Geologically, Suriname Onshore is subdivided into a crystalline basement (80%), and a coastal plain (20%). The Shield area forms part of the Guiana Shield, which stretches between the Orinoco and the Amazon rivers and includes eastern Venezuela, Guyana, Suriname, French Guiana, and northern Brazil. The crystalline basement is formed principally of igneous and metamorphic rocks, whilst the coastal plain, which stretches along the northern fringe of the shield area, is exclusively sedimentary.