Fossil Landscape Revealed: Reading the Rocks in New England Summits

It’s remarkable to think that we are discovering ice on a dwarf planet 4.67 billion miles (7.5 billion kilometers) away from Earth, at a time when we are still unraveling clues to the ice that once shaped this planet.

Two New England geologists have spent years studying the rocks and traces left by ancient glaciers in the White Mountains, the northern stretch of the Appalachian Mountains in New Hampshire and Maine. This past October, they co-authored a paper in Geology with 3 other scientists.  And in it, they revealed that the highest summits in New England were covered by solid ice during the Last Glacial Maximum.

 

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View from the summit of Mount Moriah in New Hampshire looking at the White Mountains’ Presidential Range; image by Cappi Thompson at Getty Images.

 

But what does this mean? And why should we care?

“This question about whether or not the New England summits were covered by an ice sheet is long standing, going back over 100 years,” explained Dr. P. Thom Davis by phone.  “And one reason this question is important is because continental ice sheets take a long time to build up, and as they build up, they reduce global sea level.   So ice sheet thickness has implications far beyond just New England.”

Dr. Thom Davis of Bentley University and Dr. Paul Bierman of the University of Vermont actually wrote and presented the paper at the annual Geological Society of America conference in 1999, but they didn’t publish it until 2015.

“We sent a draft around informally to some colleagues,” said Dr. Davis, “and they sort of were giving us a hard time, wanting us to go back and rethink a lot of the implications. And then one thing led to another, and we just kept rethinking and rethinking for the next 15 years.”

“It was a much more radical proposition in 1999 than it is today,” explained Dr. Bierman. “Since then a whole lot of work has come out of the Arctic. So when we finally submitted this now, I guess it was a surprise, but it wasn’t unexpected.”

That ‘radical proposition’ involved whether or not ice covered the mountains, whether it was a certain type of ice, and therefore whether it did or did not preserve “fossil” or “relic” landscapes.   These ideas have been pondered (or dismissed) by various geologists since the mid 1800s.

While the term “fossil landscape” might inspire images of preserved prehistoric environments suffused with traces of ancient life, this is not at all what it means.  Rather, it refers to the geology left behind by cold-based ice–ice frozen all of the way to the ground–that both shielded rocks from the effects of cosmic rays and slowed erosion.

Think of how much impact an enormous sheet of ice can have on an environment.  When ice is not completely frozen to the ground (warm-based ice), water runs through it, pulling dirt, rocks–and the glacier itself!–along with it.  The ground erodes; the debris is carried elsewhere. Remnants can be seen in boulders scattered throughout New England.

Notice the shape of the valley in Crawford Notch, NH.  This valley is a result of glacier ice moving through the environment, albeit at a remarkably slow speed. Image by Mark Zelasko at Getty Images.

Boulder in Salisbury, NH

 

Detail of boulder in Salisbury, NH

Images of a boulder (perhaps a glacial erratic: a rock carried by a glacier and deposited in another location in geologic terms) in Salisbury, NH; photos taken by the author

 

“The word ‘fossil landscape’ sort of worried me from the get-go because of how it might be misconstrued to having more of a biological context, like mammoth bones,” stated Dr. Davis, in reference to the press release describing their work. “We’re looking at the age of exposure, the length of time those surfaces have been exposed to the cosmic ray bombardment.”

Drs Davis and Bierman collected samples near to or on the summits of Mt. Katahdin in Maine and Mt. Washington and Little Haystack Mountain in NH during the 1990s.  Their ‘radical’ suspicion–that these summits were indeed covered by solid ice–could only able be proven recently with advanced technology.

Before that, they–like their peers in the last two centuries–relied on visible clues: the type of rock on summits and in valleys, striations (or grooves) in the rocks that may have been made by  ice, the type of sediment in the valleys and whether this indicated the type of glacier that might have helped create them.  And one of the biggest clues?

Erratics.

“That is,” Dr. Davis explained, “stones that have been transported from another location.”

“Two centuries ago, scientists might have argued [that erratics] were deposited in these high locations by great floods,” he continued. “But that pretty much ended with Agassiz’s glacial theory in the middle of the 1800s.”

He is referring to Louis Agassiz, an eminent Swiss biologist and geologist who taught at Harvard, and perhaps the first to support the idea that these summits were covered by an ice sheet.  It is important to note, however, that he believed that ice sheet was a local glacier rather than a vast continental ice sheet.

Prior to this, geologists such as Charles T. Jackson–the first NH State Geologist–or Edward Hitchcock (of trace fossil fame) believed that a flood complete with icebergs was responsible for misplaced boulders. Striations could be explained by the force of rock against rock from powerful currents within that water.

British citizens Mary Horner Lyell and her husband, Charles–another well-known geologist from the 1800s–explored these mountains in 1845, including a trip up Mt. Washington on horseback. Lyell attributed erratics to melting icebergs.

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Frozen tower and communication equipment at the summit of Mt. Washington; image by Onfokus at Getty Images.  Charles Hitchcock — son of Edward and Orra Hitchcock — helped create this year-round weather station.  He was a NH State Geologist and a Dartmouth professor. 

NH geology took a step forward with James W. Goldthwait and then later his son, Richard, in the 1900s.  They proposed that New England summits were covered by solid ice–not warm-based ice–and by a continental–not a local–ice sheet.

“[James W. and Richard P. Goldthwait] recognized this importance long ago, from the turn of the last century,” said Dr. Davis. ‘They both recognized very fresh looking erratics. The only way erratics can arrive on these summits is by continental ice sheets.”

“They made a really good case that the last ice sheet that dropped these erratics on the summits happened during our last glaciation about 20,000 years ago. [I]f the summits had been nunataks during the last major glaciation about 20,000 years ago, then the erratics should have been more weathered, the soils should have been more developed on the summit areas, and the bedrock should have been more weathered, as well.”

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Baxter Peak of Mount Katahdin in Baxter State Park, Maine. View from Knife Edge Trail; image by Posnov at Getty Images.

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Mount Katahdin is the highest mountain in Maine at 5,268 feet (1,606 m). Katahdin is the centerpiece of Baxter State Park: a steep, tall mountain formed from underground magma; image by Simon Massicotte at Getty Images.

The debate about the type of ancient ice in the White Mountains was dropped for a few decades, slowly regaining interest in the 1970s.  But it wasn’t until the recent paper by Drs. Bierman and Davis that proof lent itself to solving the issue.

“[Our method was to] count the abundance of very, very rare isotopes,” Dr. Bierman explained, “And, by that we mean isotopes of the element beryllium and the element aluminum.”

“The beryllium isotope with a total mass of 9 is the normal stuff that you find in nature. The beryllium isotope with a total mass of 10 per atom is extremely rare. And in order to measure these isotopes, we needed the technical ability to do that, and that didn’t come about until the late 1970s with a device called the accelerator mass spectrometer. These are very large, very expensive, difficult to maintain, and rare beasts. Over the past 30 years, they’ve been used increasingly by geologists to make the kinds of measurements that we did.”

“We also used cosmogenic carbon-14,” he continued, “which is an isotope with a much shorter half-life, about 5,730 years. And what that means is that when a rock is exposed to cosmic rays at the surface and then buried, that carbon 14 disappears much more rapidly than beryllium 10 and aluminum 26 isotopes.

“[Data from the accelerator mass spectrometer] tells us the [exposure] age because we can count the number of carbon-14 atoms, just like we can count the beryllium-10 atoms. We know that these are produced at a certain rate every year. It’s a very low rate.

“For beryllium-10, it’s just a few atoms per year per gram of material that we’re measuring. It’s a little bit more for carbon-14.  And since we know how quickly they’re made and we can count how many atoms there are, we can calculate an age–or a residence time–near the surface.”

“A lot of these ages from our exposure dating,” added Dr. Davis, “were coming out much older than we expected, much older than the last glaciation from the summits of both Katahdin and Mt. Washington.”

 

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A view of Mt Washington and Mt Madison along some farmland in Shelburne, New Hampshire during winter; image by Cappi Thompson at Getty Images.

“I think the Goldthwaits were primarily looking at these kinds of qualitative data, like how fresh the erratics in the bedrock were,” Dr. Davis offered. “And based on that, they probably weren’t exposed very long.  But as it turns out, weathering varies dramatically to different latitudes, so is not a very quantitative method. That’s all we had, though, until these cosmogenic radionuclides became available for measuring.”

“The main point of our geology paper is that, apparently, even at temperate latitudes, the higher elevations may have been overrun by ice sheets that were frozen to the bed, leaving what we call ‘relic landscapes,'” he concluded.

“From a geologic point of view,” Dr. Bierman continued, “it points to the complexity of the evolution of the New England landscape. It’s another piece of the puzzle in how this landscape evolved over time.”

 

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Sunrise clouds above the White Mountains’ Presidential Range in Jefferson, New Hampshire; image by Cappi Thompson at Getty Images.

Old_Man_of_the_Mountain_4-26-03

 

Old Man of the Mountain–an iconic NH rock formation, one that seems appropriate to share in a blog post on geology–on April 26, 2003, seven days before the rocks of its face collapsed. A late spring snow fell the night before. Image by Jeffrey Joseph, public domain, Wikipedia.

Dr. P. Thom Davis and Dr. Paul Bierman not only introduced me to a new science, they also piqued my interest in it. Basic geologic vocabulary was foreign to me. I delighted in discovering the meaning behind new words (nunataks, moraines, varve records, basal thermal regime!) in order to better understand their work. Thanks to their time and their research, I now look at the world around me with much more discerning eyes, especially at the many boulders erratics that scatter the landscape.  Fossils in New England may be scarce, but rock formations are not.  I extend a sincere and resounding THANK YOU to both, for their help, their graciousness and the fun three-way conversation we had discussing their paper!

Thank you to Kea Giles at the Geological Society of America for sending me a copy of the paper!

I highly recommend the book “The Geology of New Hampshire’s White Mountains” shown below, co-authored by Dr. P. Thom Davis. It is a fascinating account of NH geology and a great introduction to geology itself.

Woodrow Thompson, another co-author of that book, wrote an engaging account of the history of NH geology (paper is listed below). It was a great help to me in writing this piece, and I encourage anyone interested to read it

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References:

  1. Fossil Landscapes in New England, GSA press release, October 26, 2015
  2. Cold-based Laurentide ice covered New England’s highest summits during the Last Glacial Maximum, Paul R. Bierman, P. Thompson Davis, Lee B. Corbett, Nathaniel A. Lifton, Robert C. Finkel, Geology, October 2015
  3. History of Research on Glaciation in the White Mountains, New Hampshire (U.S.A.), Woodrow B. Thompson, Géographie physique et Quaternaire, Volume 53, 1999
  4. The Geology of New Hampshire’s White Mountains, J. Dykstra Eusden, Woodrow B. Thompson, Brian K. Fowler, P. Thom Davis, Wallace A. Bothner, Richard A. Boisvert, John W. Creasy; Durand Press, 2013

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