2006-03-17 / Columnists

On The Bayfront

By Elisa Hinken

A few years ago, students from the Department of Geology at Hunter College came out to the beach. They looked at rocks that wash up on the beaches of Breezy Point, Staten Island and Sandy Hook, NJ.

To me, it brings back memories of sitting at the seashore with my pail and shovel for hours and hours, digging and building sand castles. I always wondered how rocks got onto the beach and embedded themselves in the fine white sand we cherish so much. Why were there shiny glistening in the sand? Maybe I would find diamonds I thought. Maybe a pirate's treasure is stowed deeper in the sand. I had a wild imagination back when I was a child, which suited me just fine when I spent so many wonderful years at the seashore.

As an adult, I hate getting sand in my car or house!

Anyone interested in seeing what kinds of rocks are common in the New York City region needs only to go for a walk on the beach. Look at the wave-polished gravels that accumulate in patches around groins or on the storm deposits in the highest tidal area. Rocks from every "geologic province" in the greater New York Bight can be found mixed together, although in varying relative abundance. In general, gravel seems to accumulate on the beach in greatest abundance during the rising high tide cycles when wave energy is relatively low.

Gravels on three area beaches were examined: Breezy Point (Gateway NRA, Rockaway, Queens, NY), South Beach (Staten Island, NY), and Sandy Hook (Gateway NRA, NJ). Sandy Hook Beach displays the greatest abundance of gravel, followed by Staten Island, then Rockaway Beach. Quartz pebbles (igneous and metamorphic origin) dominate on all three beaches, but are exceedingly abundant on Sandy Hook.

The quartz pebbles on Sandy Hook were probably derived from reworked Cretaceous and Tertiary coastal plain sediments. This is also true for the abundant bioturbated, glauconitic sandstone and fossiliferous concretion materials as well.

Newark Basin-derived materials are most abundant on South Beach (Staten Island), but are also common on both Sandy Hook and Rockaway beaches.

Breezy Point's shoreline contains the least amount of gravel of the three beaches.

Sand on Breezy Point is generally more fine-grained than the other beaches. Fragments of well-worn concretion material and bioturbated glauconitic sandstone similar to Sandy Hook sediments can be found on Breezy Point, suggesting that these materials were probably derived from the same source areas offshore Sandy Hook.

These materials were probably transported shoreward onto Breezy Point during the late Pleistocene transgression. These materials managed to escape the grinding action of wave energy by remaining buried in the sediment for long periods of time.

Scattered "fresh" fragments of granite, schist, gneiss, and other rock-types (bearing rough or slightly worn edges) appear mixed in the sand on Breezy Point.

These materials were probably derived from reworked glacial till.

The other possibility, like every rock fragment or object on the beach, is that it was dumped there by humans intentionally or accidentally!

Hematitic sandstone and siltstone most likely derived from the Newark Basin or Catskills region. Many specimens are similar to the "brown stone" used in the construction of homes throughout the region. Specimens are orange, red, or brownish in color, and commonly display evidence of bioturbation.

Gneiss and Granite most likely derived from metamorphic and igneous terrain of the Highlands region.

Some samples display large "pegmatite" crystals.

Long Island beaches sometimes yield beautiful pieces of anorthosite from the Adirondack region.

Schist likely derived from metamorphic and igneous terrain of the Highlands region, probably from Manhattan. Schist samples commonly display garnet crystals.

Quartz pebbles were ultimately derived from igneous and metamorphic terrains throughout the Appalachian region.

Gravel consisting dominantly of quartz pebbles is abundant in coastal plain sediments and in Paleozoic conglomerates in the Catskills region. Quartz pebbles are extremely durable on the surface environment and can survive transport over great distances and for long periods of time.

Peat is abundant in areas where nearshore subaerial erosion has exposed older tidal flat and swamp deposits.

These materials range in age from Cretaceous (Magothy and Raritan formations under Long Island) to early Holocene sediments.

Old peat deposits can become well indurated (lignite). Some peat beds display orange-colored "clinker" (partially burned peat ignited by lightning strikes in the geologic past). Some samples contain shell fossils.

Basalt and diabase derived from Newark and Connecticut Basin volcanic sources (Triassic and Jurassic). Some samples display crystals of amphibole, pyroxene, and olivene. Dark green samples with fibrous minerals are probably serpentine from Staten Island (early Paleozoic).

Pumice possibly derived from Caribbean volcanoes and carried northward on the Gulf Stream (along with coconuts and other "drift-seeds," as well as and every plastic item imaginable!). Alternatively, it came from some pipe-dream landscaping job lost to a Nor'easter!

Chert derived from Paleozoic limestones and other sources throughout the region.

Check chert carefully.

Chert, slate, and argillite were used by Indians for an assortment of tools and weapons.

Conglomerate derived from Paleozoic metamorphic and sedimentary formations in the Catskills and Highlands region, and from cemented gravels in coastal plain sediments.

Coastal plain conglomerates are most abundant on Sandy Hook and tend to be cemented with "bog iron" (limonite). Paleozoic conglomerates tend to display evidence of metamorphic pressures. The older conglomerate is also cemented by "red" hematite. Look closely, however, what looks like conglomerate can be concrete.

Concretion derived from Quaternary marine coastal plain sediments. Concretions sometimes are encrusted with coral, each rock derived from Quaternary coastal plain sediments.

Beach rock is the host material of most beach fossils.

The matrix consists of fine to coarse sand, even conglomerate. Sand is cemented with calcite or limonite. (Shell-bearing limonite samples may be recent "rusted iron."). This cobble of well cemented beach rock contains jingle shells, oysters, gastropods, shark vertebrae, and spruce pine cones.

Coal occurs naturally in abundance in pre-anthropogenic sediments. Early reports noted "sea coal" in abundance along the shores of Long Island long before the Industrial Revolution. However, because of the incredible amount lost or dumped into New York Harbor in recent historic times mostly as cellar ash waste or ship spillage, coal is included in "anthropogenic sediments."

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