Geology

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)

Evidence of Geologic History at Bye Preserve

bye-geologyThe stony ground and prominent rock formations of Pound Ridge in Westchester County, New York elicit curiosity in many people, some of whom attended our recent guided hike focused on geology. I will summarize our tour of the geologic history that can be seen at the Bye Preserve, owned by the Pound Ridge Land Conservancy (link to map and directions).  Trails at the Preserve are open everyday from dawn until dusk.

Imagine that your arm represents geologic time, with your armpit being the Big Bang and the tip of your middle finger representing today.  The dawn of life occurs just above your wrist.  Between your wrist and the middle of your palm, all of the bedrock that shapes this land was formed by a process known as the tectonic cycle:  erosion of mountains, deposition of materials in layers, subduction (movement of the earth’s crust), and volcanism.  There was something on the earth’s crust prior to the formation of our bedrock, of course, but we cannot see evidence of that now.  Geologists have discerned that an ancient Acadian Mountain Range once rose east of us, to the height of the Himalayas, and then eroded into particles that were later metamorphosed into our bedrock.  Add folding from the movement of tectonic plates, deformation by glaciers, and erosion, and you have a punctuated record of history etched on the landscape.

Fordham gneiss bedrock exposed on the surface

metamorphic-sedimentary-layers

Metamorphosed sedimentary layers, eroded and tilted

 

 

 

 

 

 

In its western half, the Bye Preserve is underlain by Fordham gneiss, which formed in the Precambrian era, 1.1 billion years ago.  This layer measures up to 500 feet thick and is extremely resistant to weathering.  There are many kinds of gneiss, and it is variable in color from brown to buff and can be pink or green.  Fordham gneiss contains quartz, biotite mica, silicates, garnet, and other minerals.  Bedrock along the eastern side of Bye belongs to the Hartland Formation, which is half as old and is a mixture of gneisses, schists, and amphibolites.  It formed when shales and sandstones from a deep ocean metamorphosed and retains its layers of alternating color.  Many of these rocks were inverted when the Hartland Boundary Fault displaced older layers and brought in new rocks.

muscovite-mica

Muscovite mica

Granite

 

 

Subsequent to the last great tectonic shifts in the Paleozoic era, the bedrock at Bye has endured at least four episodes of folding and associated metamorphism, which is visible in many rocks where layers are curved rather than straight.  The first stage of folding produced large ENE-WSW trending ridges and valleys, while the second stage trended NNE.  Later stages produced tight folds that reoriented earlier folds.  Where rock is resistant to bending, brittle faulting may occur, which produced the Mill River Gorge just northeast of Bye Preserve and the Mianus River Gorge as well.  In the midst of these changes, magma intruded the bedrock at Bye and produced a very pretty white-to-pink granite that is rich in quartz and mica.

Metamorphosed layers, block faulting

Metamorphosed layers, block faulting

Folding

Rock tumble from brittle fault

Rock tumble from brittle fault

 

 

 

 

 

 

 

 

 

 

More recently in geologic time, between your furthest knuckle and fingertip on our imaginary time scale, the glaciers of the Ice Age left their mark.  An ice sheet more than one mile thick ground back and forth across this landscape for nearly 100,000 years in freeze/thaw cycles.  Each advance brought in new rock from afar.  Each period of retreat caused mass deposition, flooding, and a rise in sea levels.  Bye Preserve is now only ten miles from the Long Island Sound and was at times submerged by ocean, although fresh water has played a greater role in carving this landscape. We can see evidence that great torrents once ran in the boulder fields that line two now-intermittent streams, scouring away soil such that even today, only ferns and moss grow there.

The last glacial ice disappeared from Pound Ridge about 12,0000 years ago, leaving behind piles of sand, rock, and even huge boulders on the surface of our scoured bedrock.  These are the clues from which we can piece together millions of years of history on the land.  To end our story, I invite you to sit with a favorite rock and to ponder what it has endured to bring you both to this moment in time.

variation-in-gneiss-rocks-moved-by-glacier

Variation in gneiss moved by glacier

glacial-erratic-boulder-with-lichen

Glacial erratic boulder with lichen

Boulder showing evidence of glacial tumbling and erosion

Intermittent stream bed showing many phases of geologic activity

ferns-and-moss-in-a-rock-strewn-valley

Ferns and moss in rock-strewn valley

upturned-metamorphic-sedimentary-rock-with-glacial-scarring

Upturned metamorphic sedimentary rock with glacial scarring

 

faulting-caused-by-the-expansion-of-freezing-water

Cracking caused the expansion of freezing water