Mt. Dabajian and Mt. Xiaobajian: Taiwan’s Most Ancient Sedimentary Rock

Straddling the border of Jian­shi Township in Hsin­chu County and Tai’an Township in ­Miaoli County, 3,492-meter-high Mt. Da­ba­jian and 3,418-meter-high Mt. Xiao­ba­jian are located in the central section of the Xue­shan Range, and lie within Shei-Pa National Park.

Mt. Da­ba­jian’s most distinctive feature is the hard sandstone pyramid that forms its peak. Fossils of ancient Fora­mini­fera plankton found in this area prove that the geological forms here are Taiwan’s oldest sedimentary rock, with the lower layers dating back 40–50 million years. And Mt. Da­ba­jian and Mt. Xiao­ba­jian have the same geological makeup and origins.

Hard sandstone formed by tens of millions of years of sedimentation is responsible for Mt. Dabajian’s distinctive appearance.

Just imagine an ancient seabed 40–50 million years ago, layered with rock and grit as well as now-extinct marine organisms that would one day be fossilized. Over a period of more than 10 million years, strata of sandstone and shale piled up like a layer cake. Then, starting around 4 million years ago, during the orogeny of the Xue­shan Range, it was slowly lifted upward to form some of Taiwan’s most magnificent peaks.

In the view of geologists, Mt. Da­ba­jian and Mt. Xiao­ba­jian are valuable in terms of academic research, natural beauty, and education. This is indeed a national-class geological landscape.

Explains Chen Wen-shan, professor of geosciences at National Taiwan University, “After tens of millions of years, the rock of Mt. Da­ba­jian and Mt. Xiao­ba­jian still retains its original appearance, not having been strongly metamorphosed, and fossils of marine organisms are also visible. So the upper strata of these two mountains and the Xue­shan Range can be deemed Taiwan’s most ancient sedimentary rock.”

Looking at Mt. Da­ba­jian from afar, we see horizontal strata formed by the accretion of layer upon layer of sandstone and shale. The upper layers of shale were gradually eroded, leaving hard Sze­leng sandstone at the summit. This rock doesn’t fold or weather easily, so it sticks out along the ridgeline, creating the two prominent peaks of Mt. Da­ba­jian and Mt. Xiao­ba­jian.

But long ago, Mt. Da­ba­jian and Mt. Xiao­ba­jian were not the isolated mountaintops we see today, which resemble two brothers facing each other. Says Chen, “From the stratigraphic structure of the mountain range, we know that Mt. Da­ba­jian and Mt. Xiao­ba­jian originally belonged to the same stratum and were connected. But the ground between the two mountains eroded away, leaving the harder peak structures standing tall, thereby forming the terrain we see today.”

Even more incredibly, tens of millions of years ago, what is now Mt. Da­ba­jian and Mt. Xiao­ba­jian was covered with a layer of rock up to 10 kilometers thick, which was gradually worn away by relentless uplifting and erosion.

But even amid the short existence of the human species, this mountain scene may be fleeting. “It’s estimated that after just a few thousand more years of weathering, the awe-inspiring mountaintops of Mt. Da­ba­jian and Mt. Xiao­ba­jian will also be worn away to nothing!” says Chen.

Mt. Xiao­ba­jian is like a compact version of Mt. Da­ba­jian. Though it’s not as stately or prominent as Mt. Da­ba­jian, it too has a recognizable layered pyramid shape, and its cliffs are every bit as precipitous, giving the mountain its own unique grandeur.

The Atayal and Sai­si­yat peoples both revere Mt. Da­ba­jian as a sacred place of ancestral origin, and so it was long off limits to climbing. It was only in 1927 that it was first scaled by Japanese mountaineers.

NTU geologist Chen reminds us that hikers visiting these mountains, when gazing from afar at Holy Ridge and the chain of peaks, should not forget to look down at the ground under their feet and seek out the tiny traces left in the rocks around them.

Long ago, the entire Xue­shan Range was part of an area of continental shelf lying beneath a shallow sea between a few dozen and 200 meters deep. The thick sandstone of Mt. Da­ba­jian and Mt. Xiao­ba­jian came from a marine shoreface zone within 20 meters of the coastline.

Tens of millions of years ago, Mt. Da­ba­jian and Mt. Xiao­ba­jian, as well as the entire Xue­shan Range, lay in a shallow seacoast environment, accumulating sediments from the Eurasian landmass, while also preserving shells, plankton and benthic organisms.

As if riding a geological rollercoaster, fossils of ancient marine organisms were lifted from the seashore to the mountaintop.

Says Chen, most of these organisms are now extinct, but the trace fossils they left behind from the time they were active include marks from crawling, nest building, foraging, burrowing and resting, all engraved into the rocks and the cliff faces.

“The active orogeny of this young island of Taiwan is still pushing Mt. Da­ba­jian upward by about one centimeter a year. But at the same time it’s being broken down by inevitable erosion.”

Chen points out that the sides of Mt. Da­ba­jian have been crumbling recently, the original black rock layers falling away to expose bright yellow rock face. From the color differences, we can see the progress of change in these mountains. As for what Mt. Da­ba­jian and Mt. Xiao­ba­jian will look like in the future, only time will tell!