Geologic History of Fordham Gneiss Rock Formations at Armstrong Preserve in Pound Ridge, New York

In this latest installment of the Pound Ridge Land Conservancy (PRLC) blog, I invite you to explore the vast history of time written in the rocks of Pound Ridge with a focus on the Armstrong Preserve, located in the northwestern corner of town at 1361 Old Post Road. Trails at this preserve are open to hikers every day of the year from dawn to dusk.    

To begin, please imagine that your arm represents geologic time, with your shoulder joint being the Big Bang and the tip of your middle finger representing today.  Rocks began to form near your elbow and the dawn of life occurred just before your wrist.  Between your wrist and the middle of your palm, the prominent bedrock types of Pound Ridge were created by successive waves of mountain-building events.  They were then covered and contorted repeatedly, over millions of years, and eventually stripped bare by the glaciers that receded only 20,000 years ago, near the tip of your middle finger on our imaginary time scale.  We must look to the wider region and even the other side of the world to piece together this vast history, much of it obliterated by time.   

The oldest rock in Pound Ridge is the Fordham gneiss underlying Armstrong Preserve.  It was produced 1.1 billion years ago during a Precambrian period called the Grenville Orogeny, when this part of the world was located in the Southern Hemisphere and was turned 90 degree on its side from our current orientation.  This collision between then-continents Laurentia and Amazonia caused the rise of a massive mountain range that compressed and deformed existing rock into a Gneiss basement layer that is called the Grenville Province and underlies much of New York.

Gneiss is a high-grade metamorphic rock, formed from either granite or sedimentary layers under intense heat and pressure.  It is resistant to weathering and can be seen in the many exposed outcrops of Armstrong as well as in the nearby Ward Pound Ridge Reservation.  While variable in color, gneiss displays distinct foliation, or grain, created by alternating layers of its component minerals: quartz, plagioclase, biotite mica, garnet, hornblende and others.  The alternating light and dark colors that are so characteristic of gneiss do not represent fossilized sedimentary layers but rather a restructuring and realignment of minerals into layers that are called “gneissic banding.”

The basement layer we now see visible before us was increasingly buried by the sediments of the eroding Grenville Mountains over the next 400 million years, which depressed the land with their great weight and at times were inundated by shallow seas.  Then, in the late Ordovician Period, a volcanic island arc approached and slammed into this part of the continent.  Called the Taconic Orogeny, it’s pressure created our local Manhattan Schist and Inwood Marble and severely folded the existing bedrock into large ENE-WSW trending ridges and valleys.  This or a subsequent episode caused some intrusions of granitic pegmatite in Armstrong’s bedrock as well, which were subsequently folded too.

Life was difficult in New York for the next few million years as continents collided and mountains rose and fell.  The Fordham gneiss of Armstrong experienced a third wave of compression and metamorphosis in the Acadian Orogeny followed by another long period of erosion of highlands and sedimentation of lowlands.  Folding caused by this event runs in a NNE direction and is difficult to discern from the preceding episode.

The last of the great mountain-building events was the Alleghanian Orogeny, 300 million years ago in the late Carboniferous Period. This fourth collision resulted in the formation of Pangaea and was strongly felt in southeastern New York, where it produced tight folds that reoriented earlier land formations.  

The supercontinent Pangaea did not last long before it began to rift apart, resulting in volcanoes and allowing inundation of previously dry land.  Evidence from that period is visible in the Hudson River Valley and in the Palisades, but was largely swept clear from Pound Ridge in the Pleistocene:  the age of glacial advance and retreat.  An ice sheet measuring one-half mile thick ground back and forth across this landscape for nearly 100,000 years, freezing and thawing, cracking rocks and transporting them from mountaintops to the sea.  During four long periods of retreat, the glaciers dropped stone inland and created dams and flooding, and a rise in sea levels that brought the sea to our door.

The last glacial ice disappeared from Pound Ridge about 12,000 years ago, leaving behind a roughened and stony landscape that has been little changed by the thin mantle of forest now covering it.  We can readily see the glacier’s action in the exposed stony faces of east-facing hills and the till and boulder-strewn west-facing slopes.  Glacial erratics are common: there is a huge boulder perched atop a forty foot cliff at Armstrong’s Crow Ledge, tumbled for some distance but now at a high point in the Preserve.  There are also several lesser cliffs, some with wonderful gneissic banding and folding, and talus at their bases.  The vast majority of rock seen will be Fordham gneiss but there is the possibility of finding any mineral from higher elevations carried here by force of wind or water, or even human.  

Keep an eye out for history on the landscape.  It has quite a tale to tell, if only we read the clues.

Thank you to Ted Dowey for photographs and assistance with this exploration.  I have also relied heavily upon the following published resources:

Robert Titus of Hartwick College, frequent contributor to regional media and author of The Catskills: A Geological Guide. Third edition 2004. (link)

Chet and Maureen Raymo’s invaluable book Written in Stone:  A Geological History of the Northeastern United States.  Third edition in 2001. (link)

Mehdi Alavi’s 1975 Thesis:  Geology of the Bedford Complex and Surrounding Rocks, Southeastern New York. University of Massachusetts, Amherst. (link)

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