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Thursday, 5 October, 2000, 12:28 GMT 13:28 UK
Rocks reveal ancient tides
Bands Ken Eriksson
Bands indicate tidal movements billions of years ago
By BBC News Online science editor Dr David Whitehouse

The Earth's oceans were being tugged by tides more than three billion years ago, according to an analysis of rocks in South Africa.

The sandstone and shale deposits, which were found in the Moodies group of hills, have markings that scientists say were made by the ebb and flow of waters moving along a continental shoreline.

The study proves that the Moon was orbiting the Earth in a roughly similar orbit to the one it occupies today.

The tidal research has been published in Geology, the journal of the Geological Society of America.

The scientists examined rocks exposed on the banks of the Sheba River in Mpumalanga province.

Dating showed the deposits were laid down 3,225 million years ago, in a shallow lagoon on the edge of a continental margin.

Strengths of the tides

Close inspection revealed markings indicating the various times during the tidal cycle when silts would have been deposited - such as in an ebb tide when the waters moved back from the shore.

Other patterns in the rocks revealed the neap-spring-neap tidal cycle, as well as variations in the strengths of the tides.

Dr Eriksson, of the Australia National University in Canberra, explains the evidence as due to variations in the pull of the Moon depending how far or how close the satellite was to Earth in its orbit.

He told BBC News Online: "In my mind this represents unambiguous evidence for tides on Earth some 3,200 million years ago, and implies the presence of the Moon in orbit around the Earth at that time."

The analysis of the tidal patterns also suggests that the duration of the orbit, the length of the lunar month, was 20 days as opposed to the 27.5 days today.

"The nature of the cyclicity indicates that the Moon was in a near-circular orbit similar in shape to the present orbit. This is consistent with the idea that the Moon formed after a giant impact and was not a separate body that was captured by the Earth.

The Geology paper was co-authored by Edward Simpson from Kutztown University, Pennsylvania, US.

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