Another whale?

That was what the caller suspected when they looked down the beach at Norton Point and saw a dark mound in the distance. Please, not another one.

Luckily it wasn’t another deceased goliath. It was much more benign: a pile of black-looking sand. It was very good news; not a whale, just a case of poor eyesight and geologic grandeur.

Black sands make up a small portion of our beaches but they can aggregate, which is one of the reasons the pile looked black. Likely the top sands were darker than the middle and bottom of the pile.

Magnetite is the main mineral that produces that dark color, though ilmenite and hematite also can also provide black pigmentation. The big piles of sand were recently dredged from the bay bottom, and sands brought up in this way also often have higher concentrations of murkier sediments that contrast with the lighter surficial sands. 

Some minerals have a way of making themselves known more than others. In this case, size may not matter as much as weight. Weightier particles settle, while lighter ones are moved about the beach as water and winds carry the finer grains and leave the heavier ones behind.

A sandy beach is made of those aforementioned particles and other rocks, shells, organic and inorganic matter, and minerals, and these determine the color of the beach. Quartz is the main mineral component of Island sand. Along with another light-colored mineral, feldspar, our beaches appear tan or beige, sometimes even pinkish or grey. 

Other minerals in lesser amounts can add color and variety to the shore. Some North Shore beaches, including Crane Beach and Plum Island, can appear purple because of the garnet scattered among the sands. Limonite produces yellow hues and olivine projects a green shade. Of course, nearer to home, the red clay of Aquinnah adds its own ruddy hue to the beach. 

Most fun might be sparkly mica — or maybe call it Swiftie sand, for its glittery glamour — that shimmers and shines with its flat plane glistening in the sun. Organics such as seaweeds, algae, plants and animals provide greens, greys, blacks and browns, depending on the nature of the debris.

Beyond the granular colors, beach surface structure can be at the whim of wind, wave and water. When wind is the sand-moving culprit, eolian describes its action. Together with waves, physical forces can shape and sculpt the beach into myriad patterns of light and shadow. The water adds its touch of darkening wet areas to contrast with the dry areas.  

Look for ripple marks — potato chip-like ridges caused by wind — that occur between the high and low water lines. As the surf moves down the beach, rill marks may be produced and appear as eroded channels and braided streams. Behind a sand bar, shallow troughs called ridges and runnels can get created. All of these beach bumps provide shape-shifting intricacies in the sands. Note also the way the darker sands that we began with make patterns as part of these features.

A winter beach holds contours and colors and invites observation and appreciation. A hand lens, magnet for picking up magnetite, and strainers for sifting sand are terrific tools to bring to study the smallest pieces of even the biggest beach.

Suzan Bellincampi is Islands director for Felix Neck Wildlife Sanctuary in Edgartown and the Nantucket Wildlife Sanctuaries. She is also the author of Martha’s Vineyard: A Field Guide to Island Nature and The Nature of Martha’s Vineyard.