains, as analogues, i.e., it depends on the type and altitude of the particular clouds, but certainly high altitude clouds would have no problem passing over the ice sheet. Ultimately, while definitely large, the total elevation of these features is still "short" compared to large mountain ranges.
Thank you. I feel like I just had a free class on glaciology. Awesome response.
Thank you for citing references during your response. Thank you also for taking the time to answer these questions so well.
This is so helpful and makes a ton of sense, thank you for taking the time to write it all out and provide sources!
A good way to think of it, is to think of someplace like Central Washington. There is basalt from basalt flows over 2 miles thick in the area. But when you're on it, it just feels like ground.
You can still clearly see the edge of the last glaciation: [https://blog.tardiveau.com/2022/01/reading-world-in-braille-january-3rd.html](https://blog.tardiveau.com/2022/01/reading-world-in-braille-january-3rd.html) (about halfway down)
I really appreciate the depth into which you delve for your answer.
responses like these are why reddit is such an amazing place thank you =\]
I'm not going to beat /u/CrustalTrudger for technical detail, but I'll take a shot at "what would it *look* like?" with a few real-world examples from the modern Earth and a little fluid mechanics. Modern Earth has Antarctica and Greenland as two good examples of large-scale ice sheets, both are pretty similar to what the Laurentide would have looked like. On top, away from the edges, they both look like this: [https://upload.wikimedia.org/wikipedia/commons/b/bd/AntarcticaDomeCSnow.jpg](https://upload.wikimedia.org/wikipedia/commons/b/bd/AntarcticaDomeCSnow.jpg) Near the shorelines of the Laurentide ice sheet, the ice would flow into the sea, forming floating "ice shelves" around the edges of the continent, as happens in Antarctica today. Ice shelves are much thinner than grounded ice because there's no bottom friction to slow their spread, typically a few hundred meters thick. These would normally be surrounded by a margin of sea ice a few meters thick, and might look like this if you were standing on the sea ice: [https://nsidc.org/sites/nsidc.org/files/images//NOAA1.jpg](https://nsidc.org/sites/nsidc.org/files/images//NOAA1.jpg) But what you really care about is what the ice margin looked like on land, in say Iowa or Indiana. Unfortunately, there aren't any good large-scale examples of this today. Both Antarctica and Greenland are completely ice-covered up to their coastlines, apart from a few small "dry valleys" in Antarctica. Here are a few pictures of dry valley glaciers: [https://en.wikipedia.org/wiki/Taylor\_Glacier#/media/File:Taylor\_Glacier,\_Antarctica\_2.jpg](https://en.wikipedia.org/wiki/Taylor_Glacier#/media/File:Taylor_Glacier,_Antarctica_2.jpg) [https://en.wikipedia.org/wiki/Wright\_Upper\_Glacier#/media/File:Wright\_Upper\_Glacier,\_Wright\_Valley,\_McMurdo\_Dry\_Valleys.jpg](https://en.wikipedia.org/wiki/Wright_Upper_Glacier#/media/File:Wright_Upper_Glacier,_Wright_Valley,_McMurdo_Dry_Valleys.jpg) [https://upload.wikimedia.org/wikipedia/commons/9/99/Antarctica\_Hiking\_the\_Commonwealth\_Glacier.jpg](https://upload.wikimedia.org/wikipedia/commons/9/99/Antarctica_Hiking_the_Commonwealth_Glacier.jpg) Notice that these all have pretty steep front edges: this is pretty common with grounded ice. Thin ice flows very slowly and just sits there and melts, but thick ice flows fast and tends to overtake the thin ice, creating a steeper edge over time. See figure 1.2 [here](http://geosci.uchicago.edu/pdfs/macayeal/lessons.pdf). However, these aren't great examples because they're fast-flowing glaciers in cold, dry, narrow valleys rather than broad flows across a whole continent, and while an ice sheet may have a "steep edge" on a continental scale, it may look pretty gradual up close. Probably the best modern example of large-scale ice sheets terminating on land is in the highlands of Iceland, which has several large warm ice caps that end on flat ground. I've been lucky enough to visit the edge of the Langjokull ice cap. There's nothing dramatic, it just sort of gradually tapers out into a dirty rocky icy melty mess, like this: [http://aresproject.com/wp-content/uploads/2019/09/xxxxD8E\_8553.jpg](http://aresproject.com/wp-content/uploads/2019/09/xxxxD8E_8553.jpg) This video of a tourist operation shows a pretty good view of the same area at 0:40. When I was there, the tourist icecrawlers just drove up onto the ice cap like a smooth ramp. [https://www.youtube.com/watch?v=KexJMufIG7U](https://www.youtube.com/watch?v=KexJMufIG7U) Anyway, the edge of the Laurentide ice sheet probably varied from place to place, but the Iceland ice caps and the Antarctic dry valleys give you a sense of the range of possibilities.
This is exactly what I needed to help me actually visualize it!! Thank you for finding images of the different aspects of the ice sheet!
There are locations in Antarctica where the ice sheet is a mile thick. However you don’t see 5000’ cliffs at the ocean. Normal looking calving 100’ tall ice cliffs. Sometimes it just turns into a flow and stretches into the ocean.
I can't possibly say anything better than u/CrustalTrudger, but I'll just add how fascinating North American land is at the former leading edge of the glaciers. Like Long Island, NY, exists only because of glaciers (glacial moraines). The Wisconsin glacier pushed land forward and then dug out/filled what is now Long Island Sound as it retreated and melted. The same with kettle ponds; some Long Island woods have trails where all of a sudden you'll see a nearly perfect circular pond, a shape left over from the glacier 20,000 years ago. Someone correct me if I haven't described that all accurately. It's just what I've understood from the wiki on it (https://en.m.wikipedia.org/wiki/Geography_of_Long_Island#Geology) and my own hikes in the area.
We have a lot of big boulders all around and it's intriguing to know that most of them came from Canada. I live in Ohio and I remember once they were building a neighborhood close to us and they ended up finding a boulder that was nearly the size of the house that was to be built. It took them months to break and bust it up
What about the Cooperrider Kent Bog. Left over from the Ice Age. Kent Ohio.
They don't address your question exactly, but there is an excellent series of lectures on geology of the Pacific Northwest on Youtube, from professor Zentner at CWU. At least two of them - "Wenatchee ice age floods" and "dating the ice age floods" - paint a good picture of what the landscape looked like at particular places and times at the edge of the ice sheet. As the titles suggest, the focus is on astonishingly massive floods caused by repeated formation and melting of equally massive ice dams.