What makes New England’s cemeteries so creepy? The answer may have more to do with stones than bones.

The historic Market Street Burying Ground in Brighton, Massachusetts was laid out in 1764 near the site of the first American cattle yard. Today, the cemetery is sandwiched between Brighton Beer Garden and a four-story apartment complex. The tiny, eldritch burying ground broods from behind the iron bars that protect its historical integrity from modern mischief. And while its gnarled trees, dark corners and, well, rows of dead people contribute to the graveyard’s macabre mien, it’s geology, really, that makes Market Street Burial Ground and other early American cemeteries so eerie.

The dominant gravestone material of Colonial America was slate, a fine-grained metamorphic rock also used to make roof tiles, chalkboards, and pool tables. Thanks to the parallel alignment of slate’s mineral matrix, the rock breaks readily into smooth plates, and because it began its geologic life as shale—a sedimentary rock usually composed of clay minerals—it is also relatively soft and easy to carve. But these qualities also make the slate especially susceptible to elemental weathering. Centuries of rain, ice, and snow have cracked, faded, pitted and streaked the old slate markers, making the sunken stones look ghoulishly worn. The result is slate stone cemeteries that easily out-creep their modern, granite gravestone counterparts.

Market Street Burying Ground is a prime example, as are two particularly eerie slate gravestones overlooking the busy street. They mark the final resting places of Abijah Learned and his wife Abigail. The markers are pallid as zombie flesh and a black discoloration creeps in from the stones’ weathered edges.  Abijah’s stone is engraved with the solemn inscription, Behold the spirit of the just ascend to God on high, and while their bodies sleep in dust their souls shall never die. Like many of the cemetery’s gravestones, his stands nearly four feet tall and is less than two inches thick, dimensions that render slate stones vulnerable to weather and time. The wear has left the markers tilting sickly in the soft soil, or, in the case of Abigail’s gravestone, broken jaggedly in half.

Modern gravestones, on the other hand, are almost exclusively made from granite, an igneous rock that forms when magma beneath the earth’s surface becomes trapped in crustal pockets and solidifies slowly enough to form large quartz and feldspar crystals, which shimmer and sparkle in the sun.  And because granite is so much harder than slate, modern stonecutters can use stencils, lasers, computers to create varied, intricate, and near-perfect designs.

Certainly, rocks influence the look of a cemetery, but psychologists suggest that a graveyard’s feel is more of a mental matter. According to Neil Lutsky, Kenan Professor of Psychology at Carlton College, the dread associated with old gravestones stems from cultural stereotypes and people’s inherent fear of death. Humans, after all, are unique among animals in our knowledge of death’s inevitability and this creates powerful psychological insecurities. “The older cemeteries correspond in appearance to prototypes for spooky, broken down, ancient graves as represented in television and movies,” he says. “Perhaps their age reminds us of the constancy of death.” In social psychology, a popular hypothesis about why humans fear death called Terror Management Theory asserts that we erect defenses against reminders of our own mortality. Certainly, says Lutsky, “Cemeteries serve as such threatening reminders.”

But what makes older cemeteries seem so much creepier than the new? Jeff Greenberg, a social psychologist at the University of Arizona, believes that for some people, weathering equals woe. “The older [stones] are more decayed, they reflect people who are forgotten. And people are afraid of being forgotten,” he says. “Watching horror movies and ghost stories, people connect the older [gravestones] with nameless ghosts. There is a sense of obscurity.” Old gravestones, Greenberg suggests, “are no longer a part of the ongoing, meaningful world.” At least with the new ones, we think, people come to visit.

Still, not everyone agrees with the “older is creepier” notion. While he acknowledges the cultural stereotypes associated with old cemeteries, Lutsky actually finds modern ones more troubling. “The rows are too close, the sameness too threatening,” he says. Indeed, there is something disturbing in the precision of modern cemeteries: the perfectly aligned grave rows, the flawless carvings, the laser-carved verses, and the impeccably manicured grass.

Perfection to this degree feels false, as synthesized as the plastic flowers that that lie upon the markers. These modern graveyards may not be classically spooky but they are certainly dreary in their dullness. Viewed in this way, the granite gravestones, which in their geologic obstinacy seem to last forever, become testaments to our human inability to let go. While back at Market Street Burying Ground, Abijah and Abigail’s eroding slate markers now seem utterly organic. Rather than ghostly tributes to forgotten dead, the weathered, broken stones are just symbols of life’s inevitable, and natural, return to the earth.