Since Rarotonga has her short history comparing to the other Cook Islands southern group, it is geologically unlike its other islands where erosion and periodic submersions have changed mountains to gentle hills. Rarotonga's central massif is the eroded remains of a once mighty volcanic pyramid whose crags now form sawtooth peaks and razorback ridges covered with tropical jungle. These are separated by streams running down steep valleys. Around the central mountainous area of this beautiful island is a narrow band of agricultural terraces and flats which, in turn, is encircled by a ring of swamps used largely for growing taro.