A piece of South Australia's "glacial pavement", a rock that is hundreds of millions of years old, has been removed with "heavy-duty tools" in "a selfish act" at South Australia's Hallett Cove Conservation Park.

The rock, which was from a time when South Australia was under an ancient glacial lake, was reported missing in early May.

The piece was removed from a section of the geological walk that shows some exposed glacial pavement.

South Australian Museum's earth sciences senior collection manager Ben McHenry said the rock itself was about 600 million years old and the glacial scratches dated back to about 280 million years ago when the region was covered by the glacier.

The glacier is estimated to have been about kilometre thick.

Mount Lofty Ranges district manager Piers Brissenden said Hallett Cove was the only location in the Southern Hemisphere where such glacial pavements existed.

"This is an internationally significant bit of geological history and it would be a shame if people negligently destroyed something which has a value to the whole world," he said.

Mr Brissenden said he believed the culprit must have climbed up the embankment and used heavy-duty tools to break off the glacial pavement.

"I would suggest someone would have to use some kind of mechanical tools to help get this off," he said.

The Hallett Cove Conservation Park is popular with walkers. ( Supplied: Department of Environment, Water and Natural Resources )

"I think it's likely that it's some amateur collector who thinks they just like to see the merits for themselves."

He said the small rock specimen would not have any monetary value.

"It's such a pointless thing to do ... it's just someone's selfish act," he said.

He said he was hopeful the specimen would be returned and staff would be able to restore it.

"We do work hard to educate people about the values of these sorts of conservation reserves and they are there for everyone's enjoyment," Mr Brissenden said.

"We do hope that people understand those values and they look after the parks as well."

Mr McHenry said the significance of the glacial pavement at Hallett Cove was first identified by geologist Ralph Tate in 1875.