In the Northern Territory, Uluru is the tip of a buried slab
348 metres above the plain and some 6 kilometres below it. Uluru's beds are tilted almost ninety degrees; Kata Tjuta's, 25 kilometres away, only 15 to 20. They are the same deposit seen in two materials and two angles.
The image of Uluru that circulates is of an enormous stone resting on an empty plain, as if someone had left it there. That image is the one to dismantle in order to understand the place, because it describes exactly the opposite of what happened.
Uluru is not on top of anything. It is what is left of something much larger after the rest went away.
01 The word monolith is misleading
The material of Uluru is called arkose: a coarse sandstone rich in feldspar, that is, freshly eroded granite sand that had no time to break down before being deposited. That already says something: it accumulated fast and close to its source.
The deposit was an alluvial fan, the cone of material a river leaves when it comes out of a mountain onto a plain. Those fans reached at least 2.5 kilometres in thickness, and were then buried under sand, mud and the remains of marine organisms that compressed and cemented them.
From this comes the figure that changes the scale of everything. Uluru is 348 metres high above the plain and 863 above sea level, with a perimeter of 9.4 kilometres, but the slab it belongs to continues downward for up to 6 kilometres. What you visit is an outcrop.
02 The beds are standing on edge
An alluvial fan is deposited in horizontal layers, one on top of another. Uluru's are no longer horizontal: later folding turned them almost ninety degrees, so today they stand practically vertical.
You can see this without knowing geology. The parallel grooves that run up and down Uluru's flanks are not marks of surface erosion: they are the edges of the beds, cut by the present-day surface. You are looking at a cross-section of a tipped-over deposit.
And it explains the shape. A stack of vertical, strongly cemented beds erodes as a compact block, without ledges or steps, because there are no soft horizontal layers to be undercut. Hence the smooth, continuous profile.
03 Two formations, one event
Some twenty-five kilometres from Uluru lies Kata Tjuta, a group of 36 domes spread over 21.68 square kilometres. The highest, Mount Olga, reaches 1,066 metres above sea level, 546 above the plain: 198 metres more than Uluru.
The two formations are the same geological episode and came out of the same fans, but not from the same part of the fan. Uluru is the fine fraction, sand; Kata Tjuta is the coarse fraction, a conglomerate of pebbles and boulders of granite and basalt cemented in a sandstone matrix.
And there is a second difference, the one that decides the shape of each. Kata Tjuta's beds tilted only 15 to 20 degrees, not ninety. With near-flat beds and a rock of uneven grain, erosion opens fractures and separates blocks: that is why they are separate domes and not a continuous massif.
Put another way: the same story told twice, with a different material and a different angle. Visiting one without the other means keeping half the argument.
04 Why there is nothing around them
The other reasonable question is why there is an almost perfect plain around the two formations. The answer is that everything that surrounded them went away, and they did not.
Both lie near the southern edge of what geologists call the Amadeus Basin, an enormous deposit that was folded and then spent hundreds of millions of years eroding. The soft material disappeared and the most strongly cemented remained, as isolated outcrops.
The same folding is much more legible a few hundred kilometres to the northeast, in the MacDonnell Ranges, where the strata draw parallel and curving bands visible from the air. It is the same event, laid out horizontally there and vertically here.
05 A territory with 0.2 people per kilometre
It is worth putting the setting in figures, because it is hard to imagine. The Northern Territory has 1,347,791 square kilometres, almost three times Spain, and around a quarter of a million inhabitants.
That gives fewer than 0.2 people per square kilometre, and they are not even spread out: more than half live in Darwin, at the northern tip. The interior, where Uluru and Kata Tjuta are, is practically empty.
That scale has a pleasant geological consequence: there is nothing covering the ground. With no forest, no farming and no city, the rock is exposed over an enormous area, and structures that on other continents can only be inferred are looked at directly here.
06 How to travel it
The first practical consequence is that the route that pays off is the perimeter, not the summit. The 9.4 kilometres around Uluru pass hollows, walls with different dips and a permanent waterhole, and show the structure better than any distant view. Since October 2019 the top is not climbed.
The second is to give Kata Tjuta time and not treat it as an add-on. The paths that go in among the domes, particularly the one that crosses the group, are the ones that show the conglomerate close up: fist-sized pebbles embedded in the wall.
And the third changes what you look at. If at every wall you ask which way the beds run, the place stops being a postcard of a big rock and becomes what it is: two different cuts through the same deposit, emerging above the little that erosion has not yet taken away.
