Deep in the heart of the last ice age, at the bottom of a glacial lake, the clay babies were born.

Before I tell you exactly what a clay baby is, here’s how to find one. Take a stroll along the Connecticut River or its tributaries and find a bank of thick clay. Next, get dirty. Dig your hands into the muck, keeping an eye out for light-colored bumps protruding from the riverbank. Pull one of these bumps out of the ground, rinse it off, and you’ll be left with a carbonate concretion -- the stodgy geological term for what may more affectionately be called a clay baby.

A concretion is a hardened chunk of clay molded into a peculiar shape. Wherever you find one, you are standing at the bottom of what was once a frigid lake.

During the last ice age, a mile-thick glacier covered Canada and a sliver of the northern United States. As the earth warmed, around 18,000 years ago, and the glacier began to retreat north, a massive rock pile in central Connecticut backed water up all the way into northern Vermont. By approximately 15,000 years ago, Glacial Lake Hitchcock filled the entire Connecticut River Valley.

There was a seasonal rhythm to the life of that lake. In the warm months the glacier melted, releasing gritty sand and pulverized rock. Sediment-rich runoff flooded the lake, and heavy grit sank to the bottom. Then in the winter, the lake top froze and the water beneath it became death-still. Fine particles of silt -- so fine they had remained suspended in the water column during the turbid warm months -- now drifted to the bottom and settled above the layer of grit.

Today, geologists can dig through those double layers of grit and silt, called varves, to find all kinds of clues about life in Glacial Lake Hitchcock, including preserved fish nests and the imprints of sculpin lying in the lake’s muddy bottom. There are also clay babies.

Clay babies are not fossils, but the result of a unique geological process. At its core, a clay baby has a little scrap of organic matter. A dead worm, perhaps, drifted to the lake bottom and was coated by a varve. Between the varves, water flowed slowly, brushing past the organic scrap. That water was rich in carbonate -- probably dissolved from surrounding bedrock -- and dissolved carbonate was drawn like a magnet to any surface made of carbon. Like attracts like. A layer of carbonate-rich silt clung to the carbon-based worm bit, and then another layer clung to that, and then another. A few thousand years of this layering process resulted in a chunk of hardened clay.

Katherine Adams has been collecting clay babies along a stream in northern Vermont since the 1970s. (She’s cagey about the location.) Adams has almost 100 clay babies. When I visited her Brookline, Mass., apartment, she laid them out on her kitchen table in a phantasmagorical display.

Their shapes, Adams said, range "from the ridiculous to the sublime." One is a perfect ball -- almost the size of a fist -- with a divot in the middle, reminiscent of a jelly donut. Several look like marble-sized snails. There is a hotdog, a penguin, a dachshund, and a perfectly flat silver dollar. Most have layers that look like melted wax or rippled muscles. Some are perfectly symmetrical, while others reach in all directions with no repetition. No two are exactly alike, yet each looks intentional, as if an artist sculpted an abstract form to hint at a veiled meaning. As I picked up one after another, I found myself speaking softly, as if to not disturb them.

"They’re pretty magical," Adams said. "They’re expressive -- real eager to do what they’re doing and have their shapes. They worked hard on it, they strived for it, and they got it."

Adams’ stash will be on display this spring at the Brookline Public Library. Folks rushing in to pick up the latest John Grisham novel may find themselves drawn into the past. In the wilderness that planted the seed for these clay babies, there were some of today’s familiar species, including eagles and loons. But there were also bison, wooly mammoths and saber-toothed cats. Rather than forested mountains, the landscape was alpine grassland pockmarked with cold-hearty evergreens. These funky rocks formed just below our feet, yet in a wholly different place. Perhaps library-goers will see all that when they peer through the glass case.

Or maybe they’ll just see a doorknob, or a bunny, or a heart.

Jack Rodolico is a freelance writer and radio producer. He lives in a cabin on a pond in New Hampshire. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: wellborn@nhcf.org.