15:41 Mon Apr 20, 2015
Gazing Into the Rockpile's Icy Past
It is late April at the end of a long cold winter and the ravines on Mount Washington are just about full to the brim with snow. More snow may be on the way this week and it’s almost hard to imagine that it will all melt before the snow flies again in the fall. That will probably seem foolish come July, but there have been times in local memories when small pockets of snow endured the summer months tucked away in hidden corners of the mountain, and a few dedicated skiers claimed streaks of 20-30 consecutive months of turns. The lore of perennial snow in the Whites is impressive, but it wasn’t long ago that winter ruled and the mountains of northern New England were saddled with the glaciers that carved the many cirques of the Presidential Range. By some estimates mean summer temperatures would have needed to be 5-9 degrees C below modern-day value to support glaciers in the White Mountains (assuming equal amounts of winter precipitation).
Drawn in by the spectacular cirques of Tuckerman and Huntington Ravines, geologists have been contemplating the glacial history of the range for almost 200 years, although they weren’t always aware of the glacial origins of the ravines. A popular idea of the early 19th century was the “drift theory”, which attributed glacially-derived sediments and striae (grooves on rocks) to a marine flood throwing icebergs against the mountainsides. The conclusion may have been a bit off-point, but the observations and measurements on which it was founded were the initial steps in a lengthy collaboration that has, in today’s understanding, identified no less than 17 glacial cirques on the Presidentials. Some, like the Upper Great Gulf, are classic examples of alpine cirques, with steep headwalls and broad flat floors. Others, like the Ammonoosuc Ravine, are a little less convincing, perhaps due to fluvial erosion that has occurred since the glaciers retreated. All together these cirques have an average height of 452 meters and a total collective volume of 4.21 billion cubic meters!
Aside from identifying the many cirques, there have been a few large-scale questions about the glaciation of the White Mountains, and other ranges in northern New England, that have puzzled those who study it. The morphometry, or shape, of the most pronounced cirques, with defined lips at the upper edge and a high headwall-floor slope ratio, is suggestive that independent alpine glaciers carved the cirques, and that they were not formed by a continental ice sheet or tongues of ice extending down from an icecap centered on Mount Washington and the rest of the range. Continental glaciation has, however, been an active part of forming the landscapes of New England, and the question arose of whether the cirque glaciers outlasted the Laurentide Ice Sheet, which left the moraine of Long Island as it began its recession 15,000 years ago.
Dating the recession of cirque glaciers in the Whites is a difficult proposition due to the limited number of tarns (small lakes that sit at the base of a cirque in the depression formed by the weight of the ice). While the two tarns of the Presidential Range (Spaulding Pond and Hermit Lake) have not been cored for organic material to date, minimum radiocarbon ages for retreat have been determined from cores of Lost Pond in Pinkham Notch (12,870 +/- 370 years) and the lower of the Lakes of the Clouds (11,530 +/- 420 years). These dates are the ages of the first organic material that collected in the bottom of the ponds, and indicate that deglaciation occurred at least that long ago. The question of whether cirque glaciers survived beyond the recession of continental ice is still a debated topic, due in part to complex glacial features (debated provenance of till in cirque floors, presence or absence of terminal moraines on cirque floors) and the lack of high resolution dates for the local recession of the ice sheet. It is still fascinating, however, to look out our windows and imagine the glaciers draped on the sides of the surrounding ridges (and how much fun they be to ski!).
Mount Washington Summit and the Great Gulf Headwall from near Clay col.
PT Davis, Cirques of the Presidential Range, New Hampshire, and surrounding alpine areas in the northeastern United States, Geographie Physique et Quaternaire, Vol. 53, No. 1 (Image Source)
WB Thompson, History of research on glaciation in the White Mountains, New Hampshire, Geographie Physique et Quaternaire, Vol. 53, No. 1
Adam Freierman, Summit Intern