On the east and west side of the Main Library, limestone was used for its base building material. All sedimentary rock is fossiliferous, commonly known as limestone. A magnifying glass will help with seeing the fossils. Usually, small shells can be seen if you look closely. On the west side, about fifty feet down is a “rust stain” where iron pyrite was deposited in the sediment. The brown stain is where the iron has rusted.