Residual mountains occur when

Residual mountains are formed when the general level of land is lowered by agents of denudation, such as erosion and weathering, but some areas remain elevated due to the presence of hard and resistant rock. Over time, these areas are shaped by erosion into mountains, which are called residual mountains. The erosion and weathering gradually wear down the softer rocks around them, leaving the harder rocks standing tall. This process can be observed in places like the Appalachian Mountains in the eastern United States, which were once much taller but have been worn down over millions of years by erosion to their current height.