Wednesday, March 9, 2011

Ghosts of Glaciers Past? - One of Two

Instead of flying directly south, the monarchs crossing high over [Lake Superior] take an inexplicable turn towards the east. Then when they reach an invisible point, they all veer south again. Each successive swarm repeats this mysterious dogleg movement, year after year. Entomologists actually think that the butterflies might be “remembering” the position of a long-gone, looming glacier.
Annie Dillard, Pilgrim at Tinker Creek

This is one of those stories that could go in lots of different directions, but I’ll try to stay to the point. Not long ago, I was researching an unrelated topic when I encountered a title that commanded attention:

Pleistocene Glaciation in the Blue Ridge Province, Southern Appalachian Mountains, North Carolina, by James O. Berkland and Loren A. Raymond
Science 17 August 1973:
Vol. 181 no. 4100 pp. 651-653

This was an unexpected twist. I thought the Southern Appalachians escaped the glaciation of the Last Ice Age, 10,000 to 30,000 years ago, when glaciers extended no farther south than Pennsylvania. Today, that scenario might be beyond dispute. I don’t know. But it turns out that throughout the 1970s, several geologists debated the existence of glaciers in the Southern Appalachians, with areas near Richland Balsam, Sam’s Knob, Shining Rock and East Fork among those they examined.

Berkland and Raymond were geology professors at Appalachian State University, and in their 1973 article, they related a survey of Grandfather Mountain and discovery of evidence that the Boone Fork area was carved by glacial activity:

Glacial polish, grooves, and striations discovered at an elevation of 1370 meters in the headwaters of Boone Fork on Grandfather Mountain, North Carolina, indicate the former existence of alpine glaciation at a latitude of 36°07'N. The Boone Fork glacier was located 890 kilometers south of the previously recognized southern limit of alpine glaciation in the Appalachian Mountains, and 350 kilometers southeast of the nearest point on the Laurentide ice sheet. This find has significant implications for studies of Pleistocene geomorphology, paleobiology, and paleoclimatology in the eastern United States.

Rock solid evidence? A photograph of "glacial" grooves on Boone Fork, from the article by Berkland and Raymond.

Though it was a startling announcement, the prospect of glaciation had some precedent. In 1928, geologist C. K. Wentworth entertained and then dismissed the possibility that the Southern Appalachians had ever contained glaciers. (Wentworth is remembered as the inventor of the Wentworth Scale. Remember? But that’s another story for another day.)

From time to time, I have hiked the Boone Fork area, and can’t think of another place quite like it. While rock-hopping from boulder to boulder, I never stopped to consider the ghosts of glaciers on the mountain. According to Berkland and Raymond, grooves in the rock outcroppings indicated glacial activity occurring 15,000 years ago:

The glaciated surface is considerably weathered, but grooves and polish are visible on about 5 percent of the top of the outcrop (900 m2). The longest continuous grooves are about 1 m long, 15 cm wide, and 5 cm deep. At least 60 grooves are known and adjacent grooves are parallel…

Several months later Science published responses to the article, from two USGS scientists and a geology professor:

North Carolina Glacier: Evidence Disputed, By John T. Hack, Wayne L. Newell, and John B. McKeon.
Science 5 April 1974: Vol. 184 no. 4132 pp. 88-91

In their critique, Hack and Newell expressed doubts:

We visited the outcrop on 18 September 1973 and examined and photographed the reported features. We found several dozen well-formed grooves on a group of outcrops at the site. We recognized the grooves as having been made by moving cables used in logging operations. They in no way resemble the polished, striated, and grooved outcrops we have seen in glaciated areas….

To bolster their case, Hack and Newell sought out an expert witness familiar with the area in question:

…we interviewed Charles Coffey, a retired lumberman now living in Blowing Rock. Coffey was employed in lumbering operations in the area for many years and had participated in the last logging of Boone Fork during the 1930's. He had been the operator of a steam-powered cable system called an "overhead skidder," a device used extensively in Boone Fork. This system involved several cables suspended between winches at one end and a tall stationary mast as much as several hundred yards upslope. In operation, slack cables were repeatedly dragged along the ground while the aerially suspended logs traveled down the mountain. It seems to us that such a system could have produced the grooves we saw on the outcrop. When asked whether the cables of a skidder could cut grooves, Coffey told us that we could find cable grooves on rocks all through the woods.

In the same issue of Science, Berkland and Raymond responded to their critics, conceding that the primary evidence for Grandfather Mountain glaciation, long grooves carved in the rock, were most likely produced by cable logging. But in their defense they produced a logger of their own:

An 86-year-old logger, Wayne Harmon, familiar with local lumbering practices, examined our photographs and asserted that our grooves "looked natural" and that loggers valued their cables too highly to have allowed them to drag along rocks for any length of time.

If you examine the headwaters of Boone Fork on this topo map it is easy to visualize the bowl-like depression that is often associated with cirque glaciers. A cirque glacier is “a relatively small body of ice, firn, and snow, occupying an armchair-shaped hollow in bedrock. It is generally wide in relation to its length. It is actively supplied by drifting snow and therefore shows vigorous behaviour, involving rotational sliding." (From A Dictionary of Earth Sciences, 1999)

[To be continued]


Jim Parker said...

Fascinating. Anxious for the 2nd part.

GULAHIYI said...

Thanks! Part 2 is scheduled for Friday.

James Robert Smith said...

I've been in the Boone Bowl, and it certainly does look like a glacial cirque. That said, I'm no geologist. Only an uneducated laborer. However, I've been hiking in glacial cirques here in the east on both Mount Washington in New Hampshire and Katahdin in Maine. The Boone Bowl resembles cirques on both of those peaks, but especially the Tuckerman Ravine on Washington.