end arches is taken into the earth by substantial (vertical) footings at the canyon walls, or by large inclined planes forming in a sense ramps to the bridge, which may also be formed of arches. The several arches are (or single arch is) constructed over the centering. Once each basic arch barrel is constructed, the arches are (or arch is) stabilized with infill masonry above, which may be laid in horizontal running bond courses (layers). These may form two outer walls, known as the spandrels, which are then infilled with appropriate loose material and rubble. The road is paved and parapet walls protectively confine traffic to the bridge. Types of arch bridge Corbel arch bridge The corbel arch bridge is a masonry, or stone, bridge where each successively higher course (layer) cantilevers slightly more than the previous course. The steps of the masonry may be trimmed to make the arch have a rounded shape. The corbel arch does not produce thrust, or outward pressure at the bottom of the arch, and is not considered a true arch. It is more stable than a true arch because it does not have this thrust. The disadvantage is that this type of arch is not suitable for large spans. Aqueducts and canal viaducts In some locations it is necessary to span a wide gap at a relatively high elevation, such as when a canal or water supply must span a valley. Rather than building extremely large arches, or very tall supporting columns (difficult using stone), a series of arched structures are built one atop another, with wider structures at the base. Roman civil engineers developed the design and constructed highly refined structures using only simple materials, equipment, and mathematics. This type is still used in canal viaducts and roadways as it has a pleasing shape, particularly when spanning water, as the reflections of the arches form a visual impression of circles or ellipses. Deck arch bridge This type of bridge comprises an arch where the deck is completely above the arch. The area between the arch and the deck is known as the spandrel. If the spandrel is solid, usually the case in a masonry or stone arch bridge, the bridge is called a closed-spandrel deck arch bridge. If the deck is supported by a number of vertical columns rising from the arch, the bridge is known as an open-spandrel deck arch bridge.