How geology came to help Alexander the Great

Source: Nature

A natural sand-bridge made his defeat of Tyre easier.

Ned Stafford

Historians need not be quite so impressed by Alexander the Great's defeat of the island of Tyre in 332BC. Geological studies of the region show that Alexander's army had help reaching the island, in the form of a natural land-bridge lying just a metre or two below the water's surface.

Alexander the Great was just 23 years old when he stood on the coast of what is now Lebanon, gazing offshore at the tiny Phoenician island city of Tyre, a powerful commercial centre. Alexander knew that Tyre had to be seized before he could safely move south to Egypt and then turn inland to conquer the Persian Empire.

In a determined attack, Alexander's engineers used timber and ruins from the old centre of Tyre on the coast to build a 1-kilometre-long 'mole', or causeway, to the island. Months later, his army broke through the fortress walls and brutally crushed Tyre.

Nick Marriner of the European Centre for Research and Teaching on the Geosciences and the Environment (CEREGE) in Aix-en-Provence, France, says that historians and archaeologists had previously relied on ancient texts and illustrations, along with aerial photographs of the region today, to work out how this causeway was built and how difficult it was to do. "These theories were not backed by any hard data," he says.

"Before our work, archaeologists had no idea of the depth of water between Tyre and the mainland," says Marriner. Others have described the waters as "shallow", with some references saying it was perhaps 5 or 6 metres deep.

Digging deep

To investigate, Marriner's PhD supervisor Christophe Morhange, also of the CEREGE, went to Tyre in 2002. His team drilled more than 20 cores on the now urbanized isthmus that today connects the mainland with the former island of Tyre, as well as other scattered nearby locations. The cores contained sediments from as far back as 8,000 years ago. Diving surveys were done by Lebanese archaeologists.

Back in the lab, the team performed analyses of the types of sediment and tiny fossils within the cores, to learn more about the ancient near-shore marine environment; fine-grained sediments and the remains of creatures that prefer to live in sheltered environments show up when and where waters were once calm. Wave modelling and previous studies of the area helped to complete the picture.

An elongated region of sandstone reefs acted as a 6-kilometre natural breakwater in the area 8,000 years ago. By 6,000 years ago, rises in sea level had reduced the length of the island from 6 to 4 kilometres. This, combined with an increase of sediment supply due to agricultural activity and a rise in inland rainfall, particularly after about 3,000 years ago, created a natural sandbar that sat an average of 1-2 metres below mean sea level in Alexander's time, they report in Proceedings of the National Academy of Sciences1.

Sediment build-up

The idea that a natural sand-bridge came to Alexander's aid has been proposed before2.

Marriner's work highlights how the natural causeway started to grow faster sometime before Alexander arrived, and accelerated again after Alexander's construction blocked sediment transport in the area. This latter point is hardly surprising, says Liviu Giosan, a marine geologist at Woods Hole Oceanographic Institution in Massachusetts. "Any coastal engineer could tell you about sedimentation around dikes," he says.

Marriner adds that their study should help archaeologists to pick sites in the area for further investigation. "We can use the geological record as an aid to understanding the evolution of the coastline, and identify areas of potentially rich archaeology," he says.

References

1. Marriner N., Morhange C. & Meulé S. Proc. Natl Acad. Sci. USA, doi:10.1073.pnas.0611325104 (2007).
2. Nir Y., et al. Geoarchaeology, 11 . 235 - 250 (1996).