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I'm writing a piece of fiction in which I want to describe the journey taken by a large glacial erratic. Leaving the wisdom of this literary device aside, how might I realistically do that?

The erratic in question would need to end up in eastern Ontario and preferably be visually distinctive compared to the local rocks. The stone in question, if it matters, would be about 10ft in diameter.

I want to send my rock on a long voyage. Would this scenario be scientifically plausible? For instance, the erratic became absorbed in the glacier in the far north of Canada and then got carried far to the south. The glaciers retreated and eventually deposited the stone in mid-latitude in Ontario.

Also, what would be reasonable timelines for this? I'm guessing the journey could have started around 20,000 years ago and finished 10,000 years ago. How thick would the glacier have been during this period?

Also, would the stone have likely been left on the ground or covered in eroded soil? The soil in the region in question is light sandy, and bedrock isn't near the surface.

Sorry if this request seems a bit odd. Hopefully, it might be fun for an expert to think about.

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The history of the Laurentide ice sheet has been described by e.g. Batchelor et al (2019). Their results have been used in a nice graphical presentation by the University of Toronto. As can be seen it is thought that the ice sheet grew out of two centres on Baffin Island and Labrador. Eventually it grew to a large dome centred roughly over Hudson Bay. At this stage ice flowed from this central area towards the margins, essentially radially. The ice sheet reached a maximum around 21 000 years ago and a rough estimate is that the ice melted away from eastern Ontario around 10 000 years ago as you also state. The thickness of the ice sheet center would have been 3 to 4 km thick with decreasing thickness towards the margins. This is the glacial framework.

Now a rock would be picked up by the ice at some point during the ice age and transported with the ice towards the margin. Exactly how long this transport would take is difficult to state since it could depend on many variables. I do, however think that the transport time could be significantly longer than what you mention.

There are a few complications that must be mentioned. One is that material transported by the ice is likely to slowly get crushed into smaller pieces. The longer the transport distance the more crushing will occur but it does not mean a boulder can not be transported for long distances. Another issue is that there have been several ice ages so it is conceivable that a boulder may have been picked up and deposited several times. How likely it is a boulder will survive several ice ages is not easily determined but chances are slim but not zero.

The type of bedrock in a boulder also matters. Boulders from sedimentary rocks such as sandstone or limestone are more easily broken down than igneous rocks such as granite. Anything picked up from non-sedimentary areas will survive longer.

So these are some of the frames within which you have a lot of possibilities to have your boulder sourced and travel. Depending on how much artistic freedom you add it could be a long and complex journey with some perils on the way.

Reference Batchelor, C.L., Margold, M., Krapp, M. et al. The configuration of Northern Hemisphere ice sheets through the Quaternary. Nature Communications 10, 3713 (2019). https://doi.org/10.1038/s41467-019-11601-2 (The article is open access and can be downloaded from https://www.nature.com/articles/s41467-019-11601-2)

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  • $\begingroup$ Thanks for this fantastic and detailed answer. The only questions I have re how to describe the way the glacier picked up the boulder and how it got deposited. I'll probably go with 'plucking' for the pickup. As for deposition, how does this happen? Does the glacier just melt back and deposit everything in place? I imagine that smaller material may be carried out by melt water and dispersed more, while larger pieces kind get dropped in place. As for the composition, I'll probably go with a durable material such as granite. $\endgroup$
    – pnadeau
    May 28 at 11:29
  • $\begingroup$ @pnadeau The deposition will either subglacially if the boulder gets stuck at the bed (there can be melting also beneath an ice sheet) or commonly by simply melt out at the terminus as the ice retreats. As a whole, the envelope of possibilities is very large and filled with unknowns and local circumstances. $\endgroup$ May 28 at 13:45
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You say "the erratic became absorbed in the glacier in the far north of Canada, and then got carried over far to the south. The the glaciers retreated and eventually deposited the stone in a mid latitude in Ontario."

I don't know if you are suggesting some retrograde action, i.e. movement from the far south back to mid latitude. This would not be possible. The ice, and its load of rock, can only flow "down hill" or down and out from the piled up center. The "retreating" is only of the melting edge; no ice or rock retreats; the ice melts in place and any rock falls down, maybe being farther moved by running water, and or covered by stuff that had been higher in the ice.

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    $\begingroup$ Good point, but a glacier could still pick up a rock and deposit it far from the point where it absorbed it right? Meaning it conceivably could carry a block of stone from the far north to a lower latitude. $\endgroup$
    – pnadeau
    May 28 at 21:07
  • $\begingroup$ @pnadeau Quite correct. The word absorbed is not a term used in glaciology or glacial geology, the term would be incorporated or entrained. But that is a se.mantic issue $\endgroup$ May 31 at 17:55
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There are three vital points you need to note:

  • The Origin, where, and how, was the erratic eroded from its parent formation and entrained into the glacier which took it onward. If you want to go farther back you can look at where the parent formation itself comes from.

  • The Frozen Journey, how and where did the glacier take the erratic and over what timescale?

  • The Final Deposition, many erratics are found where the ice first left them but many others are moved repeatedly by pulses of glacial motion and even shifted by melt water and/or rainfall after glaciers have long left the area.

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