I'm working on a world that hasn't had an ice age in 250 million years and looking at how to shape the coasts. I'm thinking about fjords (also glacial lakes) and trying to figure out if they would have eroded.

I've tried to look at average erosion rates and concluded that "between 0.016 and 0.024 mm/yr" is 0.02 mm/yr but in 250,000,000 years that's equivalent to 5 km so I'm guessing there's other things going on.

So would it be possible to see glacial remnants after that long or would they all have eroded away?

Thank you

  • $\begingroup$ 5km at every point and surface on the fjord, not just 5km across the entire thing. $\endgroup$
    – John
    Apr 9, 2021 at 4:36
  • $\begingroup$ Even without an ice age, a normal glacier will carve a lovely valley for itself. The valley will just not be able to form a fjord, because making those requires the sea level to be much lower, allowing the ice to move further, before rising sealevel now make pretty contours out of the valley. That, plus if your climate does not vary then this neat new valley of yours stays inconveniently filled with glacier, which is quite pretty by itself but is also not a fjord. $\endgroup$
    – PcMan
    Apr 9, 2021 at 4:50
  • $\begingroup$ My world has not had polar ice caps in 120 million years $\endgroup$
    – Nierninwa
    Apr 9, 2021 at 11:34

3 Answers 3


250 Million years is MUCH too long.

in 250 million years, odds are that the coastline the fjord used to be on, has rammed full speed into another continent, gotten subducted, and is already being spewed out again in a volcano. Very few land features, especially so coastal terrain, would be at all intact after 250MY.

For example... 250 million years ago, India was in the southern Arctic circle, part of what we would call antartica.The southern side of the himalayas was 1500m under water. The northern side of the himalayas was nuzzled up in the armpit of what is now Ethiopia, in the form of swampy lowlands.


As for the Scandinavian Fjords? A mere 90 million years ago, they were hundreds of kilometers from the nearest ocean. And you could walk from 'finland' through 'greenland' and into 'canada' without ever seeing the sea.


250 million years would totally erase fjords

Much bigger formations are erased during such time.

The only option for them to survive (so to speak) is to get filled with sediment which would last that long. Then, after 250 million years, future geologists may figure out that this geological formation was, in fact, a sea fjord long time ago.

This is assuming that regular geological calamities, like plate tectonics and volcanism would spare this area for 250 million years.


This would certainly depend on a variety of factors, as the other answers have mentioned, including plate tectonics, latitude of this particular fjord, and importantly the rock that makes up the fjord walls.

If we make the assumption that plate tectonics have not destroyed the area, and that the area is located in a fairly dry, cold part of the world, and that the underlying rock is resistant to erosion, a fjord could certainly still exist in some form after 250 million years.

According to this abstract from a University of Vermont study, granite will erode at rates ranging from 1 Meter/Million Years up to about 20 Meters/Million Years, so quite slowly.

The fjords on Baffin Island, for example, have huge granite cliffs (almost a mile high in some cases), and the island is quite cold and dry, and so an assumption could be made that their erosion rates will be towards the lower end of that range. So in the best case scenario, these fjord walls would have eroded 250 Meters in 250 million years, and so they certainly would retain a lot of the characteristics that make them fjords.


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