The geology and geomorphology of the islands is varied. Some, such as Skye and Mull, are mountainous, whilst others like Tiree are relatively low-lying. The highest mountains are the Cuillins of Skye, although peaks over are common elsewhere. Much of the coastline is machair, a fertile low-lying dune pastureland. Many of the islands are swept by strong tides, and the Corryvreckan tide race between Scarba and Jura is one of the largest whirlpools in the world.