Bedrock Geology of the Boquet River Watershed

Background

Geologists have discovered that the oceans and continents are not fixed in place. Instead, the surface of the earth is broken up into large plate-like masses, and on top of some of these float the lighter rocks of the continental masses. These plates and their accompanying continents skitter around on the core of the earth like water on a hot griddle, occasionally bumping into each other. This concept is known as plate tectonics or "continental drift." The Champlain valley was right at the edge of one of these continental masses and the Boquet River watershed was at its east coast.

Every continent has a rocky core which floats on the deeper and darker rocks of the earth's mantle. Usually this core is covered by younger sedimentary rocks, but in North America this core is exposed in a few places. It is known as the Canadian Shield and is exposed in large areas of northern Canada and at an outlying exposure which forms the Adirondack Mountains.

You can link here to find out more about the geological time scale.

The Canadian Shield is the oldest rock on the North American continent. It is from the geological period known as the Proterozoic Era. Some of the rocks in the Adirondacks are igneous rocks that were squeezed and heated (metamorphism) to the point that they melted. Radioactive dating shows that this happened about 1.2 billion years ago. This is about the time when the first modern single celled organisms evolved. Interlaced with these metamorphic igneous rocks are metamorphic sedimentary rocks - these were not heated quite to the point that they completely lost their original form. Study of these metamorphic sedimentary rocks shows that they were derived from rocks formed about 2.7 billion years ago in mountains to the west. This is about the time when the very first living things evolved on earth. Ancient rocks indeed.

This core of rock that became the underlying rock of the Adirondacks formed about 1.1 to 1.2 billion years ago in what geologists call the "Grenville Orogeny" (orogeny means mountain building episode). The area that is now the Boquet watershed was at the east shore of the primitive North American continent and this continent was drifting toward another continent. As the continent drifted large mountains like the present day Andes in South America were pushed up on the leading edge. When our coast hit the approaching continent even larger mountains, comparable to today's Himalayan Mountains were thrown up. These were the Grenville Mountains and their rocky core is what forms the base rock of today's Adirondacks.

Today's rocks were buried at the bottom of the Grenville Mountains under more than 20 miles of overlying rock. During the process of metamorphosis they were subjected to pressures more than 7000-8000 times normal and temperatures of 750-800 degrees C. The pressure and temperature not only melted or almost melted the rock, it also deformed the rock. Movements deep under the mountain mass caused ductile deformations such as folding and ductile sheer. In many places in the Adirondacks you can see these swirling patterns in the rocks.

Over the next hundreds of millions of years nothing much happened in what was to become the Boquet watershed except that by about 660 million years ago these huge mountains were eroded down to a flat plain. But elsewhere other continents were colliding and sticking together until all of the continents had been joined together in one mass - a supercontinent called the Grenville supercontinent.

But supercontinents make the earth unbalanced and they are unstable. This one began to split apart. At about 660 million years ago the first break in the supercontinent began to occur - right here in the Champlain Valley. Rifts occurred and the continental masses began to move apart and an ocean filled in to our east. When this rifting and movement began the crust was thinner and cooler. Because the rock was more brittle it did not bend (ductile deformation) - instead it broke (brittle deformation). Much of the faulting in today's Adirondacks probably first occurred at this time, although it was added to later. Sediments eroded off the continent into this newly formed ocean from about 660 to 460 million years (the Cambrian and Ordovician geological ages, both of which are part of the Paleozoic Era).

Next, about 550 million years ago, an island arc formed in the middle of this new ocean and began to move toward us. It collided with us about 460 million years ago and produced another mountain chain comparable to the Himalayan Mountains to our east. This mountain building also reinitiated the faulting in our Adirondack rocks, of both the older Grenville rock and also the more recent Cambrian and Ordovician rocks. The rock from these mountains is the present day Taconic Mountains and forms the underlying rocks of western New England. These mountains eroded in turn, only to be replaced by another set of mountains (comparable to the Andes) further east when yet another continent drifted toward us. When it collided (between 410-350 million years ago it built a massive mountain plateau similar to the Tibetan Plateau in what is now eastern New England. And then this began to erode.

And finally the continent we now call Africa collided with all of this (about 330-250 million years ago) and produced yet another set of Himalayan type mountains. The rock of these forms the Appalachian Mountains we see today, though a great deal of erosion has occurred and only subsequent uplift has made them into mountains again.

Throughout all of these events since the Grenville supercontinent broke up the continental fragments have again been colliding with each other and sticking together. By 220 million years ago all of the land masses were again combined into a supercontinent called Pangea. As in the earlier case, the supercontinent was not stable and began to break up.

About 180 million years ago a rift formed near our present-day east coast, the continents began to move apart, and the Atlantic Ocean was formed. The rocks to the west of the rift became eastern New England; the rocks to the east of the rift became England and Ireland. The sea floor continued to spread on each side of the rift and the Atlantic Ocean widened. Once the rift became separated from the east coast of North America there was no longer any mountain building and the area became geologically inactive except for the continuing erosion of the mountainous regions.

As you can see the Boquet watershed has a complicated geological history. Supercontinents formed, split apart, reformed, and split apart again. The bottom line is that our rocks were formed early on, and then got buried by the sediments from the successive episodes of mountain building to our east. Most of these sediments, except for the older ones on the bottom were then removed by erosion after the Appalachian mountain building episode.

As we come closer to the present day the Boquet watershed was a flat region of sedimentary rocks (Cambrian and Ordovician ages) underlain by an older core of Canadian Shield (Proterozoic) metamorphic rock. Then, 20 million years ago (at the last minute geologically speaking), mass of subsurface rock called the Adirondack Dome began to rise. For some unknown reason the mass of heated rock began to rise, pushing our present day rocks upwards. In fact the Adirondacks may still be rising - about 2 to 3 mm per year, which is lightning speed in geological terms.

references:
Chernicoff, S. & R. Venkatakrishnan. 1995. Geology: An Introduction to Physical Geology. Worth Publishers, New York. 593pp.
Hunt, C.B. 1974. Natural regions of the United States and Canada. W.H. Freeman, San Francisco. 725pp.
Isachsen, Y.W. & D.W. Fisher. 1970. Geological Map of New York - Adirondack Sheet. New York State Museum.
Isachsen, Y.W. et al. 1991. Geology of New York - a simplified account. New York State Museum Educational Leaflet 28. 284pp.
Rogers, W.B. et al. 1990. Geological Highway Map. New York State Museum Educational Leaflet 33.

 


 
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