Argentine Patagonia is for the most part a region of steppe-like plains, rising in a succession of 13 abrupt terraces about 100 m (330 ft) at a time, and covered with an enormous bed of shingle almost bare of vegetation.[16][17] In the hollows of the plains are ponds or lakes of fresh and brackish water. Towards Chilean territory, the shingle gives way to porphyry, granite, and basalt lavas, and animal life becomes more abundant.[16] Vegetation is more luxuriant, consisting principally of southern beech and conifers. The high rainfall against the western Andes (Wet Andes) and the low sea-surface temperatures offshore give rise to cold and humid air masses, contributing to the ice fields and glaciers, the largest ice fields in the Southern Hemisphere outside of Antarctica.[17]