Key takeaways
- There is no meteorite in this story. The bowl was made by folding and then by erosion.
- The rock was pushed up into a dome, and the softer middle wore away faster than the hard rings around it.
- Layers laid down flat are now close to vertical, which is why footprints appear on walls.
- Large enough that the structure only resolves properly from the rim.
Why it is called a crater
Because it looks like one. From the rim you are looking down into a broad circular bowl with a raised edge running all the way round, and that is precisely the shape a large impact leaves.
The name stuck, as names do, and a good many visitors arrive expecting a meteorite story. There is none. Nothing struck the earth here.
What did happen is slower, larger and more interesting, and once you see it the landscape stops being a curiosity and becomes legible.
Folding first
These rocks were laid down flat, as sedimentary rocks always are: layer upon horizontal layer of sand and mud on the floor of ancient water bodies, compressed over tens of millions of years into stone.
Then the Andes happened. The collision that raised the range squeezed the whole region, and rock that had been flat was buckled into folds, the way a rug buckles when it is pushed from one end.
At Maragua the fold was a dome: the layers pushed upward into a broad blister with the oldest rock at the top of the arch and the younger layers wrapped around it.
It is worth pausing on the timescale, because the numbers are the part that makes it real. The sediments that make these rocks accumulated grain by grain over millions of years on a floor that was flat and wet. The folding that lifted them happened over millions more. Nothing here was sudden, and the only fast event in the whole story would have been the meteorite that never arrived.
Concentric ridges of upturned rock curving around the crater floor
Then erosion
A dome, once raised, is under attack from the moment it exists. Rain and rivers take the highest ground first, and the top of the arch is the highest ground.
Crucially, the layers are not all equally tough. Some are hard sandstone that resists; others are soft mudstone that does not. Water finds the soft ones and removes them, working down into the middle of the dome and leaving the hard layers standing as ridges.
Given enough time you end up with exactly what is here: a hollow with a flat floor of soft rock, ringed by concentric ridges of the hard layers that used to arch over the top of it. Geologists call the eroded remains of a dome like this a breached anticline; the local word for it is a crater.
Why the rock stands on end
Because the flanks of a dome are steep. Around the edge of the structure the layers that once lay flat have been tilted through most of a right angle, so their edges now point at the sky.
That is why the ridges around the crater look like fins or teeth rather than like gentle slopes, and why the surfaces you walk past are cross-sections through time rather than the tops of beds.
It is also, entirely, the reason the dinosaur footprints here are on vertical rock. The animals walked on a horizontal surface. The surface was tilted afterwards.
A useful way to picture it is to think of the layers as the pages of a closed book pushed up into an arch and then sanded flat across the top. What you would see afterwards is not the cover but the edges of every page, arranged in rings. That is exactly what the rim of Maragua shows you, and every ring is a different moment in the same sequence.
Cretaceous
The age of the surfaces carrying the footprints. The rock is far older than the shape it makes: the folding that raised this dome came tens of millions of years later.
The scale, and the view from the rim
The formation is several kilometres across, which is too large to grasp from the floor. Inside it you are simply in a wide valley with unusual hills around the edge.
From the rim it resolves. You can see the rings, follow them round, and read the whole structure as one object: a dome with its heart removed. It is the single best argument for approaching from above, which is what the road does.
The colours help. The exposed strata are banded in red, ochre, grey and white, and in the hard highland light the rings are obvious rather than subtle.
How old the rock is
Old enough to contain dinosaur footprints, which places the relevant surfaces in the Cretaceous, the last period before the extinction that ended the age of large dinosaurs.
The folding is very much younger. Andean mountain building is a continuing process measured in tens of millions of years rather than hundreds, so the rock is far older than the shape it now makes.
That gap is the point. The footprints record a flat wet plain; the shape records a mountain range arriving underneath it long afterwards.
One consequence is worth stating plainly for anyone weighing the trip. Almost everything on this tour is one story told twice: the footprints prove the rock was flat, and the rings prove it was bent afterwards. A visitor who grasps that in the morning spends the afternoon looking at evidence rather than at scenery.
Who lives inside it
The floor is farmed, and the crater is home to Jalqa communities. This is not an empty geological curiosity: there are houses, fields, animals and paths, and the tour lunch is eaten inside it.
That changes how the place reads. A landform this strange might be expected to be uninhabited, and instead it is ordinary ground for the people who live on it, which is the more interesting arrangement.
Walking in and finding it worked and lived in is, for most visitors, the thing that stays with them longer than the geology does.
