35,000 years ago, nature set about cutting gorges

By measuring damage to rocks from cosmic rays, scientists have nailed down the age of scenic gorges along the Susquehanna and Potomac rivers - and rewritten the ancient history of both waterways.

The two popular hiking spots, Mather Gorge on the Potomac near Great Falls, Va., and the Susquehanna's Holtwood Gorge near Peach Bottom, Pa., began forming about 35,000 years ago when ice, snow and rainstorms started to pelt the East Coast, spawning centuries of heavy flooding, researchers say.


The Potomac and the Susquehanna were influenced more by floods than by melting glacial ice, contrary to previous theory, according to a University of Vermont study.

"Climate played a significant role. There were colder temperatures, a greater snowpack, and that meant more water," said Luke J. Reusser, a University of Vermont graduate student and the lead author of the study published in today's issue of the journal Science.


The study is the first to measure how quickly water cuts through rock in stable geologic formations and carves out riverbeds such as those of the Susquehanna and Potomac - two of the East Coast's largest rivers.

Funded by the National Science Foundation and the Department of Energy, the research is expected to help geologists better understand the long-term effect of climatic forces.

"Does it change the way I look at the world? It does," said Richard Alley, a Penn State geologist who reviewed the findings. "It adds something we didn't know about how rivers move rocks and how they make the landscapes we see all around us."

Mysterious process

Geologists have extensively studied the effects of rivers on sandbars, sediment and debris that flow into them during heavy rains. But much less is known about how rivers cut through stable rock and about the forces of nature involved.

"Rivers are not just water pipes. Debris collects in waters as they pass by, and rivers move rocks as well as water. A lot of the dynamics of that process remains a mystery," Alley said.

Using data from previous studies of Greenland ice cores and East Coast lake sediments, the researchers showed that the gorges formed at the onset of a deep freeze that occurred when glaciers advanced as far south as Central Pennsylvania 20,000 years ago. Colder temperatures brought ice that clogged rivers and snow that caused spring flooding.

"There were a lot more flood events. They lasted longer, and they were capable of moving a lot more rock," said Paul R. Bierman, a geology professor at the University of Vermont and a co-author of the study.


The study began four years ago when Reusser and Bierman chiseled samples of exposed rock known as schist from the Mather and Holtwood gorges. They knew the rock samples were between 400 million and 500 million years old. But they wanted to know when the rivers began sculpting them into gorges.

"We didn't know if they were a couple thousand years old or a couple hundred thousand years old," Bierman said.

They ground up the schist and separated the quartz that it contained. With a spectrometer at the Lawrence Livermore National Laboratory, they measured the amounts of a beryllium isotope that forms in quartz over time when it is exposed to cosmic rays in Earth's atmosphere.

Geologists have been using such beryllium 10 isotopes to date rock exposures since the 1980s.

The researchers found both gorges were carved over a relatively rapid 20,000-year stretch, starting about 35,000 years ago when floodwaters cut deeper and narrower channels into the rocks.

Reusser said the Susquehanna and Potomac were natural choices for the study. Although the upper reaches of the Susquehanna were covered by Ice Age glaciers, the Potomac is farther south and was never under glacial ice.


The contrast made the rivers ideal because researchers thought that they would find evidence that the waterways formed during different time periods, at different paces, and that Holtwood Gorge was carved out, or "incised," at a quicker pace than Mather Gorge.

"Rates of glacial incision are generally much faster," Reusser said. "You have tons and tons of water from melting ice."

Instead, researchers found that the rivers formed at about the same time and pace, cutting into rock at rates that ranged from a half-inch every 1,000 years to a relatively speedy 2.5 feet every 1,000 years. Researchers have no idea why the pace varied so much.

Natural choice

"It's like these rocks just sit where they are for a while and nothing happens, and then something changes and the incisions begin again," said Reusser.

The gorges also were a natural choice for the study, Reusser said.


Mather Gorge is 2 miles long, studded with outcrops and a familiar haunt to Bierman, who was born and raised in the Pimlico section of Baltimore.

"It was a lot of fun to be out hiking around in an area that I remembered from childhood," Bierman said.

Holtwood Gorge is about 3 miles long, sits downstream from a dam and features three distinct levels of exposed bedrock.

"They were well-defined, well-preserved terraces, just waiting to be studied," Reusser said.