Bedrock Geology

Bedrock geology is the study of the rock masses beneath your feet. Where did those sedimentary rocks come from? How long ago did they form? How far underneath the earth did those molten igneous masses form and how did they get to the surface? These are all questions bedrock geology addresses. They are fascinating questions. But our interest in bedrock geology is more about how the bedrock influences the surface world we see today. On this part of our website we will explore how the bedrock influences the Boquet River: how it determines the course the river takes from the mountains down to Lake Champlain. The material below gives a brief description of how the Adirondack Mountains were formed and how this affects the drainage pattern of Adirondack rivers in general and of the Boquet River in more detail. Anyone who wishes more information about the geological origin of the rock underlying our region can link to our page of background information.

Perhaps the easiest way to understand the basic structure of the Adirondacks is to visualize an ancient chain of mountains known as the Grenville Mountains which arose about 1.2 billion years ago during the Proterozoic Era and were then worn down to a completely flat plain. Later, during the Paleozoic Era, this flat plain was covered with sediments.

Adirondack region before uplift

Very recently in geological history, only 20 million years ago, a mass of subsurface rock called the Adirondack Dome began to push up the overlying Grenville rock and the younger sediments. As the younger softer sediments eroded away the harder core of old Grenville rock was exposed. This core kept on rising and formed a central mountain mass overlooking the surrounding lowlands.

Uplift of the Adirondack Mountains

 

Map of Bedrock in the Boquet River watershed

A geological map of the Adirondacks shows this older core of metamorphic rocks in the center. Surrounding this on all sides are rings of successively younger sedimentary rocks. In the Boquet River watershed we see these younger sedimentary rocks on the eastern side.

One consequence of the high central core of rock for all of the rivers of the Adirondacks is that it promotes a radial drainage pattern. All of the rivers tend to radiate out from the central highlands like the spokes from a wheel. The diagram below confirms this general pattern:

Map of Radial Drainage in the Adirondack Mountains

As you can see, the Schroon and Hudson Rivers flow to the south; the Raquette and St. Regis flow to the west; the Salmon and Chateaugay flow to the north; and the Chazy, Saranac, Ausable, and Boquet flow to the northeast. All tend to flow away from the central highlands.

 

Map of Faults in the Boquet River watershed

However, the pattern is not perfect. The broken rocks of the Adirondacks, affected by two episodes of faulting which produced long cracks in the bedrock, distort the radial drainage pattern. In our area most of these deformations are long straight faults lines running in a north or northeast direction.

In some cases valleys were formed by parallel faults on each side. The central block then dropped down forming what is known as a graben valley. One example of this is Pleasant Valley above Elizabethtown; another example is the lower portion of the North Branch and the combined North Branch/Main Stem down to the falls in Willsboro.

Other valleys are formed along single faults where the rock has been weakened and is more easily eroded. One example is the valley between Hurricane and Giant Mountains which contains the tributary known as "the Branch."

As you can see in the diagram above, many sections of the Boquet River and its tributaries parallel these fault lines.

Sometimes, in areas known as fracture zones, the rock is extensively broken up and easily eroded. A good example of a fracture zone can be seen in the shattered rock at Split Rock Falls south of Elizabethtown.

Photograph of fractured rock at Split Rock Falls

 

Let's look at the Boquet River using this information. The map to the right integrates the information on faulting and bedrock of the Boquet River Watershed.

If you hold your mouse over the map to the right it will display numbers to help you with locations. Moving the mouse away will make the numbers disappear.

The Main Stem of the Boquet arises (1) on Dix Mountain at the southern end of the watershed. But rather than flowing in an easterly direction, when it crosses Route 73 the river is captured and funneled to the northeast by the two faults which form a graben valley (2). Farther downstream, around New Russia (3), the graben valley ends but one of these faults continues until it reaches Elizabethtown (4). On the eastern side of the fault the ground tilts down toward the fault and so forms a barrier which funnels the water to the northeast.

At Elizabethtown the Main Stem is joined by the Branch (5), which has also eroded a valley in a fault running between Hurricane and Giant Mountains. After Elizabethtown the combined Main Stem/Branch continues to flow northeast along a fault line, again diverted to the northeast by the higer ground on the eastern side of the fault. Then, about where Steele Woods Roads meets County Road #10 (Lewis-Wadhams Road), the river breaks out (6) of this fault and begins to flow east toward Lake Champlain.

At the Falls in Wadhams (7) the river is backed up by a dam of igneous rock associated with another fault. Behind this dam the Boquet is a slow moving and meandering river. After the river flows over this dam it picks up speed again, but rather than continuing east the river is again diverted to the northeast, this time by a combination of faults and the younger Paleozoic sedimentary rocks which are tilted up into a low ridge to the east. In several places, such as the ledges at Whallonsburg and Boquet (8), the river flows over the edge of blocks of these sediments. The river continues to the northeast, constrained by this ridge of younger sediments until it reaches the confluence with the North Branch (9).

The upper portion of the North Branch (10) is almost an entirely different river. The stream flows to the east, away from the central highlands. In this region the eastern side of the faults are lower, so the river crosses over the faults rather than runs parallel to them. This changes in the lower part of the North Branch. At Reber Road in Willsboro the stream enters a long graben valley (11). It flows down this until it meets the Main Stem, then the combined rivers continue to follow this graben valley all the way down to Willsboro.

At Willsboro Falls (12) the river flows over a drop formed by another fault, then turns east (13) toward Lake Champlain, funneled by this same fault until it reaches the lake.

As you can see the bedrock plays an important role in determining the route the river takes. Glaciation and erosion were ultimately responsible for forming the landscape we see today, but the bedrock - weak in some areas, resistant in others - determines where those forces can act.
 


 
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