A column of ice, topped by a pebble, resembles a mushroom on Dec. 12, on a trail in Acadia National Park. Credit: Courtesy of Aislinn Sarnacki

At first glance, I thought it was a mushroom. Frozen solid in the frigid December air, the tiny object had a lumpy cap and arcing stalk. But when I crouched down, I realized it wasn’t a mushroom at all. It was a pink granite pebble perched atop a bent column of ice. Fascinating.

Ice, in its many forms, can be absolutely mesmerizing — and confounding.

I’ve written about ice formations a few times before. Each winter, I stumble upon nature-made ice sculptures that beg the question: How did the ice form like that?

I’m sure you’ve thought the same thing a time or two.

The thing is: I don’t know who to ask. As a journalist, I’m used to bugging experts to explain all things scientific. But who does one turn to in order to explain an ice-rock mushroom? A glaciologist? A forensic meteorologist? A fifth-grade science teacher?

So, I did what any floundering writer might: I turned to books.

One particularly helpful book, “Ice Engineering” by the U.S. Army Corps of Engineers, was published in 2002, in Hawaii, of all places. It’s a 475-page manual, a dry read, but it does contain some interesting ice-related terms and facts.

A “slush ball,” for example, is a ball of snow and ice particles that’s produced by wind and wave action along the shore of lakes. Boy do I want to find a slush ball.

An “ice pancake” is a circular, flat piece of ice with raised rims that can sometimes be found on the surface of streams and rivers. The shape and rim are caused by repeated collisions as it’s pushed around in the current.

“Rotten ice” is ice in an advanced stage of disintegration, while “mush ice” is a floating accumulation of very fine ice fragments that is somewhat cohesive. “Glare ice” is highly reflective ice, while “skim ice” is the initial thin layer of ice that forms on the surface of water.

I searched through the glossary for a term that might explain my ice-rock mushroom, and I nearly settled on “candle ice,” which are columns of ice that can form when ice melts. But it didn’t quite fit the circumstance.

Finally, in the 2004 “Encyclopedia of Geomorphology” edited by Andrew Goudie, I came across “needle ice.”

Needle ice, also known as pipkrake, is formed when ice crystals grow up from the ground in the direction of heat loss, according to the encyclopedia. These needle-like columns typically grow perpendicular to the ground, and they often bend due to wind and gravity.

Icicles hang from a rock ledge in Acadia National Park in December. Credit: Courtesy of Aislinn Sarnacki

This type of ice best develops in moist, fine-textured sediment — such as the kind found surfacing an old road in the forest of Acadia National Park. That is where I discovered my ice-rock mushroom.

Typically, needle ice starts developing within an hour of ground temperatures dropping below freezing. In Maine, people see a lot of it in late fall.

Most often, needle ice forms on ground that’s not covered in snow, according to “Winter: An Ecological Handbook,” written by James C. Halfpenny and Roy Ozanne and published in 1989. The ice builds from the bottom, pushing the first crystals up and up, like hair or fingernails. In the process, small objects such as stones are lifted up by the ice. Sometimes, this can be destructive.

“Roots of plants and burrows of animals are destroyed by the cutting ice,” Halfpenny and Ozanne state in the book. “In cold regions, buildings are often torn from their foundations and concrete is broken as the ground heaves 30 cm (12 inches) or more. Needle ice is extremely powerful.”

A 2021 study published in the “Proceedings of the National Academy of Sciences” examined how needle ice can move stones into patterns over time, through multiple freeze-thaw cycles. Basically, the ice forms underneath the stones, lifts them up, then bends, moving them to new locations. Under certain conditions, this can cause patterns to emerge, circles and lines of stones, that are so uniform that they appear to be man-made.

So, I think I’ve solved the ice-rock mushroom mystery. But that leaves about a million other ice formations to unpack. I think I’ll put down the books and let them remain a mystery. At least until my curiosity gets the best of me once more.

A thin sheet of ice has formed over a bed of moss and appears to hover over it on Dec. 12, in Acadia National Park. Credit: Courtesy of Aislinn Sarnacki

Aislinn Sarnacki

Aislinn Sarnacki is a Maine outdoors writer and the author of three Maine hiking guidebooks including “Family Friendly Hikes in Maine.” Find her on Twitter and Facebook @1minhikegirl. You can also...