Final answer:
The Appalachian Highlands were once very high mountains that have significantly eroded over time, due mainly to natural weathering and erosion processes known as orogeny.
Step-by-step explanation:
The mountains that were once very high but have been eroded over time are the Appalachian Highlands. Over millions of years, weathering and erosion have worn down these once towering peaks. This geographical phenomenon is referred to as the orogeny, which is a term that encompasses various forces and processes leading to the formation of mountains, followed by their eventual erosion.
Learn more about Mountain Erosion