A valley is a long narrow depression in the earth's crust, flanked by well defined ridges and usually due to the erosive action of rivers or glaciers but sometimes due to trough-faulting.
Longitudinal valleys are the hollows between the up-folded mountain ranges, parallel to the mountains, and they usually contain a largee river. Similar valleys occur between upfoldod mountains and the crustal plateau which has resisted upheaval. The Indo-Gangetic valley between the upfolded Himalayas and the Deccanplateau is the largest example of this type.
The valley cut by vertical erosion is usually V-shaped in cross-section and irregular in its course, its gradient being punctuated by sudden drops and long shelves. These irregularities represent local base levels which are gradually removed by denudation, so that as the falls are worn back and lakes infilled the breaks in the profile are reduced. In southern England the valleys of the Severn and the Thames show the results of denudation, which has carved away the softer rocks, and left the more resistant ridges of the Cotswolds, Downs, and Chilterns, which confine the drainage system.
With lateral erosion and mass movement, the valley broadens. Deposition occurs as the gradient slackens, and floodplains fill the valley floor. Rejuvenation leaves remnants of old floodplains above the new ones in the form of terraces, the highest of which are the oldest. A lowering of the water-table may leave dry valleys, and sudden uplift may leave hanging valleys, while the flooding of valleys by the sea gives rias or 'drowned valleys' which are existing estuaries where the sea has encroached upon the lower courses of rivers, such as the Gulf of St Lawrence. Research Valley
Abbreviations
Research Valley
