 The plain ranges from 26 to 77 kilometers wide and extends from the Corentyne River in the east to the Venezuelan border in the northwest.The coastal plain is made up largely of alluvial mud swept out to sea by the Amazon River, carried north by ocean currents, and deposited on the Guyanese shores.A rich clay of great fertility, this mud overlays the white sands and clays formed from the erosion of the interior bedrock and carried seaward by the rivers of Guyana.