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In All Directions, Many Stones

Jeff Meyers, LCMM

 

As soon as we had reached the fall [we] went on shore to see whether we could pass this place; but we went some league and a half without seeing any prospect of being able to do so, finding only water running with great swiftness, and in all directions many stones, very dangerous, and with but little water about them...Having returned, and seeing the slight prospect there was of passing the fall with our shallop, I was much troubled. And it gave me especial dissatisfaction to go back without seeing a very large lake, filled with handsome islands, and with large tracts of fine land bordering on the lake…After duly thinking over the matter, I determined to go and fulfill my promise, and carry out my desire. Accordingly, I embarked with the savages in their canoes…

--Voyages of Samuel de Champlain, 1604-1618 Grant (ed.), page 155.

Bedrock is the solid rock that underlies soil and other loose material on the landscape, providing the physical foundation for landforms and for the natural and cultural events that make up what we know as "history". In our minds, it forms the solid framework, the "terra firma" that seems to have always been here, unchanging and immovable.

In the summer of 1609, Samuel de Champlain accompanied Algonquin and Abenaki allies to enter the "River of the Iroquois" (today's Richelieu River) in a large rowing and sailing boat known as a "shallop". He was excited to explore the waters now known as Lake Champlain. When the party reached a stretch of impassable whitewater, however, Champlain stared in consternation at the bedrock and boulders that blocked his way. There was no possibility of his thirty-foot boat being able to pass through the "water running with great swiftness...in all directions many stones, very dangerous, and with but little water about them".

Champlain continued with his native allies, leaving the shallop behind in favor of one of the twenty-four swift and portable birchbark canoes on the voyage. While the rocky falls had nearly stopped him, Champlain traveled on with satisfaction. Perhaps the time staring at the stones and boulders gave him greater reason to wonder about the many rocks and rock forms that were frequently in his view for the remainder of his trip.

For centuries, geologists have examined the rocks that make up the land we move upon. With discovery after discovery they have come to realize that although the rocks around us may seem silent and immobile, in fact every crystal and layer of sediment echoes with a story of dramatic change. Bedrock tells a tale of time made up of moving oceans and shifting mountains, rivers, volcanoes, earthquakes, magma and ice. Landforms and the rocks and sediments that compose them are in a state of constant change.

Evidence of Ancient Seas

Champlain paddled by, peered at, or walked upon the evidence of geologic change every where he went in the New World. On Lake Champlain, the fleet of birch bark canoes passed by "four fine islands ten, twelve, and fifteen leagues long" and for mile after mile, his eyes could not have missed seeing the gray, layered rocks of low shoreline cliffs topped with northern white cedars. Countless chunks of this rock had broken off to form rocky beaches where lake waves lapped along the island shores. The same rocks covered the bottom of the lake into which he looked.

The bedrock seen along Isle la Motte, Cumberland Head, North and South Hero Islands, and Valcour Island formed about 500 million years ago when sediments deposited in an early precursor of our modern Atlantic Ocean that was forming a gap between the North American and Eurasian continents. (This ancient sea should be distinguished from the much more recent "Champlain Sea" that existed after the recession of the Pleistocene glaciers.) Shelf-like rock layers formed over millions of years as the sediments compacted and solidified into sedimentary rock formations. During this period the Vermont region was much closer to the equator and tropical latitudes. The region had a hot and humid climate and the sea was filled tropical marine life. The limestones and dolomites of the Champlain lowlands are formed largely by the calcareous hard parts of the marine species that lived during this period.

If Samuel de Champlain had climbed up one of the limestone ledges of Isle la Motte, he would have seen fossil coral reef structures chock-full of fossil organisms. Along the route of his lake travels he may have picked up fossil cephalopods (extinct relatives of squid), trilobites (extinct relatives of the horseshoe crab), brachiopods (an extinct phylum of shelled creatures) among others. Further down the lake, if he had stretched his legs at present-day Willsborough Point, he would have observed large fossil ripples in the rock – ripples with an asymmetry from trough to crest that indicate a strong local tidal current had once flowed there hundreds of millions of years ago.

Shifting Continental Plates

Why does so much of Lake Champlain bedrock contain fossils of coral and other marine life when the nearest such life today is thousands of miles away? It was not until the 1960s that geologists began to agree that the earth's crust is not an unbroken sphere, but is actually composed of gargantuan, solid "plates" that "float" and drift in a semi-solid region below the earth's surface. The drifting of these plates offers a convincing explanation for the puzzle: the continental plates are in constant movement. As they slide by, pull apart from, or smash into one another, tremendous forces are generated. Plate colliding with plate can lead to crustal folding that builds mountains. Plate sliding by plate can result in earthquakes; plate separating from plate can cause volcanoes or block faulting. The geologic history of the Champlain Valley is characterized by a repeating sequence of plate collision (mountain building events, also called "orogenies") alternating with plate separation (down-dropping blocks and rifting of crust leading to valleys and new ocean basins).

Mountains Old and New

As he progressed down the lake, Champlain noticed in the distance "some very high mountains on the eastern side, on the top of which there was snow". Since this was the beginning of July, many historians believe that it was probably rock—not snow—that he observed high on the top of what is today known as the Green Mountains. Champlain could not have imagined that there were once much loftier mountains here. Over one billion years ago a major mountain building event, the "Grenville Orogeny", raised an extensive range of very tall mountains that once covered the areas of western Vermont and the Adirondack region.

Over a great deal of time, these mountains eroded away to nothing, leaving behind only their underlying "basement" rock. Before the next orogeny, crustal separation led to rifting and the formation of the "Iapetus Ocean" where the limestones described earlier were deposited.

About 440 million years ago, another plate collision event—the Taconic orogeny—first raised the mountains we know as the Green Mountains. This was a period of unbelievable landscape compression, and the bedrock at the mountain zones folded and folded again, crumpling and pushing bedrock both up in elevation and down into the depths of the earth. Another mountain-building event 100 million years later (the Acadian orogeny) added to and compounded the folding, raising the mountains again. The rock strata to the west of the growing mountains were affected as well. Layers of bedrock broke as a result of pressure in many places forming "thrust faults". In some locations, older layers sheared off and rode up over younger layers of bedrock. Thrust faults occur in many Champlain Valley locations and account for a chain of small but prominent mountains along the Vermont shoreline of the lake (Cobble, Arrowhead, Pease, Philo, and Snake Mountains) as well as major waterfalls at the Lamoille, Winooski, Otter Creek, and other tributaries.

Champlain also saw to the south "other mountains no less high than the first, but without any snow." These were the Adirondack Mountains, a circle of geologically very young mountains, but made of the same billion plus-year old rocks as ancient as the basement rock of the Green Mountains and the Canadian shield. The Adirondack rock originally formed at great depth from molton rock that cooled very slowly. Igneous in origin, these rocks were covered by many kilometers of overlying mountains and were therefore exposed to extreme pressure and heat that reconstituted their original mineral arrangements to form new mineral assemblages. Adirondack rock is therefore metamorphic rock. The ancient mountains overlying Adirondack rocks gradually eroded down to nothing, and they were then covered by the same sedimentary layers that surround the Adirondacks and which compose most or all of the Champlain Valley.

In relatively recent geologic time, the Adirondack region uplifted to form a mountainous dome. Erosion removed the sedimentary layers from the region and eventually created a sort of "window" through the sedimentary rocks that allows us to observe the older basement rocks below. In 1609, the sculpted peaks of the Adirondack dome showed their distinctive profiles to those looking up to their summits from the waters of the lake. We now know them as Whiteface, Giant, the Dix range, and about forty other peaks that exceed 4,000 feet in elevation. Interestingly enough, these peaks are actually higher now than when Champlain observed them, for the Adirondacks are still slowly rising at a rate of about 3 millimeters per year. That's 12cm!

Block Faulting

The party of canoes eventually paddled over the very deepest section of Lake Champlain where the depth surpasses 400 feet. Here it is actually nearly 700 feet deep to bedrock, as the lake is filled with huge volumes of sediment. They then passed in close proximity to a hunk of Adirondack anorthosite as they paddled by Split Rock Point on the lake's western shore. This "island" of rock, however, is exposed at the surface not because of the uplifting of the Adirondack dome, but as the result of block faulting—the result of one of several periods of crustal stretching or rifting. This type of faulting is the result of major tensional stresses, which stretch and pull apart the earth's crust. As the bedrock pulls apart, sections drop down while others raise up. Split Rock Point is an upfaulted block of anorthosite and metagabbro that stands not far from the downthrown, younger limestone of Whallon Bay. Successive block faulting associated with the stretching of continents and the opening of the Atlantic Ocean about 200 million years ago helped to form the canyons of Lake Champlain. A few miles south of Split Rock Point is a cliff known as the "Palisades", also the result of block faulting. The party of canoes must have passed close by this impressive cliff that rises 150' above the water and sinks about the same distance below.

Erosion

Today, the main lodge of the Basin Harbor Club stares across the lake at the Palisades. This building is exposed to strong winds and harsh winter weather of Lake Champlain. Left alone, without constant painting and maintenance, this and any other lakeside buildings would quickly decompose into a pile of rubble in a mere hundred years or so. Although it takes a much longer time, solid bedrock is also subject to weathering and eventually disintegrates into fragments. Weathering of bedrock occurs by both mechanical and chemical means. The action of water and ice exploits rock joints to expand and fracture rock sheets in a sort of natural quarrying process. For 10,000 years prior to Champlain's arrival, Native Americans in this region had been picking up certain fragments of rock—mainly quartzite and chert—to fashion them into projectile points and other tools with which to hunt, fish, work hides, and perform other tasks necessary to survival.

We see the results of weathering everywhere we look. Rock bodies are broken apart and modified into smaller and smaller spherical shapes. Wedging, chemical dissolution, and other actions form a blanket of loose, decayed rock debris known as regolith, a widespread, discontinuous cover over solid bedrock. In Vermont, there is not much regolith as recent glaciers scraped the landscape down to bedrock leaving instead a layer of glacial till over the bedrock. Over this layer of till small particles of rocks and minerals, plus amounts of decomposed organic matter have created soil. The type and thickness of soil depend on the underlying rock material, topography, climate, and the passage of time.

Champlain noticed "many rivers falling into the lake," his route probably passing within proximity or view of the Saranac, Ausable, Boquet, Winooski Rivers, Otter Creek and other major tributaries. Running water is by far the most significant agent of erosion. Rivers collect water precipitated on the surface and funnel it back to the ocean. As water flows, it picks up weathered rock debris. Champlain may have noticed the difference in water clarity between the deep, clear waters of the Lake and the murky brown waters at the mouth of each tributary where sediments—eroded from the surrounding landscape—entered the lake. Water moves rocks large and small, enabling the aspirations of explorers and continuously modifying the surface of continents.

Bibliography

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