In this area, most of the sand comes from glaciofluvial sand deposits situated along the shore behind the beach, and some comes from the erosion of the rocks on the headlands. There are no large rivers bringing sandy sediments to the west coast of Vancouver Island, but there are still long and wide sandy beaches there. An obvious example is where the Mississippi River flows into the Gulf of Mexico at New Orleans another is the Fraser River at Vancouver. On coasts that are dominated by depositional processes, most of the sediment being deposited typically comes from large rivers. These areas have relatively little topographic relief, and there is now minimal erosion of coastal bedrock. The coasts of the United States along the Atlantic and the Gulf of Mexico have not seen significant tectonic activity in a few hundred million years, and except in the northeast, have not experienced post-glacial uplift. This coast has also been uplifted during the past 15,000 years by isostatic rebound due to deglaciation. A coast like that of British Columbia is tectonically active, and compression and uplift have been going on for tens of millions of years. The main factor in determining if a coast is dominated by erosion or deposition is its history of tectonic activity. On deposition-dominant coasts, the coastal sediments are still being eroded from some areas and deposited in others. This is clearly evident in the Tofino area of Vancouver Island (Figure 17.1), where erosion is the predominant process on the rocky headlands, while depositional processes predominate within the bays. But on almost all coasts, both deposition and erosion are happening to varying degrees most of the time, although in different places. Some coastal areas are dominated by erosion, an example being the Pacific coast of Canada and the United States, while others are dominated by deposition, examples being the Atlantic and Caribbean coasts of the United States.
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