A river's relentless work
The Cilician Gates is a narrow gorge cut by the Göksu River through the Taurus Mountains of southern Turkey. This mountain range, a product of the collision between the African and Eurasian tectonic plates, is composed predominantly of limestone. Over millennia, the river, known in antiquity as the Calycadnus, has acted as a powerful erosional force. Carrying abrasive sediment from its headwaters, the river has sliced through the soluble limestone, steadily deepening the gorge that connects the high Anatolian Plateau with the coastal Cilician plain.
The rate of this geological process is not uniform. The river's power to cut into bedrock depends on its flow and the amount of sediment it carries. The calculation that the pass has deepened by 50 meters in the last 10,000 years provides a specific measurement of the river's long-term incision rate into the landscape. This equates to an average deepening of about 5 millimeters per year. This measurement comes from studying river terraces—remnants of former floodplains left on the valley sides as the river cuts downward.
Measuring deep time
Scientists determine the age of these terraces using geochronological methods. One such technique is cosmogenic nuclide dating. This method measures the concentration of specific isotopes, like Beryllium-10, in minerals such as quartz. These isotopes are produced when rocks are exposed to cosmic rays at or near the Earth's surface. By measuring the concentration of these nuclides in the terrace sediments, geologists can calculate how long the material has been exposed, and thus, the age of the abandoned floodplain.
By dating a series of terraces at different elevations, a timeline of the river's down-cutting can be constructed. The height difference between a dated terrace and the current river level reveals the total incision over that time period. This data allows for the calculation of an average erosion rate, providing records the geological pace of landscape change.
A path for armies and apostles
The slow, persistent erosion by the Göksu River created a feature of immense historical importance. The resulting pass became one of the most important commercial and military arteries for millennia. It was the primary route for armies and trade moving between central Anatolia and the Levant. Historical records show a long succession of armies marching through its narrow confines.
In 401 BCE, the Greek mercenaries of Cyrus the Younger passed through. In 333 BCE, the army of Alexander the Great marched through the Gates uncontested before the Battle of Issus. Roman legions later improved the road; Emperor Caracalla widened the pass in 217 CE to better supply his campaign against the Persians. The pass was also traveled by Paul the Apostle on his journeys through Cilicia and to the Galatians. Crusaders in the 12th and 13th centuries knew it as the "Gate of Judas." The pass's geography, with steep rock walls, made it a natural defensive chokepoint, influencing the strategy of countless military campaigns. Today, a modern highway, the O-21, follows this ancient, river-carved route.