Three limestone islands make up the Aran Islands: Inishmore, Inishmaan and Inisheer (respectively, the Great, Central and Eastern Island). The three islands of Aran stretch across the mouth of Galway Bay, forming a natural breakwater against the Atlantic Ocean. The largest of the three, Inishmore, is about nine miles long. In many places it is quite barren. Geologically, the islands are an extension of the Burren in Clare, on the mainland to the southeast: an uplifted limestone block, striated by gashes ranging from inches to hundreds of feet deep. Water percolates right through the stone, leading sometimes to water shortages and preventing the formation of the typically boggy western Irish land form. Peat for fires had to be imported from Galway. The resulting flora are unique; Mediterranean and Alpine species meet here, attracting masses of tiny multicoloured butterflies.