Monday, January 19, 2009

Red Rock Basin: glacial cirque or fluvial ravine?


According to Richard Goldthwait (as he expounded in his 1970 paper, Mountain Gaciers of the Presidential Range in New Hampshire), late geologist of Ohio State University, there are nine "steep-walled cirques" in the White Mountains, three “incipient basins” that were “ice-deepened,” and several other “névé basins holding firn”—an intermediate material between snow and ice.


As this map shows , Goldthwait believed all of these alpine glaciers, which he established were active before the onset of continental glaciation (a topic of great academic controversy a century ago, Woodrow Thompson tells in History of Research on Glaciation in the White Mountains), lay in the Presidential Range, with the possible exception of one or two on Moosilauke. Alex MacPhail’s recent series on glaciation beginning with this post on his excellent blog, White Mountain Sojourn, highlighted Goldthwait’s work and spurred my interest in the subject.

Goldthwait omitted one or two ravines that are strikingly similar to glacial cirques in their morphology. In particular, Red Rock Basin, as Alex calls it, a massive bowl on the west side of Guyot, has typically steep sides (note the fresh slides in the photos) and a relatively flat bottom. I wrote about bushwhacking up to and out of the ravine in my June 2007 post, Finding Red Rock Pond. It is far larger and more carved than any fluvial ravine I can think of in the Whites, and it splits into two distinct lobes. The photo at the top of the post, taken from West Bond's summit, clearly shows the bifurcation. Within the northern ravine sits a small, shallow body of water, Red Rock Pond, possibly a small tarn (cirque lake). Indeed, just below it, there is a “little headwall” similar to that found below Hermit Lake in Tuckerman’s Ravine.

Problematically, Red Rock Basin, opening to the southwest, lies in the opposite direction to which most local glaciers formed, which is probably why Goldthwait discounted it. Goldthwait diagrammed the direction of glacial movement in the accepted White Mountain cirques around the points of the compass, finding that they are distributed from northwest to southeast, on average pointing northeast. Consequently, prevailing winds usually blew from the opposite direction, the southwest (today it usually blows from the west or northwest), when these alpine glaciers formed. The wind pushed snow off the ridges into the ravines, where it accumulated until the pressure and cold transformed snow into ice, creating the alpine glaciers. Tuckerman Ravine’s enormous powder stash derives from the same circumstances. If Red Rock Gorge was glaciated, then how did it accumulate enough snow to create its glacier?

I propose two possible solutions: either the wind was more variable than Goldthwait believed, or perhaps snow was transported off the unnamed ridge to west into the Basin. These are both precarious hypotheses; there needed to be a great deal of snow consistently blowing into the ravine to cause glaciation. For example, Goldthwait speculates that the Valley Way on the north side of the northern Presidentials never developed into a cirque because nearby feeder ridges were too small to provide sufficient snow, whereas King’s Ravine, just to the west, gorged on the drifts blown off massive Durand Ridge and the large alpine expanse just south of Mt. Adams’ summit. The unnamed ridge may not have enough surface area or to have provided sufficient feeder material to Red Rock Basin.

Nonetheless, the physical evidence of the ravine—i.e. the U-shape rather than the V-shape typical of fluvial erosion—still supports glaciation. I like this final picture, taken by Alex two winters ago from Lafayette, because it contrasts the minimally eroded, clearly fluvial ravines on the north side of West Bond in the center of the photograph with the far grander Red Rock Basin on the far left. Perhaps significantly, all the major ravines on the west side of the Twins and Bonds - the Twin Brook valley, Red Rock Basin, and Hellbrook Ravin - are far more developed than their cousins on the east side of the Franconias.

1 comment:

Samuel said...

Really nice blog man!

http://samsglaciationblog.blogspot.com/