Words commonly heard by students walking between Sackett Building and Willard Building might be, "What is that?"

No, their eyes are not gazing upon the Willard Preacher; rather, they're directed up at a 33-foot-tall tower of rock.

However, this "tower" is not just a public sculpture in place solely for aesthetic value.

Terry Engelder, professor of geosciences, said this tower, which is more appropriately know as an obelisk, is probably the "longest continuous university experiment still running."

In 1896, Magnus H. Ihlseng, a professor and also the first dean of the School of Mines -- now the College of Earth and Mineral Sciences -- erected the obelisk to test the durability of different rocks.

"There was a general interest in the construction industry concerning the rocks' long-term resistance to weathering," Engelder said. "He [Ihlseng] collected 281 stone blocks from quarries throughout the commonwealth of Pennsylvania."

Engelder said there are also a few rocks from other places like New York, Ohio and Massachusetts.

He said the obelisk can be seen as striped. There are alternating red sandstone layers and gray carbonate layers that create this pattern.

There are also rocks that vary from the common black to the more unusual green.

"The green rock comes from the mantel of the earth. Those are the very deepest of the rocks," Engelder said.

He said green rocks are in fact the most common in the earth, but they're typically not seen because they are from such deep layers.

The obelisk rests on top of a sandstone base and, excluding this base, is arranged in geological order with the oldest rocks near the bottom. It resembles a pyramid shape with four sharpened corners.

"The real experiment is to watch the beveled corner disappear with time," Engelder said. "When this obelisk was set in place in 1896, every rock had that beveled corner. So if the rock does not have the beveled corner now, that means that it's no good for building stone. To my knowledge there's no better monument representing geological time than this particular monument," he said.

This might come as a surprise to many students who never thought about the monument before.

Justin Herman (sophomore-film) and Ryan Roseborough (sophomore-film) said they have no idea what the large tower is. However, after reading the plaque, Roseborough said it's Pennsylvania building rock, and Herman said that all he knows is it's arranged in geological order.

In fact, Engelder said, the rocks are not entirely in geological order.

He said the type of rock that soldiers hid behind on the fields of Gettysburg are known as diabases, and they are the black rocks located near the base of the obelisk.

"A hundred years ago geologists could not date rocks easily, and it was always assumed that diabases -- the crystalline rocks -- were the oldest rocks in the state of Pennsylvania. It turns out though, this really should be on the top of the obelisk because we later knew, this is one of the youngest rocks in the state of Pennsylvania," Engelder said.

He added, "so although it says arranged in geological order, it's to the best of the knowledge of the geologists in 1896, but we later learned that they goofed."