Great river gorges are some of the world's most impressive landforms.
A great suite of landforms comprises the planet's topography. Taken on a variety of scales, they include everything from the continents and the oceans to small kettle ponds and playas. Derived from layers of rock and sediment sculpted by geologic forces, erosion, weathering and other natural agents, they exert major ecological and climatic influence.
Hills and Mountains
The world's highest topographic points are mountains: whether the soaring seamounts and exposed volcanic islands of the oceans or the great terrestrial summits. Collisions of tectonic plates crumple them into existence, as do the violent, chronic releases of magma from under the Earth's crust that form volcanoes. Some mountains may simply be masses of resistant rock that have been revealed by erosion stripping away more yielding surrounding layers. The lower and more subdued landforms roughly classified as "hills" may represent ancient mountain ranges heavily weathered and eroded; old sand dunes stabilized by vegetation; mesas or buttes molded by differential erosion; or any number of other origins.
Plains
By contrast to the ruggedness of mountains and hills, the world's plains are level and sometimes nearly featureless. Vast accumulations of sediment build the mysterious, little-known abyssal plains of the ocean bottom that trend between marine trenches or ridges and the rims of the continental shelves. On land, broad flats mark the former basins of lakes or inland seas now vanished. Big rivers build floodplains by spilling their banks during high-flow events. The sediments deposited during these inundations grant the floodplain rich, fertile soils.
Defiles
The active burrowing of high-gradient streams and rivers creates the narrow valleys called canyons, gorges, ravines, gulches, arroyos and a host of other geographic monikers. These rends in the landscape constitute some of the most impressive terrain on the planet: gorges like the Grand Canyon in Arizona, Hells Canyon in Oregon-Idaho, Copper Canyon in Mexico, Fish River Canyon in Namibia and the Kali Gandaki Gorge in Nepal are renowned for their wildness and scenery.
Glacial Landforms
Glaciers are masses of ice that accumulate in high mountains and latitudes where cold temperatures and generous precipitation encourage the accumulation of large amounts of snow. Extreme pressure and slope inclinations encourage glacier movement, and their advances and retreats bulldoze the terrain and form distinctive landforms. These include teardrop-shaped hills called drumlins, sinuous ridges called eskers, great rubble piles called moraines that mark glacial fringes and scoured-out depressions called kettles. Mountain glaciers also significantly shape alpine landscapes, carving peaks into sharp conical horns and excavating huge slope-side amphitheaters called cirques.
Tags: most impressive
