Large Stones, salt and Topsoil have all been postulated as the cargo. The stones might have been the ones used to build the megalithic temples found over the islands and therefore would have had the weight necessary to scrape such channels in the rock, though it is difficult to imagine stones weighing 50 tons being transported on a frame.
Salt wouldn’t have been needed in such quantities to warrant it being transported in the amounts required to create the Ruts. Topsoil is a distinct possibility as the ruts do conform to the patterns of the island’s oldest settlements. This would mean the topsoil was transported from the coast, which seems unlikely due to the exposed nature of the clifftops and the friable nature of the rock, which wouldn’t support good quality topsoil.
However, the coastline may have changed since the ruts were created, the land having fallen away dramatically due to seismic activity. This idea is supported by the fact that in other parts of the country, some ruts disappear into the sea on one side of a bay and reappear on the other side.
Those under the sea tend to be deeper than the ones on the land, they go on for longer in a straight line and stand proud of the seabed in places.