The geographical resources of the Indonesian archipelago have been exploited in ways that fall into consistent social and historical patterns. One cultural pattern consists of the formerly Indianized, rice-growing peasants in the valleys and plains of Sumatra, Java, and Bali, another cultural complex is composed of the largely Islamic coastal commercial sector, a third, more marginal sector consists of the upland forest farming communities which exist by means of subsistence swidden agriculture. To some degree, these patterns can be linked to the geographical resources themselves, with abundant shoreline, generally calm seas, and steady winds favouring the use of sailing vessels, and fertile valleys and plains—at least in the Greater Sunda Islands—permitting irrigated rice farming. The heavily forested, mountainous interior hinders overland communication by road or river, but fosters slash-and-burn agriculture.