VALLEY FORGE, Pa. (AP) — The national park on the site where George Washington and the struggling Continental Army endured a tough winter during the American Revolution boasts a new feature that’s a couple of hundred million years old — dozens of fossilized dinosaur footprints discovered on rocks used to pave a section of hiking trail.

The trace fossils, as they are known, are scattered along a winding trail at Valley Forge National Historical Park, on slabs purchased in 2011 from a nearby commercial quarry.

To the untrained eye, they appear as indistinguishable bumps in the sandstone rock, with the largest about 9 inches long. On a recent weekday, hikers, joggers and dog walkers used the trail, oblivious to the marks of prehistoric animals beneath their feet.

Those marks drew the attention of Tom Stack not long after he began working as a volunteer park ambassador at Valley Forge in 2017.

Stack, who has a background in geology and paleontology, recognized the approximately 210 million-year-old rocks known as argillite as being similar in age and type to fossil-bearing rocks used to construct a 1930s-era bridge on the Gettysburg battlefield, about 100 miles (161 kilometers) to the west.

Most of the tracks left in what were once muddy flats consist of three-toed foot impressions from the early days of dinosaurs, although Stack also found footprints from a non-dinosaur reptile, a relative of the modern crocodile. The largest would have been a bipedal theropod that was 6 (1.8 meters) to 9 feet (2.7 meters) long and 4 (1.2 meters) to 6 feet (1.8 meters) high.

“They’re subtle, they’re not easy to spot, but once you learn the characteristics of them, given the right sunlight angle and, at times, the moisture on the rock, then they are easier to identify,” Stack said.

There are also distinctive patterns in the rock thought to be caused by the cracking of dried mud, and from the ripples of a lake or river.

The National Park Service requested the exact location of the rocks not be publicized, to help protect them from being damaged or removed. Officials said visitors will be told about the rocks and how park resources are protected, but not where to find them. The 5-square-mile (13 square kilometer) park has about 30 miles (48 kilometers) of trail.