of working against gravity are required when the thickness of the crust (not the lithosphere) changes. Lithospheric flexure Lithospheric flexure is the process by which the lithosphere bends under the action of forces such as the weight of a growing orogeny or changes in ice thickness related to glaciation. The lithosphere rests on the asthenosphere, a viscous layer that in geological time scales behaves like a fluid. Thus, when loaded, the lithosphere progressively reaches an isostatic equilibrium. For example, the lithosphere on the oceanward side of an oceanic trench at a subduction zone will curve upwards due to the elastic properties of the Earth's crust. Orogenic uplift Orogenic uplift is the result of tectonic-plate collisions and results in mountain ranges or a more modest uplift over a large region. Perhaps the most extreme form of orogenic uplift is a continental-continental crustal collision. In this process, two continents are sutured together, and large mountain ranges are produced. The collision of the Indian and Eurasian plates is a good example of the extent to which orogenic uplift can reach. Heavy thrust faulting (of the Indian plate beneath the Eurasian plate) and folding are responsible for the suturing together of the two plates. The collision of the Indian and Eurasian plates produced the Himalayas and is also responsible for crustal thickening north into Siberia. The Pamir Mountains, Tian Shan, Altai, Hindu Kush, and other mountain belts are all examples of mountain ranges formed in response to the collision of the Indian with the Eurasian plate. The Ozark Plateau is a broad uplifted area which resulted from the Permian Ouachita Orogeny to the south in the states of Arkansas, Oklahoma, and Texas. Another related uplift is the Llano Uplift in Texas, a geographical location named after its uplift features. The Colorado Plateau which includes the Grand Canyon is the result of broad tectonic uplift followed by river erosion. When mountains rise slowly, either due to orogenic uplift or other processes (e.g., rebound after glaciation), an unusual feature known as a water gap may occur. In these, erosion from a stream occurs faster than mountain uplift, resulting in a gorge or valley that runs through a mountain range from low-lying country on one side to similar country on the other. Examples of such water gaps include the Manawatu Gorge in New Zealand and the Cumberland Narrows in Maryland. Isostatic uplift The removal of mass from