This article aims to understand the socio-economic and religious changes in early mediaeval Surma valley, and the role of Brahmanical and Buddhist religious institutions in effecting them. This valley was the most forested and marshiest part of Bengal. It received very heavy rains and was under substantial tribal influence. At the beginning of the period under study, it was a peripheral part of two fringe states. By the eleventh century ce a local state evolved, resulting from centuries of agrarian expansion. Brahmanical religious institutions played a very important role in effecting this transition. Well before the arrival of Islam in the eastern/north-easternmost sector of the Bengal delta, local society had devised its own ways to tame the jungle, cope with fluvial volatility, and cultivate rice, even in the marshy areas. These developments force us to question those historiographical models which explain the Islamization of eastern Bengal in terms of Islam being the ‘religion of the plough’ and ‘the harbinger of rice revolution’ in the region.