The Coastal Mountain Range is geologically unlike the rest of Taiwan's mountains. Where the Central Mountain Range to the west is built largely from metamorphic and sedimentary rocks pushed up by the continent's edge, the Hai'an Range originated as the Luzon Arc — a chain of volcanic islands that formed above a subduction zone in the open Pacific. As the Philippine Sea Plate carried these volcanic islands northwestward into the Eurasian margin, they accreted onto Taiwan's eastern edge rather than subducting beneath it.

The result is a mountain range whose basement is ancient Miocene volcanic rock, overlain by a thick sedimentary sequence — four to six kilometers deep — deposited in deep to shallow marine environments over millions of years. The range is essentially a former island arc now sutured to the side of a continent. Its highest point, Xingangshan, reaches 1,682 meters. No volcanoes are currently active within the range, but the tectonic forces that created it are very much ongoing.

West of the Hai'an Range, between it and the Central Mountain Range, lies the Huadong Valley — one of the most dramatic geographic features in all of Taiwan. This long, flat-floored trough is not a river valley in the ordinary sense; it is a rift created directly by the collision of the two plates, a zone of crustal weakness and active faulting that runs for much of the island's length. Rivers flow through it, and farms occupy its floor, but the underlying ground is in constant slow motion.

The Hai'an Range is converging with the Central Mountain Range at an average rate of 2.3 centimeters per year. To put that in perspective: in a human lifetime, the two ranges move closer by roughly a meter and a half. The peaks themselves rise at about 3 millimeters per year — a pace that sounds negligible until you calculate what it means across geological time. These are not old, settled mountains. They are actively becoming.

From a human perspective, the Hai'an Range functions primarily as a wall. It separates the inhabited rift valley from the Pacific coast, concentrating settlement on the valley floor and limiting east-west movement to a handful of passes and coastal roads. The mountains are forested, steep, and receive heavy rainfall on both slopes — conditions that have kept large-scale development at bay.

The communities at the base of the range, from Hualien County south through Taitung County, are among the most culturally distinct in Taiwan. Indigenous Amis and Bunun communities have lived here for centuries, farming the valley floors and fishing the rivers that drain the mountains. The landscape they inhabit — paddies against a mountain backdrop, rivers threading between peaks and sea — has made the Huadong region one of Taiwan's most photographed corridors, celebrated precisely for the juxtaposition of cultivated flatland and wild vertical terrain.

Despite its overall ridge-and-slope character, the Hai'an Range contains several small intermontane basins where flat land briefly appears within the mountains themselves. The largest of these is Taiyuan Basin, a relatively protected area where settlement and agriculture have taken hold amid otherwise rugged topography. These pockets of flatness in the hills have historically served as refuges and waypoints, places where communities could establish themselves slightly apart from the main valley corridor.

The range's coastline, where it meets the Pacific on its eastern face, is equally dramatic — a cliffed shore where mountains descend directly to the ocean, broken occasionally by small coves and river mouths. The Huadong Coastal Reserve protects stretches of this shoreline, preserving habitats shaped by the same tectonic restlessness that built the mountains behind them.