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With the intention of completing this discussion about the felsenmeer I want to reinsert the timetable projected by Richard Goldthwait (from The Geology of New Hampshire, 1951) and what he felt is a realistic chronology for the arrival of the last Wisconsian ice sheet and it's ablatement:
180,000 years ago and again more than 50,000 years ago, short mountain glaciers slowly scoured out the deep cirques and valley of the higher White Mountains;
Between 40,000 and 60,000 years ago a great ice sheet gathered in northeast Quebec and grew southward overwhelming mountain glaciers in New Hampshire;
More than 20,000 years ago this ice sheet began to melt and waste away (ablate);
Less than 14,000 years ago the last significant ice mass seems to have completely diappeared from New Hampshire.
This timeline was developed by three generations of Goldthwaits in collaboration with Ernst Antevs, Marvin Billings and Katherine Fowler-Billings, Randolph and Carleton Chapman, D.W Johnson, Armin Loebeck, and many others. It evolved from the best scientific practice during a period stretching more than 50 years up to the early 1960s. The timeline and conclusions drawn by this formidable group of geologists relative to the glacial history of the White Mountains have been challenged by a few scientist, most notably Will F. Thompson, a physical geographer, in 1960, but the challengers didn't produce an alternative chronology so I feel that the timeline developed by Goldthwait, et. al.,is accurate for it's time (the 1960s). It's only a matter of time before we have more accurate measurements some of which are becoming available as I write. In a personal conversation on 11-07-09 with Thom Davis of Bentley College (Boston), a geologist, he observed that cosmogenic measurements taken during the past few years focusing on the age of the felsenmeer on the summit of Mt. Washington point to the accuracy of 14,000 years as the time the continental ice sheet had fully ablated, but he said that there are variations, too, and that they were working out some of the bugs in their measurements. This underscores the difficulty in obtaining an accurate number even with the superior technology. I personally trust the number of 14000 years BP to designate the time that the last Wisconsian ice sheet had completely melted back from the White Mountains.
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The image, then, of the continental ice sheet between say 18,000 to 14,000 years ago is important in the discussion of the felsenmeer. I've had to correct my perspective several times as I've read through all the literature. In The Geology of New Hampshire (1951) Richard Goldthwait created a reasonably accurate picture of what the ice was doing and what it looked like during those 4000 years that it was melting and evaporating free of the White Mountains. The correct image is of the ice shrinking in place. It did not recede or retreat back to Canada (one image I had). It merely stopped, or, perhaps, slowed down, and melted in place. There were probably times, a thousand years, when it advanced a bit, but like the Greenland ice cap today, it was hemorrhaging most of those years. It was melting from the bottom up and from the surface down while billions and billions of gallons of water poured off the ice in turbulent cascade to form huge lakes and rivers as it rushed to the ocean.
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The melting caused the glacier to thin and as it did the high peaks of the White Mountains emerged as islands. Goldthwait wrote, "The highest peaks-Mt. Washington and Mt. Adams-protruded through the ice first as it grew thin and looked much as they do today when they rise above a sea of clouds." (1940, pg. 28)
For thousands of years, then, the higher peaks were Nunataks (from the Inuit: Nunataq), islands protruding above the ice with their summits exposed to the elements while their flanks and the valleys between them were still encased in glacial ice.
For thousands of years, then, the higher peaks were Nunataks (from the Inuit: Nunataq), islands protruding above the ice with their summits exposed to the elements while their flanks and the valleys between them were still encased in glacial ice.
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Looking at Mt. Madison with this dusting of snow it's possible to see where the bedrock protrudes along the ridge to the very summit and the felsenmeer appears to have fractured from that spine and fallen down both sides of the mountain from the ridge.
If you picture Mt. Madison with a vast sea of ice coming to within a few feet of the summit and you could sit on J.Q. Adams for a thousand years while the ice melted you might have the illusion that it's the summit that's rising when it is actually the ice surface that's decreasing in altitude and exposing more and more of the land mass. Imagine being able to run from the summit of "J.Q." straight across to the summit of Madison, or Carter Dome, on the ice! Think how easy it would be to do the '4,000 footers', just wait until the ice melted down to 3,999 feet and then do them all in a couple of days on skis. No more sweaty, tiresome hiking up mountain after mountain wondering what possessed you to do something so pointless!
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Goldthwait observed that the continental ice sheet melted faster on the south and east sides of the White Mountains, again due to a generally warmer macro/micro-climate, which might be a significant factor in the low concentration of felsenmeer on the east-facing flanks of the Presidentials (again, not completely as on Mt. Washington's east slope).
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Another limiting factor in the way 'timberline' evolves is soil. As Monihan and others have pointed out the plants making up the highest communities in the White Mountains, mainly black spruce, alpine birch and balsam fir, reproduce by "layering". Black spruce and balsam fir rarely produce seed cones above 4800-5000 feet. Layering is the predominant form of propogation and utilizing root-forming (meristematic, or root apical meristematic) tissue in the plants' lower branches, those closest to the available soil, and eventually these branches grow as separate plants. Since reproduction is absolutely necessary for these alpine plants that depend so much on each other in creating a niche the soil itself becomes a limiting factor.
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Goldthwait (1940) wrote of the felsenmeer: "The blocks form a distinct blanket over till (soil left by the melting glacier) as on the summit of Mt. Washington. Where not to numerous the blocks are arranged in a pattern simulating nets or stripes or horseshoe-shaped lobes and enclosing areas of open fine soil matted with grass. These soil cells are common on Boott Spur, Bigelow Lawn, and parts of each peak above 5000 feet altitude. Steeper slopes like those on the west and northwest sides of Mt. Washington, Mt. Jefferson, and Mt. Adams have horseshoe-shaped banks of blocks 20 to 60 feet long and 15 to 50 feet broad. Each lobe encloses a high grassy center inclined at 10 degrees to 20 degrees (16 degree average).
"Two kinds of motion are evident in such soil cells. The stretching of the block nets into block stripes on slopes which increase downwards from 3 degrees to 5 degrees and viscous shape of the block lobes clearly showdownhill creep of masses of soil by soliluction. The segregation of the big blocks into nets, stripes, and walls leaving open grassy coneters ia distinct but little understood sorting process. This sorting does not involve a deep circulation of blocks upward from within the soil because several excavations have revealed the very shallow nature of these forms. The blocks did not move far up out of the till in any sort of up-freezing process. However, the sorting does involve an actual expansion and outward push from each soil center or soil stripe towards the blocks because the blocks tend to be pushed together like dominoes on edge. This is undoubtably due to the fact that the fine soil retains more water than the loose blocks. When this freezes and expands it exerts a horizontal push on the blocks" ((Goldthwait, 1940; pgs 35-37).
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Goldthwait, in his 1939 paper, asks: "When was the frost so active? One can only speculate how long features like upended long blocks might remain in place and how long patterns of blocks might remain in place and how long blocks might retain their definity. The best guess is that the preservation of soil cells has not lasted longer than the 10,000 years and more since last remnants of the ice sheet lingered in northeast Quebec. It should be noted also, that all side of the blocks have been pitted by the chemical action which lichens promote and etched by the driving snow and frost. For a long time these blocks were toppled and turned end over end to become uniformly etched." (Goldthwait, 1940; pg. 30)
Ernest Antev, a colleague of James Goldthwait, Richard Goldthwait's father, referred to the felsenmeer as "archaic" and "prehistoric".
In a summary written for the 1951 publication of "The Geology of New Hampshire, Part I Surficial Geology" Goldthwait writes: "During the uncovering of summits of the White Mountains by the melting ice sheet, intense frost action split ledges into angular blocks, moved them about, and redistributed the till and boulders. Millions of these frost-cracked blocks make the cone of Mt. Washington look like a great rock pile even though solid ledge lies only a few feet down. For a considerable time while only the very surface of the mountain was thawing and being loosened from the frozen ground under it, all the high-level blocks and juicy dirt masses were slowly moving downhill.
"The most striking result of this prehistoric frost action is the gathering of rough stones into belts which form patterns like a net over the flatter surfaces. Apparently the stones and blocks were forced outward from dirt centers. Upon freezing, the whole soil mass expands outward from the clay-rich centers which hold more water, and as it thaws, some of the fines (smallest soil particles) seem to fill spaces occurpied formerly by rocks thus crwding them outward. As adjacent soggy soil centers expand reatedl, stone and blocks from opposite directions are squeezed together to form boulder-filled troughs. The slabby pieces are pushed together so tightly that many stand up on end like tombstones." (Goldthwait 1951, pgs 51-52)
In a 1968 paper presented at Dartmouth College Goldthwait made minor changes to this nut-shell description of the post glacial geology of the Presidential Range. The thrust of that paper was to respond to the question of whether the small local alpine glaciers returned to the northern Presidential peaks in the period of time following the melting of the ice sheet.
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"These large soil cells or groups of blocks are not moving today. To make sure of this, two hundred blocks, in all sort of position were painted and accurately located by measurements with steel tape from solid ledges. After a year these were remeasured and none of them had moved beond the small limit of error of the measurements. Even long pointed blocks tipped on end and small blocks balanced on top of larger ones stayed in place." (Goldthwait, 1940; pg. 30)
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The ferocity of the frost that must have occurred after the ice sheet honed the mountains down to a smooth ledge of bed rock and then melted away must have been extreme. It would have had to occur after the ice sheet had melted considerably. One theory speculated that the frost action occurred during Younger Dryas (YD) event that featured several hundred years of extremely cold weather. Part of the theory is that the YD occurred because of a Deglaciation Climate Reversal (DCR) that occurred about 14,000 years ago that was triggered by massive amounts of fresh water re-entering the oceans during the melting of the Wisconsian ice sheet. Mean temperatures may have dropped quickly by several digits, as much as 15 degrees (C) based on ice core samples in Greenland. The YD certainly could have caused the magnitude of frost necessary and for a great length of time and it would also have increased solifluction markedly so it may be the event that formed the felsenmeer. (One way to look at this is that in a round about way the Wisconsian ice sheet caused the frost, which caused the mantel to fracture and produce the felsenmeer.)
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If we paint a picture, then, based on Goldthwait's description of the final years of the glacial ice sheet the landscape includes the high summits poking up out of the ice sheets resembling islands. These islands, or Nunataks, had somewhat smooth surfaces of polished bedrock which in the White Mountains would be granite in most places and schist across the Presidential Range. The bedrock is covered with detritus left by the ice sheet including tons of glacial "till" that included sand and gravel, and thousands of glacial "erratics" dumped out of the melting ice. Erratics (also part of the till) are represented by pebbles, stones and, in some cases, boulders weighing thousands of tons. Some erratics are round and smooth like river stones, some are rougher because they were lodged higher up in the ice or didn't travel as far as others. Some traveled in the ice a few hundred yards and some for a dozen or so miles from their points of origin. Goldthwait had a small collection of erratics he and his graduate students found across the Presidentials that are now at the Mt. Washington Observator Museum. One, my favorite, is a pink ball of Scrag granite that weighs 3 or 4 pounds and that came from Jefferson, a few miles away.
Then, even before the ice sheet is completely melted, an event occurs causing frost of a magnitude that has not occurred since. The frost causes fracturing (or quarrying of the solid bedrock surfaces everywhere that the mountain's summits are protruding up through the ice sheet. The frost also causes intense, on-going solifluction in the nascent soils that have been evolving from the till and from weathering of the bedrock.
For a period of years, perhaps a thousand years or more, the upper surface of the mountains, the mantel, is going through a profound change and a new landscape is evolving as the smooth rock breaks into large and small blocks that either stay in place where they were or they move a few inches, or yards, or roll down the mountain some distance.
I'm still impressed with the enormous mass of the blocks and have questions not answered by Goldthwait's summary. As the summits began to appear above the melting ice, forming Nunataks, were they higher then they are today by 25 or 30 feet? Has all the quarrying by the frost lowered the altitude of some of the highest ridges? If you could put the blocks back where they came from (reverse the film so to speak) would they just form a mantel of surface bedrock 3 or 4 feet thick? Or, would they add height to the existing summits?
When I look at the above this photo of Madison (taken in 1967) it's possible, because of the angle of sunlight, to see the folds of Littleton Schist arching up towards the south. The blocks of schist emanating from those exposed folds is all about. Some of it sits close to the bedrock and some has spilled downwards towards the hut. The summit is an outcropping of one of the folds and in this photo, the next photo up of Madison in shadow and the picture taken on this trip, 11-07-09, may present evidence that the summit, after the ice shrank away from the upper part of the mountain, may have lost some of it's height due to the quarrying along the summit ridge and it might explain how the existing felsenmeer, on both Madison, Adams and Jefferson, moved down the mountain and also how the blocks tumbled into their present placement stacked up on each other as many of them are.
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Another example of this idea of "rock glaciers" is on the southwest side of Carter Dome where broken rock has created a glacier-like stream of rock that extends down into Carter Notch. The terminus of this stream is often referred to as the 'ramparts' and it is laced with caves that sometimes have ice in them all summer. On the Wildcat Mountain side of Carter Notch there are also caves that the Carter Hut croos used to store perishable food during the summer. Some of the caves are quite deep and extensive.
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This is pretty much the extent of what we know about felsenmeer:
It's derived from the local rock. (glacial erratics of various pedigrees, shapes and sizes are mixed in with the felsenmeer. The erratics are relatively local.)
The vast areas of lichen encrusted felsenmeer that we see on the northern peaks of the Presidentials is made up of millions and millions of separate boulders that were once the top 3 or 4 feet of original bedrock which in this case was predominantly Littelton Schist.
Schist, a metamorphic rock, is more likely to fracture into blocks and form felsenmeer than igneous rocks like granite.
The felsenmeer is probably more than 10,000 years old. The rock mantel, meaning the uppermost part of the bedrock that makes up the mass of the Presidentials, that broke up to form the felsenmeer had been smoothed by the Wisconsian glacial ice sheet, or continental glacier, which completely covered the White Mountains for 40,000 years.
The felsenmeer was formed as the mantel, or surface of the bedrock, was subjected to intense frosts over a long period of time, maybe as long as 1,000 years, that began when the glacial ice was melting downwards from the higher peaks. The frost caused "quarrying" (cutting or splitting:breakage) of the surface rock. The cold temperatures may have been associated with a the Young Dryas event beginning about 14000 years ago which, according to theory, was a climatic change sometimes referred to as "the little ice age."
The energy (work) involved in the breaking up of the bedrock came from a consistently cold, damp climate associated with a widespread climate change that caused intense frost to form in higher elevations of the white mountains. The frost, through on-going freeze-thaw cycles caused existing cracks in the bedrock slabs to expand until the slabs broke into smaller blocks.
The felsenmeer formed, generally, only above 5300 feet in elevation in the White Mountains (according to R. Goldthwait) . It's possible that the Wisconsinan ice sheet had not down wasted below the 5000 foot level at the time, or because of the intense cold was not down wasting at all.
The activity associated with the formation of the felsenmeer on the Presidential Range has been dormant for thousands of years. While there is inevitably some frost splitting of rock occurring throughout the White Mountains during particularly cold winters there is relatively little movement in the felsenmeer at this time.
"Mass Wasting" is a term used to describe the inexorable process by which all mountains eventually comply with gravity and wash, wear, erode, and/or fall down. This includes large dramatic events like huge landslides and small, barely noticeable events like a small bit of rock breaking off an outcropping and tumbling down a few yards into some bushes. It's like a clock ticking. It usuallys takes hundreds and hundred of millions of years for a mountain to be "born" (called orogenesis) and then mass waste back to a level plain again. The felsenmeer represents a stage in the mass wasting of the White Mountains and their transformation to smaller and smaller particiles eventually becoming small enough to wash away.
My last thoughts revolve around the part the felsenmeer plays in the natural history of the White Mountains. The story of the felsenmeer is a signifcant chapter in the geologic history of the mountains and it still has a signficant presence, e.g. it represents a dramatic change in the landscape. Continuing research will shed more light on how old the quarried blocks are and may provide valuable information about the climate that caused the transformation and, in turn, give us clues about the period of time immediately following the down wasting of the ice sheet.
For the moment the felsenmeer represents a very unique environment occurring near the ground and all across the range. It both hinders and supports attempts by vegetation to colonize the alpine areas, and it changes the way moisture moves on the ground and changes in the atmosphere close to the ground. It plays a significant role in the growth and long term stability of the krummholz which in turn plays a role in soil development and stability, moisture retention, shelter for birds, mammals, insects, etc. So there's quite a bit to think about.
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1 comment:
I am always completely mesmerized by your posts and pictures. Thanks!
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