Nature Notes: Glacial Erratics

By Larry Penny
Stephen Talkhouse Park glacial erratic after a rain

   In October, while Three Mile Harbor was being dredged by a Suffolk County contractor, Steve Brennan and Chris Martin were using side-scanning radar to follow the course of the dredging work. Side-scanning radar allows one to look sideways along the bottom of a water body and see objects that rise off the bottom such as old wrecks, sunken 55-gallon drums, and other debris. What Brennan and Martin found among other things was a very large boulder in the channel, mostly submerged but sticking up out of the bottom.
    The boulder turned out to be one of those subsurface glacial erratics carried here from the north country some 25,000 years ago or earlier. As the glacier melted away it dropped all sorts of earthen debris including sands, pebbles, stones, and several large boulders, almost all of which were ground smooth by abrasive actions taking place during their transport here and subsequent erosion by wind and water when left behind. Locals have a name for these large relics of the ice age that are roundish, at least not jagged like quarry rock. They’re called “potato” rocks by virtue of their smoothed shapes and predominantly tannish color, the color of sandstone from which they were derived.
    Around 1991 when a Suffolk County Water Authority contractor was directionally drilling under Lake Montauk’s outlet to Block Island Sound during the installation of a water main from the west side to the Gin Beach side of the inlet, the driller hit a big boulder some 15 to 20 feet below the inlet bottom and so had to start over from the beginning.
    The situation of these foreign stones beneath the surface of water bodies is of interest. Certainly, glacial erratics are not uncommon around harbors and embayments. Just take a look at Sag Harbor east of the Route 114 bridge on an ebb tide and you will see dozens of them along the shore and out in the water.
    The edges of Lake Montauk have a few, Accabonac Harbor sports several, and Three Mile Harbor has its share of them, too. One of the larger ones sticking out of the water along the west side of Accabonac Harbor was used by ospreys to build a nest several years back. When the osprey population was booming during the 1940s and 1950s before special breeding platforms were erected for them, ospreys frequently built nests on the glacial erratics rising out of the water along the shores of Robins and Gardiner’s Islands.
    Both the South Fork and North Fork are boulder strewn, but only wherever the glaciers came to sit. Walk along the ocean bluffs of Montauk and you will find a load of them, some inbedded in the face of the bluffs, some dropped down to the shore after washing out of the bluffs, some out in the water, indicating that the bluffs were once much farther seaward than they are now. They were all left by the ice sheet, which traveled the farthest south during the last ice age and deposited the “terminal” moraine, or Ronkonkoma moraine. The Long Island Sound cliffs edging the North Fork tell a similar story. Their boulders and the ones sitting out in the sound are part of the leavings of a second glacial advance, the one that deposited the Harbor Hill moraine, which runs all the way west to northern Nassau County.
    Where these massive stones sit on fastland, say in the Northwest area of East Hampton Town, or along the moraine separating Noyac from Bridgehampton in Southampton Town, they have been further weathered and have been subject to freeze-thaw actions during cold winters. Many of them have fissures in which water accumulates and freezes. The power of expanding ice is enough to actually split these boulders and there are several called “split rocks,” such as the famous one in Hither Woods or one that sits in the center of Stephen Talkhouse Park in Springs where Gerard Drive and Springs Fireplace Road meet. Many of them have human names such as Joshua’s Rock off Bull Path in Northwest and Lionhead Rock in Gardiner’s Bay, named after Lion Gardiner, some say. Perhaps the largest of them is the anonymous one perched high on a hill in Noyac west of Stony Hill Road. The part that is showing above the surface of the ground is more than 15 feet high and 30 feet across. It’s the closest thing to Tyrannosaurus rex on Long Island.
    In some places these glacial erratics occur one right after another in what are called boulder trains. In other spots they are solitary, sometimes appearing like brooding earth gods bearded with an array of grays, greens, and yellows, from lichens that festoon them. Lichens are mutualistic duets consisting of a fungus and an alga. The alga seeks cover in the lichen and manufactures food by means of photosynthesis, the lichen feeds on some of it.
    The age of these lichens on eastern Long Island glacial erratics is not known precisely, but where rock lichens have been studied in detail in other parts of the world some of them are said to be thousands of years old. Some of ours are probably a few thousand years old, as well. Unfortunately, however, glacial erratics are not protected by any kind of local or state statutes. If they are on one’s property they can be moved, decorated, broken up, or otherwise desecrated. You will ride along, say, on Brick Kiln Road in Bridgehampton and see one that’s been painted or fixed with a house address.
    On the other hand they must have a special appeal for most property owners because the majority of them have been left in situ where they came to rest thousands of years ago. They dot the landscape the way large trees do and during the winter when the trees are mostly leafless they stand out like old men on the mountain.
    Although their roots are Canadian and New England, they are now an important part of our heritage and the largest ones will be here long after we perish, hopefully, with their complementary colorful lichens but no chips of paint.
    If you want native rock, you’ll have to drill 1,000 feet down or travel to Queens and Manhattan.