Huge fossilized trees still stand rooted in their original but compacted soil, surrounded by the litter of leaves that once fluttered down.

Primitive, lizardlike reptiles were then evolving in the swamps, but there are almost no animal fossils in the Springfield forest — save for the occasional cockroach wing — since such creatures easily fled the rising waters.

Such snapshots of the very distant past — tens of millions of years before the age of dinosaurs — are hard to come by. “It is extraordinarily rare to get fossil forests of any extent at all,” said Kirk Johnson, a paleobotanist at the Denver Museum of Nature and Science. “It’s usually just a few trees here and there. But here is an ancient geography — effectively unheard of.”

Dr. DiMichele and colleagues have explored a five-mile path, or transect, starting at the ancient riverbank and arrowing through the swamp. Just as if this were a living forest, they have stopped along the route to identify individual leaves or study fallen trunks. Moving away from the river, a dense thicket of seed ferns gives way to tree ferns and low ground cover. Farther out, tree ferns are dwarfed by forest giants called scale trees.

“It was a Dr. Seuss world,” Dr. Johnson said of the scale-tree forests: sun-washed quagmires studded with giant green stalks like asparagus spears, hundreds of feet tall. (Scale trees did not unfurl spreading crowns until the very end of their life cycle.) Dr. DiMichele has followed a fallen scale tree for 100 feet, before it disappeared behind coal not yet mined away. Six feet wide at the base, it was hardly any narrower at that great height.

Scale trees had reptilian-looking, photosynthetic bark that coal miners sometimes mistake for dinosaur remains. Tube-shaped with spongy pulp inside, the trees snapped in two when storms ravaged the swamp. Immense, cylindrical roots kept stumps firmly upright, as seen in the mines.

By coincidence, the earliest ancestor of these scale trees has just been discovered in a fossil forest in New York State. Repairs at Gilboa Dam, north of the Catskills, uncovered the floor of what may be the world’s oldest forest, scientists reported last month in the journal Nature. Dating from the Devonian Period, it is 78 million years older than the Springfield find, but the mapped remains are much smaller, covering about a third of an acre.