A somber reminder of slavery has been found hiding in plain sight at some of South Carolina’s historic sites: The fingerprints of slaves left by accident in bricks.

South Carolina State Parks officials revealed the little-known detail in a recent Facebook post, and included a photo of one such brick, found at Rose Hill Plantation, 65 miles northwest of Columbia.

“During the early 1800s, bricks were handmade, often by enslaved people,” says the state’s post. “Occasionally, their fingerprints were imprinted in the clay as they pulled it from the mold....A solemn, lasting reminder of the people who made their lives at Rose Hill.”

Nate Johnson, park manager at the plantation, told the Charlotte Observer several fingerprinted bricks have been found at the site, including one in a garden wall and one that was found buried. The garden wall has been dated to the 1850s, he said.