A succession of towers of steel, timber, or reinforced concrete supporting the horizontal beams of a roadway, bridge, or other structure. Little distinction can be made between a trestle and a viaduct, and the terms are used interchangeably by many engineers. A viaduct is defined as a long bridge consisting of a series of short concrete or masonry spans supported on piers or towers, and is used to carry a road or railroad over a valley, gorge, another roadway, or across an arm of the sea. A viaduct may also be constructed of steel girders and towers. It is even more difficult to draw a distinction between a viaduct and a bridge than it is between a viaduct and a trestle.