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48 FUNDAMENTALS OF PHYSICAL GEOGRAPHY cutting becomes less dominant and lateral Mature erosion of banks increases and as a During this stage streams are plenty with good consequence the hills and valleys are reduced integration. The valleys are still V-shaped but to plains. deep; trunk streams are broad enough to have wider floodplains within which streams may Is complete reduction of relief of a high flow in meanders confined within the valley. land mass possible? The flat and broad inter stream areas and Overland flow causes sheet erosion. swamps and marshes of youth disappear and Depending upon irregularities of the land the stream divides turn sharp. Waterfalls and rapids disappear. surface,theoverlandflowmayconcentrateinto narrow to wide paths. Because of the sheer Old friction of the column of flowing water, minor or major quantities of materials from the Smaller tributaries during old age are few surface of the land are removed in the directionwith gentle gradients. Streams meander freely of flow and gradually small and narrow rills over vast floodplains showing natural levees, will form. These rills will gradually develop inoxbow lakes, etc. Divides are broad and flat long and wide gullies; the gullies will further with lakes, swamps and marshes. Most of deepen, widen, lengthen and unite to give the landscape is at or slightly above sea level. rise to a network of valleys. In the early stages, down-cutting dominates during which E ROSIONAL LANDFORMS irregularities such as waterfalls and cascades will be removed. In the middle stages, streams Valleys cut their beds slower, and lateral erosion of valley sides becomes severe. Gradually, the Valleys start as small and narrow rills; the rills valley sides are reduced to lower and lower will gradually develop into long and wide slopes. The divides between drainage basins gullies; the gullies will further deepen, widen are likewise lowered until they are almost and lengthen to give rise to valleys. Depending completely flattened leaving finally, a lowland upon dimensions and shape, many types of of faint relief with some low resistant remnantsvalleys like V-shaped valley, gorge, canyon, called monadnocks standing out here and etc. can be recognised. A gorge is a deep valley there. This type of plain forming as a result of with very steep to straight sides (Figure 6.1) stream erosion is called a peneplain (an almost and a canyon is characterised by steep plain). The characteristics of each of the stagestep-like side slopes (Figure 6.2) and may be of landscapes developing in running water as deep as a gorge. A gorge is almost equal in regimes may be summarised as follows: width at its top as well as its bottom. In Youth contrast, a canyon is wider at its top than at its bottom. In fact, a canyon is a variant of Streams are few during this stage with poor gorge. Valley types depend upon the type and structure of rocks in which they form. For integration and flow over original slopes example, canyons commonly form in showing shallow V-shaped valleys with no floodplains or with very narrow floodplains horizontal bedded sedimentary rocks and along trunk streams. Streams divides are gorges form in hard rocks. broad and flat with marshes, swamp and Potholes and Plunge Pools lakes. Meanders if present develop over these broad upland surfaces. These meanders may Overtherockybedsofhill-streamsmoreorless eventually entrench themselves into the circular depressions called potholes form uplands.Waterfallsandrapidsmayexistwhere because of stream erosion aided by the local hard rock bodies are exposed. abrasion of rock fragments. Once a small and Reprint 2026-27