The waves are increasing in size and frequency
|
The waves rip huge boulders from cliff faces and sweep them up to 50m inland in exposed areas such as Shetland, Orkney and the Western Isles.
This process creates piles of boulders tucked away inland behind cliffs.
The study shows these are not deposited by tsunamis (tidal waves generated by volcanic eruptions or earthquakes) as had been thought, but by storm waves.
Dr James Hansom, a geologist the University of Glasgow, told delegates at the International Geographical Union congress in the Scottish city that the erosion was expected to accelerate because sea levels are rising and the coastline is sinking.
 |
The distance these large boulders are being moved is
spectacular 
Dr James Hansom, University of Glasgow
|
"The boulder ridges were formed in recent times and there are no records of recent tsunamis.
"Because sea levels are continuing to rise and larger waves are hitting the cliffs, the rate of modification of the coastline and creation of these boulder ridges is increasing."
Boulder tossing
These boulder beaches occur only in exposed, remote areas where the sea is deep because the waves become smaller in shallower depths.
Most of the cliffs are about 20m high but researchers have found boulders weighing two tonnes tossed a few feet inland on clifftops 50m in height.
In the North Atlantic, waves 20m (66 feet) and over occur more than 100 times a year, and 95ft waves occur on average once every 100 years.
The average wave height during winter is said to have increased 15% between 1985 and 1995 in the area to the west of Shetland.
Data also suggests that global sea levels have increased by one millimetre per year over the past century.
Previous research in Australia and the Caribbean suggested similar effects there were caused by tsunamis.
But the last known tsunami in the North Atlantic occurred 4,900 years ago while the Lisbon earthquake of 1755 produced only slight waves in Scotland.