As promised, here is a guest post from Dr. Moshier. He sent some links along with the text and photos but I can't get them to work -- my apologies, I'm pretty sure it is past my bedtime. I'll try to make the links available tomorrow.
Enjoy!
The Final Battle of Ashkelon
One of the perks of digging at Ashkelon is the beach. Ancient Ashkelon was a major port along the Mediterranean coast. Residents of modern Ashkelon enjoy the beach for fishing, swimming and surfing. But the sea offers both benefits and dangers. A winter storm this past December 2010 whipped up waves that crashed against the Israel coast, causing damage to marinas, beachfront properties and many archaeological sites. Waves up to 40 ft (12 m) high and winds up to 60 mph (95 kph) battered the shoreline. The sea cliff of a very narrow stretch of beach north of the Ashkelon marina was badly eroded during the storm. The discovery of a Roman statue that washed out of the cliff made international news.
Tel Ashkelon did not escape the storm’s wrath. Most spectacular was the further destruction of Islamic-Crusader structures originally built to defend the ancient city along the coast. These before and after photos show the loss of prominent installations on the south rampart of the city.
If you look closely at the two photos you can see that the high waves washed away more than 3 ft (1 m) of the cliff.
Apparently, the intensity of the 2010 storm repeats about every 30 to 50 years. The Crusader defensive seawall at Ashkelon gives us a clue to the dramatic rate of beach erosion. This photo shows all that remains of a structure that probably extended along most of the beach at the tel. Granite columns reach out of the wall toward the sea like cannons. But, the columns were originally embedded within the seawall to give the structure extra strength.
Four to five ft (about 1 ½ m) of the wall have been pealed back by wave attack over the past 900 years. But, this wall now sticks out more than 32 ft (10 m) beyond the edge of the adjacent sea cliff, so cliff retreat has been much faster than is evident by the horizontal columns. We believe the beach cliff has retreated much more than 40 ft (12 m) over the past two millennia. And with that, the sea has stolen from us many historical treasures that were buried in the tel.
Why is the sea cliff of Ashkelon so easily eroded? The lower half of the cliff is held up by soft sandstone called kurkar, the remains of sand dunes, possibly tens of thousands of years old. Kurkar is the most common building stone found in the excavations at Ashkelon. We have noted that the stone is harder along the sea cliff. At Ashkelon, the kurkar sandstone is covered mostly by what archaeologists call occupational fill, the muddy sand and debris that make up the tell. This is what the archaeologists remove to reveal ancient buildings and artifacts. Fill is even softer than the kurkar. Our study of the beach cliff shows the stratigraphy of the tell, with successive layers of occupational fill over kurkar bedrock.
When storm waves pound the base of the cliff, the energy of the waves and infusion of water into the soft material causes it to act like a thick liquid that is quickly swept to sea. This action undercuts the cliff so that it rapidly slumps into massive cones of sediment and debris. Waves continue to remove the slumped material and undercut the cliff until the storm fades.
While storm activity is devastating to the coast, we took advantage of the fresh exposures along the cliff to learn more about the natural history and archaeology of Ashkelon.
The Government of Israel plans to protect archaeological sites along the coast with breakwaters, beach replenishment and structures to cover some of the soft sea cliffs. Yet, experience along other coasts around the world suggests that these efforts will only slow the inevitable impact of sea storms here. The possibility of sea level rise forced by global climate change can only make things worse.
The remains of epic military battles to claim Ashkelon over the millennia have been exposed by careful excavation. The final battle of Ashkelon will be won by the relentless grind of sea waves.
Stephen O. Moshier
Team Geologist
July 5, 2011