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I have a large continent. Most of the continent’s perimeter is enclosed by cliffs, creating a ‘wall.’ The cliffs are not continuous and periodically terminate in canyons that connect to water bodies within the continent and extend to the wider ocean. From a worldbuilding perspective, the cliffs are meant to act as natural protection from outside forces, while the ravines allow outside interaction.

  • Most of the continental perimeter is enclosed by cliffs, with few exceptions.
  • The cliffs are hundreds of meters tall and are very wide, thick, and steep.
  • The outer cliff faces, which face the ocean, are extremely steep, with near-vertical slopes. The inner faces, which face the continent, are still relatively steep but include some terraces below.
  • The crests of the cliffs are very flat and have notable outer-facing overhangs.
  • The ravine paths have minimal bending or zigzagging, remaining very straight and linear.

My problem is a geological explanation for how this cliff barrier came to be. Individually, I can justify each trait. Collectively, I can't find a potential justification for their formation. Is there a holistic geological explanation I can use?

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  • $\begingroup$ Does it have to be purely geological? $\endgroup$ Commented Apr 15 at 14:02
  • $\begingroup$ @MontyWild No, I just mentioned geological since I figured those are the only related processes that could create this. If there's a different viable explanation, I might accept it $\endgroup$ Commented Apr 15 at 14:13
  • $\begingroup$ Frame challenge: these cliffs need to only be on the side of the continent that faces outside forces. If a large ocean is on the other side of the continent precluding any invasion from that side, then the story holds up. For geological example, see the island of Oahu, HI which has very steep cliffs on one side. $\endgroup$ Commented Apr 15 at 14:31
  • $\begingroup$ I'm having trouble with that last bullet and I'd be interested to know how individually you can justify it. I've seen arroyos that were somewhat straight, but over any significant distance, everything zig-zags... unless you have an unbelievably homogeneous crust, every boulder will create zig-zags as most ravines are caused by water erosion. $\endgroup$ Commented Apr 15 at 16:16
  • $\begingroup$ Raise sea levels and much of southern Africa would meet your requirements. $\endgroup$ Commented Apr 15 at 18:24

6 Answers 6

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Glacial ice, a wandering continent, and rough seas.

The continent in question has spent some time at one of the poles, where its central region has accumulated a significant amount of ice in the past, though the continent is large enough that its coastal regions have been warm enough that they haven't accumulated much ice.

The mass of all that ice has pushed the continent down and caused glacial scouring of the central areas, leaving a dished central region. Around the edges of the continent, meltwater flows through ravines to the sea. Rough seas around the continent has caused the edges of the continent to erode into cliffs.

Then, the continent began to move from the polar regions to more tropical areas. As the temperature rose, the ice began to melt, the melt-water raced down the ravines, straightening them, and the reduced weight on the continent allowed it to float higher upon the underlying mantle's magma. This caused the cliffs to rise out of the water, though the action of the sea continued to erode them.

By the time the continent reached its current position, all the ice has melted, leaving a dished central region with ancient meltwater ravines, while the sea continues to erode the cliffs. Given that the continent has been rising due to the immense weight of all that ice having been removed, the sea no longer reaches the top of the cliffs, and so undercuts them somewhat.

This continent would need to be somewhat larger than Antarctica so that it could both accumulate a thick layer of ice, and have its coastal regions warm enough to melt the ice.

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    $\begingroup$ large continet and edges all have the same geology are mutualy exclusive clifss in one are will be smooth slopes in others, soft sediment erodes too fast to form cliffs. also rebound happens MUCH faster than a continet can move significant distance. $\endgroup$ Commented Apr 16 at 0:45
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    $\begingroup$ @John Sure, rebound happens quickly, but with enough ice, it still takes time to melt. $\endgroup$ Commented 2 days ago
  • $\begingroup$ at the scale of tectonic movement the ice basically melts instantly. the ice actually melts faster than the rebound. $\endgroup$ Commented 2 days ago
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Meteorite crater

I believe an impact crater from a meteor would be what you are looking for. Wall thickness and continent size is dependent on meteor size. It will cause:

  • Pointy walls/cliffs to surround it.
  • Near vertical slopes can be caused by erosion of the cliff faces by the sea. You see this in cliffed coasts.
  • An overhang can be created by erosion too, the water washing away the bottom stone of the cliff.
  • Mostly circular, no weird bending.
  • Ravines can be created through geological processes ripping part of the cliffs apart, weaker materials wearing away, or already having been there.

Buoyant continent

Another explanation could be a very buoyant crustal block that resists subduction almost entirely. It would get pushed up on top of the other continental plates. If it is light enough, and push in on it from all sides, and it is sturdy enough, it might get pushes up as a whole on top of the other crustal plates.

The continent, from a meteorite/small moon, can be in one piece by joining the planet late in its during crustal formation. Having it join through a tidal inspiral to bleed of a lot of the impact energy.

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    $\begingroup$ Just a note: to make the crater continent wide it would require an impact with enough energy to melt the whole planet $\endgroup$ Commented Apr 15 at 13:09
  • $\begingroup$ Maybe a microcontinent then. $\endgroup$ Commented Apr 15 at 13:15
  • $\begingroup$ actual continetal crust is too light to subduct, thats why we have continents. $\endgroup$ Commented Apr 16 at 0:48
  • $\begingroup$ @jbh do a minor spelling Grammer pass? $\endgroup$ Commented yesterday
  • $\begingroup$ @Trish There we go! SE just needed something (or enough of something) to indicate a change to the post. Thanks! $\endgroup$ Commented yesterday
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Tectonic uplift — on a massive scale

The antipode of your continent experienced a massive meteor strike. Not large enough to destroy the planet, but large enough to send a substantial pressure wave coursing through the planet, pushing magma...

... and lifting a modest-sized tectonic plate straight up!

For my basic worldbuilding I'm using Mount Thor for its overhang and the Book Cliffs for their width. Let's review this per your bullets.

  1. That meteor hit in exactly the right place, at the right time, with the right amount of force, to lift an entire tectonic plate, allowing surrounding plates to subduct (at least a bit) beneath the new, raised continent. Ocean waters cooled the lifted plate and, to a small degree the magma beneath it around the edges, allowing the uplifted plate to principally stay at the height of several hundred meters above the ocean surface. Areas around the perimeter that are lower or, (if chosen by the worldbuilder) at ocean level are due to natural variations in the previously submerged coastal topology.

  2. As we're dealing with lifting an entire plate, how high the plate is lifted is more an issue of suspension of disbelief than it is factual geology. However, considering terrestrial tectonics' capability to create mountains kilometers high, the hundreds of meters you're asking for are well within suspension of disbelief.

  3. Lifting an entire plate would lend to vertical or near-vertical cliffs. I suspect (but, not being a geologist, can't prove) that at tectonic fault lines, lifting basically vertical would result in sheer cliffs.

  4. Conveniently, the modest plate was in the middle of an ocean. Now surrounded by ocean, the currents move consistently around it, undermining the edges of the continent and causing rock slides that left many overhangs. Further, the plate, now released from the pressure of its surrounding plates, the top edge of the new continent has gently relaxed outward. These cliffs would be a world-class bounder to climb. The cliff tops are reasonably flat because, being originally ocean floor, there's a tremendous amount of silt. Near the cliff faces (and over geologic time) that silt is reasonably expected to wash away due to rain.

  5. Ravines are easy to rationalize over geologic time. While early rain would run over the cliff edges reasonably evenly (washing away the silt in #4 and sanding the edges into reasonable flatness), natural heterogeneity would lead to ravines. Due to the sharp verticality of the cliff faces, those ravines would be naturally straight in the beginning. However, guaranteeing a reasonable straight ravines over geologic time requires a homogeneous crustal makeup than will likely exist. I'm ambivalent about this requirement. You could claim that, being initially underwater, the majority of the non-silt mass would be made up of bedrock, which could have enough homogeneity to justify straight ravines. But in reality, even bedrock would have heterogeneous makeup. Eventually, your ravines will zig-zag.

Conclusion

Tectonic uplift. Yeah, baby!

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Magic

Thats your only workable answer, you can't have a large continent with uniform geology, large continents are many sperate slabs of rock stuck together over millions to billions of years. Each with their own history and minerology. You can't make it uniform enough to have uniform coastal features. Even things like the roation of the planet and coriolis forces means there will be drastically different forces on different sides. The forces that move continents around also will create drastic differences, while one side is subducting while another will be a spreading which will give you drastically diffrent forces and erosional timeframes. You can't do this on a small continent much less a large one. Large continent and uniform just can't happen through normal geology you need magic or tech so advanced its indistiquishable. Sometimes the answer is you can't get there through science.

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Lots of places like this on just one side in the Pacific Ocean caused by multiple tsunamis over the millions of years. The reason they're like this is the fault lines creating successive tsunamis are all from one side. However if your continent was surrounded by fault lines that periodically spawn tsunamis every few hundred years, then you could have steep cliffs on all sides.

Because this process takes millions of years or hundreds of millions of years to create the cliffs, rivers etc have time to cut paths to the ocean.

Better still, have the plate your continent is on have fault lines right around it close enough to the cliffs to do it.

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Aliens stole a few hundred meters planetary diameter

Let's try the phantastical: the planet shrunk by a about the height of the cliffs in radius. Tge aliens that stole the planetary core material lifted the continent out of the planet, and after extraction, placed ot back. Now it is wedged up by a rim of the other plates that crushed in as the planet was shrunk. Over geological scales the center of the continent sagged down, the cliffs still are supported by other plates.

The ravines formed where plates keeping the rim up diverged, leading for the unsupported plate to sag and break.

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