Many of us are captivated by the Adirondacks and the reasons for this likely span the continuum of human interests. My own appreciation for the region comes from the study of geology which began in Brown Hall on the St. Lawrence campus. My professors, Drs. Bill Elberty and Bill Romey, enthusiastically introduced me to the geologic mysteries and wonders of the area around campus. While the flat-lying Paleozoic rocks of the St. Lawrence River Valley lie north of campus, most of the area in other cardinal directions is underlain by Precambrian bedrock of the Grenville Province. Earlier in my career I helped lead field trips for the International Geological Congress in 1991 and 1992 with over fifty, mostly international participants, attending both. Below I hope to explain why the area has always held so much interest for me and those in my profession.
The Grenville Province is part of the Canadian Shield, the ancient, continent-scale core of North America. Rocks of the Grenville Province occur mostly in southern Quebec and eastern Ontario but are also found as inliers along the spine of the Appalachian Mountains and beyond. The Adirondacks are nearly encircled by much younger rocks like the Potsdam sandstone but connect at the surface, through the pink granite islands of the Thousand Islands Region to the rest of Grenville. Rocks of the Grenville Province share a common history of continental collision and welding to North America in a series of events that culminated in the assembly of the supercontinent of Rodinia, a precursor to Panagea, about a billion years ago. This resulted in a mountain belt whose proportions rivaled any other before or since.
While not the oldest part of North America by far, the Adirondacks contain rocks as old as ~1.35 billion years ago (bya). By 1.3 bya much of the area was covered by a shallow sea or seas. In this sea, limey sediment accumulated in warm, clear tropical waters. With time and burial, the sediment cemented into solid rock known as limestone. About 1.25 bya a series of collisional events along the coast of North America, transformed the limestone into marble: the distinctive white and grey striped rocks often observed when traveling south along Rt. 11. The grey layers are often composed of graphite, a reliable indicator of the past presence of organic matter long since turned into the platy mineral form of carbon.
Consider the travel log of these particular rocks. From their origin in the shallow oceans through a series of tectonic or mountain building events, they were buried to depths equal to the base of the crust where conditions exceeded 650oC/1200oF and unimaginable pressure. Under these conditions, many rocks begin to melt and deform, much like silly putty. Some produce copious amounts of magma as they start to melt. In the Earth marbles undergo ductile deformation, folding of their layers, the growth of new minerals in equilibrium with current conditions and recrystallization of their textures. In order for us to see them in clean blasted outcrops along the highway, they also had to make the return trip to the surface. The main point is that not only can these rocks tell us about the conditions of their origin as sedimentary material, but they can also tell us what conditions they experienced deep in the crust.
One of the longstanding mysteries of the Grenville Province is the large bodies of exceptional coarse-grained, plagioclase feldspar-dominated igneous rock, known as massif anorthosite. With few exceptions, massif anorthosite is almost entirely restricted to the Grenville Province and within a relatively narrow time window between 1.4-1.0 bya worldwide. Massif anorthosite makes up the High Peaks region of the Adirondacks, where it forms a moderate-sized body intruded 1.15 bya. The high intrusion temperature of massif anorthosite (1100oC/2000oF) indicates that it originated from the mantle and when it intruded the crust, it melted its way upwards producing the vast volume of granitic rocks found enveloping it. This intrusion event was synchronous with the close of Shawinigan Orogeny (ca. 1.2-1.15 bya) and may have been triggered by the breaking off and catastrophic sinking of a cold, subducted slab of oceanic crust balanced by the corresponding ascent of hot mantle in its place.
In a final Himalayan-type collision, Amazonia and North America collided about 1.07 bya. Eventually, the uplifting forces associated with convergence began to wane and gravitational collapse and erosion of the mountain belt led to shedding vast volumes of sediment. Some of this sedimentary material made it to the very edge of northwestern North American and forms part of the sedimentary rocks on Victoria Island, in the Canadian Arctic. As erosion slowly continued, the crust gradually returned to its original thickness, and the rocks of the region, once at the base of the crust, were uplifted and eroded to sea level. By about 0.5 bya the Adirondack region was again inundated by a shallow sea and the Precambrian basement rock covered by a thin veneer of the Potsdam sandstone and overlying limestones. These sedimentary rocks were laid down along the coast of the predecessor to the Atlantic Ocean (the Iapetus Ocean) and in a narrow, shallow, interior seaway known as the Appalachian Basin. Eventually, these newly deposited sedimentary rocks, especially those to the east, would experience deformation and metamorphism as the Appalachian Mountains achieved their final form during collision with Europe and Africa.
For many mountain belts, their ultimate fate is erosion to sea level. However, something strange and unusual began beneath the Adirondack Region about 180 million years ago. As the Atlantic Ocean began to open (rifting Europe and NW Africa away from North America) the Adirondacks began to rise. As they rose, the overlying sedimentary rocks where stripped away except in fault-controlled valleys that now contain many of the long, linear Adirondack lakes, exposing the very roots of the highly metamorphosed Grenville Mountains. This rebirth has led to mountains that now exceed 1665m/mile in height. Many questions, beyond this short summary, remain about whether the region is still rising, and if so, what is its rate of uplift and ultimate cause?
So why do geologists have a special appreciation for the Adirondacks? It stems from wonder and amazement. We can see things under foot and along a cliff face or road cut that were once at the base of the crust. We can study what is happening under Mt. Everest without the frustration and cost of drilling deeper than man has ever drilled before. The Adirondacks have mineral deposits and rocks that are not found in many other places and often at scales not found elsewhere. The mountains expose rocks that have made that trip from the surface to the base of the crust and back, and they are in our backyard here at St. Lawrence! And, as a friend and mentor, Dr. Yngvar Isachsen, used to say, “The Adirondack Mountains were born, died and have been resurrected.” I am extremely grateful to have experienced and studied them through the lens of geology, but with a sense of wonder at their complexity, age and beauty.