Mountains

When lava erupts from a volcano or fissure, it flows downhill like any other liquid, into low spots in the landscape. This is why a river valley makes a convenient path for a lava flow. When the lava has solidified in the lowest part of the valley, it may be harder than the rocks that form the valley's walls. So when water again can flow through the valley, it flows around both sides of the lava flow. Eventually the lava flow has two valleys on either side of it, getting deeper every year. After thousands of years, a mesa will have been created, for the lava flow has become a caprock. This form of mesa is called inverted topography, because the low places become high places.

Outliers and monadnocks: Another term for a mountain made from a plateau worn by erosion is outlier. Not necessarily flat-topped, an outlier can be any hill or mountain left standing as the plateau with which it was once joined erodes farther and farther away. The Tepuis of Venezuela are outliers of a once-widespread plateau. A hard-rock mountain left standing after an entire mountain range has eroded away around it is a monadnock.