There are some differences in the physical environment of mainland and island Southeast Asia. The first feature of mainland geography is the long rivers that begin in the highlands separating Southeast Asia from China and northwest India. A second feature is the extensive lowland plains separated by forested hills and mountain ranges. These fertile plains are highly suited to rice-growing ethnic groups, such as the Thais, the Burmese, and the Vietnamese, who developed settled cultures that eventually provided the basis for modern states. The highlands were occupied by tribal groups, who displayed their sense of identity through distinctive styles in clothing, jewelry, and hairstyles. A third feature of mainland Southeast Asia is the long coastline. Despite a strong agrarian base, the communities that developed in these regions were also part of the maritime trading network that linked Southeast Asia to India and to China..