In geography, a plain is a flat area. Plains occur as lowlands and at the bottoms of valleys but also on plateaus or uplands at high elevations. In a valley, a plain is enclosed on two sides but in other cases a plain may be delineated by a complete or partial ring of hills, by mountains or cliffs. Where a geological region contains more than one plain, they may be connected by a pass (sometime termed a gap). Plains may have been formed from flowing lava, deposited by water, ice, wind, or formed by erosion by these agents from hills and mountains.Mountainous regions are characterized by steep elevation (altitude) resulting in a large diversity of elevation (altitude), resulting in a large diversity of ecotones, also called transition areas, and complex climatic patterns. Climate projections are therefore more uncertain in mountain regions compared to areas with more uniform landscapes. In mountainous regions, warming exceeds the average continental trend.