why are fold mountains so called?

Fold mountains are mountains that form mainly by the effects of folding on layers within the upper part of the Earth's crust. Before either plate tectonic theory developed, or the internal architecture of thrust beltsbecame well understood, the term was used for most mountain belts, such as theHimalayas. The term is still fairly common inphysical geography literature but has otherwise generally fallen out of use except as described below. The forces responsible for formation of fold mountains are calledorogenic movements. The term orogenic has derived from a Greek word meaning mountain building. These forces act at tangent to the surface of the earth and are primarily a result of plate tectonics.