What goes up must come down. So why are many mountain ranges, such as the Appalachians, still standing tall long after rivers should have eaten them down? Paradoxically, the erosive forces that should wear them away might instead carve them into a stable shape. Recent research has it that rivers only chew through bedrock if their waters carry lots of abrasive sand and grit. When mountains are being forced upwards by tectonic processes, their slopes are generally steep and liable to collapse in landslides (see picture).
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