Location
Kalbarri National Park, WA
Rock
Tumblagooda Sandstone
Age
Silurian? · ~400–430 Ma
Process
Joint-guided erosion
Status
Open · The Loop trail
Overview
High above the Murchison River, a band of red-and-white sandstone has been worn through into an arch. The rock around it preserves some of the earliest footprints of animals on land.
400–430Ma
Approximate age of the Tumblagooda Sandstone
1fossil
Body fossil found: Kalbarria
11leg pairs
On Kalbarria, a euthycarcinoid
Chapter 01
The Tumblagooda Sandstone is a banded red and white sedimentary rock laid down by rivers and tidal flats. Its age is still poorly constrained, but Ordovician to Silurian, roughly 400–430 million years, is currently most likely.
Chapter 02
The rock is crowded with trackways left by myriapods, eurypterids, euthycarcinoids, xiphosurids and scorpion-like animals. Only one body fossil has been found: Kalbarria, a euthycarcinoid with 11 pairs of legs.
Chapter 03
Erosion exploited joints and weaker bedding in the sandstone, undercutting a layer until a window opened through it. It now frames the gorge on the Loop trail.
Chapter 04
The banded red and white layers still carry ripple marks from the currents that shaped them, and the surfaces around the gorge preserve the tracks of animals that walked on this shore.
Chapter 05
The Murchison River has cut down through the Tumblagooda Sandstone to form the gorge. Nature's Window sits high on the rim, framing the river's meander below.
How it forms
Sand is laid down in layers on ancient river and tidal flats, rippled by currents.
Buried, the layers harden into sandstone and are broken by vertical joints.
A weaker layer erodes faster beneath a harder band, guided by the joints.
The hollow breaks through while the harder band above holds as a natural arch.
A window cut by time, beside the footprints of animals that first stepped out of the sea.
The question
Because the layers are unequal. The weaker band below erodes into a hole, while the harder, well-cemented band above is strong enough to span the gap and hold as a bridge of rock.
Joint-guided erosion
Deep time
~430–400 Ma
Shoreline
Sands and trackways are laid down on Silurian (or Ordovician) river and tidal flats.
Deep time
Stone
The sands are buried and cemented into the Tumblagooda Sandstone.
Later
Gorge
The Murchison River incises the sandstone, exposing its layers.
Today
Window
A joint-guided hollow has broken through to frame the gorge.
Field record
Field data
IGMA simulation · move to heat
Rock Lab
Turn a 3D model of Nature's Window with hand gestures, heat its surface with your fingertip and weather it through 250,000 years.
Enter the Rock LabReferences
Photography
Next case · 07
Limestone stacks in retreat