Westchester County

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)

Native Plant Propagation at Pound Ridge Land Conservancy Preserves, Westchester County NY

Richard Chianella

Nature lovers who garden are almost as numerous as gardeners who love nature, and they are teaming up for a great cause in our Native Plant Propagation Lab at PRLC’s Armstrong Preserve and Education Center.  With all the buzz about declines in wildflowers and their insect pollinators, now is the perfect time to share what we have learned over the last three growing seasons.   Join me for a tour of the Prop Lab and Armstrong’s native plant restoration areas and outdoor classrooms this Saturday, June 25, and read on for details on how to volunteer to raise plants for our other nature preserves as well as your home.

Cardinal flowerThis year, our volunteer team of land stewards, master gardeners, and students raised more than 500 plants from seed and another 20 from cuttings.  We start by collecting seed from plants of known local origin in the summer and fall.  If we can avoid cultivars and garden escapes, we are more assured of the hardiness and overall suitability of the plant for restoration areas, which are tended and watered far less often than a typical garden.  We also know that local insects and birds will have evolved in concert with these plants and are best adapted to make use of them.  The seed collector may have to find and mark the location of a desired plant during its flowering time, and return later when the seeds have matured.  He or she carries a stash of paper bags and a marker to label each with the species name, location, and date.  Seeds are shaken into the bag, or entire flower heads thrown in and dealt with later.  Ethics dictate that only 5-10% of seed be collected from any one plant, and that harvesting be dispersed over a large and healthy population.

At home, the chaff is separated from the seeds to prevent molding.  All material excepting the seed should be removed.  I use a paper plate for this task, brushing and blowing the chaff to one side. Store the cleaned seeds in their paper bag until late fall, when we will attempt to mimic the conditions of nature by putting them into cold storage.  Consider the life cycle of the seed:  it will likely drop from the plant to the ground during fall rains, or pass through the intestinal tract of an animal, and then to the ground.  If it is lucky, it will land in soil and be covered by organic debris over the winter.  We bank the seed over winter in shallow trays of soil that are covered to protect from animals and stored outside.  Plastic take-out containers work well for this as they are sturdy and stackable;   just be sure to poke holes in the bottom for drainage.IMG_20160321_122115533

In mid-February, we bring the seed trays in to a heated space and watch for germination. At this point, we are artificially hastening their development so that theplants can grow large enough to be transplanted into the great outdoors in May, before hot weather sets in.   When the tiny seedlings break through the surface, they must be moved into the light.  I keep florescent grow lights on them for 16 hours per day and water them gently, every day.  Young seedlings thrive on consistent heat, light, and moisture.

IMG_20160411_115634987Our first major task is to transplant each individual seedlings into its own pot.  This is delicate work, best done with latex gloves or none at all.  A chopstick or knitting needle makes a useful tool for easing each tender stalk out of the cluster of young seedlings.  Use recycled plastic containers of any kind for pots (remember the drainage holes).  Fill them to the top with a light mixture of mainly leaf litter, with some compost and sand.  Carefully label at least one of the batch for reference (and old window blinds make great labels).  Water well, and keep the transplants under light and warm conditions for a minimum of two days.

hoop houseI do not have much heated space and so move the plants out early into makeshift green houses, where they at least have abundant sun and protection from wind and rain.  Conditions can be harsh however, ranging from freezing to 90 degrees on some days.  Daily watering is essential.  Growers with heated greenhouse space will grow plants at more than twice the rate that I can, or more, but a simple plastic covering is enough to keep them alive.  By last frost date, all plants can be moved outside of covered areas, although they will need to be dampened off (transitioned slowly) to full sun, wind, and rain.

IMG_20160619_164701432By June 1, our plants were ready to be moved out to fenced restoration areas in PRLC’s nature preserves.  One of our local partners, the Rusticus Garden Club, generously sponsored the hiring of a local college student intern to aid us in getting all 500+ wildflowers into the ground this month, and he will continue to water and weed planting areas through the summer.  Volunteers are needed at a number of our preserves to provide supplemental water and to assist with weeding until these plants become established.  We also welcome the donation of native plants, either from nurseries or areas that are slated for disturbance.  Please contact me at 914-205-3533 or by email at landsteward.educator@prlc.net for details on how you can help out at your favorite preserve.

Endnote:  Our efforts to restore native trees, shrubs, and pollinator plants to our nature preserves is a direct response to the incursion of overabundant deer and alien invasive plant species in Pound Ridge.  It is our hope that by creating reservoirs of protected plants, we can preserve current levels of biodiversity and provide continued seed stock for colonization of unprotected areas.  Deer management is therefore an integral aspect to our program.

Restoration area at Armstrong Preserve, before and after:


2 2014 (2)3 Sunny side June 2016 2

 

PRLC Native Plant Propagation Lab 2016
Total Count and Distribution
Common name 5/21/2016 Armstrong Grove Clark Russell Halle
Count
Giant hyssop 8 2 2 2 3
Aster, heart leaved 20 4 8 3 5
Tick trefoil 10 6 4
Wild yam 5 2
Woodland sunflower 2 2
Tall Sunflower 3 3
Meadow rue 44 8 16 20
Evening primrose 27 8 6 3 10
Pokeweed 5 3 2
Scarlet smartweed 4 3 1
Bluestem goldenrod 7 3 4
Canada goldenrod ? 28 3 12 3 10
Meadow goldenrod ? 30 6 12 12
Gray goldenrod 36 6 12 3 3 12
Goldenrod Bridge St 10 3 4 3
Joe Pye weed 8 8
Common boneset 42 22 20
Dogbane 27 16 2 6
Swamp milkweed 1 1
Cardinal flower 36 10 6 6 6 8
Great blue lobelia 30 7 8 2 3 10
New York Ironweed 18 8 10
Monkeyflower 34 14 20
Baptisia 3 held over
Buttonbush 19 held over
Swamp rose 11 held over
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