A glacier carried this boulder more than fifty kilometers.
It ended up on a beach in Adelaide, at a place called Hallett Cove.
Up on the clifftop above it, the rock is polished smooth, and scratched in straight lines.
Stones frozen into the ice cut those lines. That was more than two hundred and eighty million years ago.
Back then, Australia was still joined to Antarctica, near the South Pole, and a sheet of ice covered this land.
In eighteen seventy-seven, Professor Ralph Tate showed one of these scratched rocks at a meeting in Adelaide.
At first, people thought the ice was recent, like the last ice age in Britain.
Then the geologist Walter Howchin argued it was far older.
And the rock still shows which way the ice went.
The scratches all run one way. Small humps of rock drop steeply on the side the ice was heading.
Together, they say the ice moved north-northwest.
Follow them back the other way, south, and you reach Encounter Bay. That is where this granite came from.
A boulder on a beach, and lines on a clifftop, still pointing the way the ice went.
