Helston, the nearest town to the Lizard peninsula, is said to have once headed the estuary of the River Cober, before it was cut off from the sea by Loe Bar in the 13th century. It is a matter of debate as to whether Helston was once a port, albeit no actual records still exist. Geomorphologists believe the bar was most likely formed by rising sea levels, after the last ice age, blocking the river and creating a barrier beach. The beach is formed mostly of flint and the nearest source is found offshore under the drowned terraces of the former river that flowed between England and France, and now under the English Channel. The medieval port of Helston was at Gweek, possibly from around 1260 onwards, on the Helford river which exported tin and copper. Helston was believed to be in existence in the sixth century, around the "Dowr Kohar".