Many rivers, bays and estuaries indent the 805 km (500 miles) of the coastline. Sediment formed the red sandstone on the Island deposited between 250 and 300 million years ago. White sandy beaches are predominant along the northern shores while red sandstone cliffs edge its southern exposure. The redness of the soil is due to its high iron-oxide (rust) content. The Island shows clear traces of the Ice Age. About 15,000 years ago, the ice retreated and the sea level rose severing it from the mainland.

Description borrowed from
InfoPEI
Visit them for much more information