FMF - Friends of Minerals Forum, discussion and message board

[Grant M.]

Recently I arrived in Long Island, New York. This week, a couple close friends and I are planning a trip to Cedar Beach to look for unique minerals in the area and pure forms of igneous rocks smoothed out by the sand and currents of the sound and harbor. I was looking for advice in which areas of the beach to look in. I know for a fact that large rocks lay around certain parts of the shoreline. My friends and I are planning to go around a time in which the tide will be low. Also, are there any sites online where I can find mineral locality maps of the beach? Thanks.

[Bob Harman]

So I grew up in the Queens section of NYC and during the summers of my youth I spent much time at the beaches on Long Island. There actually are 2 Cedar Beaches. The well known one is on the South shore of Long Island which faces the Atlantic Ocean. It is near the East end of Jones Beach Island or near the West end of Fire Island, rather near the town of Babylon. The less well known Cedar Beach is on the North side of the island facing Long Island Sound, in about the middle of Long Island and somewhat further East than its namesake on the South Shore.

Assuming you mean the Cedar Beach on the South side of the Island, I can tell you that there are only sand beaches. The rock you are seeing are break waters. Large boulders trucked in from afar and strategically placed along the south shore beaches in the 1950s to prevent beach erosion from Atlantic storms. There is no natural native rock on the south shore of Long Island.....just sand. After WWII the commercial and residential population of Long Island exploded so beach erosion became a serious concern. The break waters, or jetties as they are also called, were put in place to stop the erosion.

As a kid I hunted thru these rocks during my summers at the beaches (mostly in the Rockaways and Jones Beach). They are interesting and I found many small garnets along with muscovite mica, but you must remember that all the rock along the South shore is non-native to the location. It was trucked in probably from Westchester County NY or Northern New Jersey. At high tide much of the rock is under water. At low tide much more is briefly exposed, but by now most of the rock is covered with algae and related organics.
Maybe other folks have more up to date info, but I hope this helps.

[alfredo]

Gravel beaches at the far eastern end of Long Island have lots of pebbles carried south from New England or even Canada by glaciers. I've found rutilated quartz, and one clear quartz pebble with a black tourmaline inclusion! It would be fun to try tracing that back to its origin.

[Bob Harman]

As an addendum, Alfredo is correct. At the far Eastern tip of Long Island is the Montauk Point Lighthouse. I believe the whole area is either a national historic site or a state park.
In any case the beaches there are composed of water worn cobbles and gravels. If you google the light house and look thru the pictures, you will see that there are boulders around the base of the light house, placed there in 1961 to slow to beach erosion as the light house was getting dangerously close to being engulfed by the ocean. Consider driving out there to collect. It is quite an interesting place of history.

[Peter Megaw]

There are almost NO "native" rocks on Long Island at all because Long Island is the terminal moraine of the Wisconsinian glacier that stopped and melted away about 11,000 years ago. (There are isolated basement rock exposures in northern Queens, but most are built over since that is one f the few places on Long Island with solid enough substrate to build anything tall on.) The Wisconsinian glacier was the last of four major ice advances (Nebraskan, Kansan, Illinoisan and Wisconsinian...nifty pneumonic I learned in school to remember the progression was Never Kick Indian Women) over about 2.5 million years. There is no reason to think that we live in anything except another interglacial period, so don't sell your long-woolies yet.

The huge pile of sand and gravel that makes up Long Island (and Cape Cod, Martha's Vinyard, Nantucket, Block and Fire Islands) was scraped off the basement by these glaciers as they ground their way across the terrain north and northwest of Long Island. So rocks from across the Appalachians, Grenville and Abitibi Provinces are included...rocks dating from Cretaceous to Archean (130Ma to 3 Ga). A huge sampling of these diverse geologic terranes to select from. As Alfredo noted, it would be fun to trace the cobbles you find to their source...with a good geologic map you may be able to come up with decent candidates in some cases.

That is actually exactly how the Canadian diamond pipes (kimberlites) have been found over the last 25 years. Working with the sand-sized fraction of glacial deposits, explorationists sample and sort out the "pathfinder" minerals (specific Chromium-rich garnets, spinel, ilmenite, diopside and enstatite especially since they survive glacial weathering and mashing) that are characteristic of kimberlites...making maps of their abundance, proportions and size and using them to track back to the 1 km diameter (at best) kimberlite pipes that carry the diamonds. At least 20 pipes have been found including the huge Ekati pipe.

Maybe you should get out your sieves on Long Island and see if you have indications of a diamond pipe??

[Grant M.]

Wow, thank you all for replying. It's a great deal interesting to know where these giant boulders actually came from. It is also truly astounding to know that specimens found on Long Island are almost no rocks are indigenous here. Thank you, Alfredo for the advice. Can't wait to head out there and follow your advice. Your remark about the quartz gives me hope.

[Peter Megaw]

Get out onto east facing or E-W aligned stretches of beach early in the morning when the pebbles are wet and the sun angle is low. Walk towards the sun and you will quickly find that the more translucent and transparent pebbles have a different appearance than the others. This makes it lot easier to find the kind of thing Alfredo is talking about. Some folks find it works better with the sun at their back...try both and see what works best for you.

Same works in reverse in the afternoon...

High sun angles make this MUCH harder

[Grant M.]

Thanks a million. Can't wait to get out there.