Oceans are comprised of saltwater with varying salinity levels. The continental shelf is a shallow, sloping plain along coastlines that transitions into a continual slope further from shore, cut through in places by trenches. Pressure increases and light/temperature decrease with depth in the ocean, while surface currents are driven by wind patterns and deep currents are caused by density differences in water. Waves erode coastlines through hydraulic action and abrasion, carrying sediment that is deposited to form beaches through longshore drift.