A little while back, Henry made this great MinutePhysics video about how the weird symmetry

of light and shadows on a landscape can make it really hard for your brain to tell which

features are popping out and which ones are dented in, especially if you're not sure

where the light is coming from.

This re-ignited an old debate we had here at MinuteEarth, about THIS photo of the surface

of Mars.

That sinuous pattern on the landscape is an ancient stream channel – which is cool because

it means that liquid water used to flow on Mars – but that crater above it has a shadow

in the top left, which means the light in the image must be coming from the top left,

which seems to suggest that the stream channel is actually sticking up above the surrounding

landscape.

But...that can't be right, right?

I mean, river channels should obviously be valleys, since they've been carved into

the ground.

This seemed like such a basic fact to me that, even when everyone else on my team said they

saw this as a ridge, my brain kept telling me it was a valley.

It turns out that, not only was my brain lying, but this stream channel is an example of a

weird geological process that essentially turns landscapes inside out, creating inverted

relief.

If you live in a cold climate, you may have actually seen something like this happen with

compacted footprints in the snow – as the sun melts the fluffier snow around them, they

can end up sticking out, like this.

The same thing can happen to a river valley.

For instance, say you've got a stream running through a desert.

Sometimes there is so little rain that the stream dries up.

As it does, some groundwater is actually drawn upward by capillary action, and as it rises

and evaporates, dissolved minerals in it are left behind and precipitate out, coating the

sediments in the riverbed and cementing them together.

As the stream flows and dries up over and over again, more and more cement accumulates,

making the riverbed harder and more resistant to erosion than the surrounding rock.

Over the eons, wind erodes that softer rock, turning the channel into an inverted version

of its former self.

We can't know exactly how this Martian riverbed got turned inside out without getting down

on the ground and digging around a little, but we can make some good guesses by studying

inverted relief on our home planet.

You can even find your own examples on Google Earth; just be warned that trying to figure

out what's rightside-up and upside-down could turn your brain inside-out.

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