"The computer model showed that the island of Tyre acted as a natural coastal barrier to the wind and swell [coming off the Mediterranean Sea]," Marriner explained.
Beginning in the Late Bronze Age about 3,000 years ago, an increase in deforestation and farming on the island caused more sediment to flow into the sea, he explained.
The geological cores show that along the sheltered, leeward side of the island, this sediment collected together and formed a spit.
The new layering of sediment was also enhanced by a slowdown in sea-level rise that began around 4000 B.C.
By the time Alexander arrived, the sandbar extended almost all the way to the mainland, submerged under 3.3 to 6.5 feet (1 to 2 meters) of water.
"[The formation] would probably have been known to sailors, for whom it might have hindered navigation," Marriner said.
Using the sandbar as a foundation, Alexander's engineers piled up timber, stone, and rubble to construct a causeway.
"It would have been very difficult, as they only had access from one shore and would have had to build out incrementally from the mainland," said Gordon Masterton, former president of Britain's Institution of Civil Engineers, who was not involved in the new study.
"It would have been very unusual to build something like this at this time."
(Read a related article about catapults and the technology of ancient war.)
Nonetheless, Alexander persevered, and after a siege lasting seven months, he marched his army into the island city.
Alexander's Sandy Legacy
By the time Alexander's army founded Alexandria in Egypt the following year, it had gotten its causeway-building skills down to a fine art, Marriner added.
"His engineers probably benefited from the savoir faire they had acquired just a few months earlier at Tyre to complete the Alexandria causeway," he said. That bridge connected Pharos island, once home of a great lighthouse—one of the Seven Wonders of the Ancient World—to the Egyptian mainland.
After Alexander's victory at Tyre, the causeway there irreversibly changed the flow patterns in the water surrounding the former island.
"Both north and south of this causeway, two bays were formed, which have slowly silted up, because the long-shore currents were interrupted by Alexander's causeway," said Olaf Schuiling, a geo-engineer from Utrecht University in the Netherlands.
Around 7.5 million square feet (700,000 square meters) of new land were created, he said, forming the broad peninsula that can be seen today.
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