About 15,000 years ago the final of four glacial advances stopped in central Connecticut and a large end moraine was established. As the glacier retreated a large lake formed in what would become the Connecticut River Valley. Varved (seasonal) clay/silt layers were deposited. Debris (fossils, sticks, leaves, etc.) carried in with the clay/silt became nucleation points for calcite cementation and concretions grew in the clays. These unique concretions are now being exposed by erosion. The locals call them “Fairy Stones” or “Mud Babies”.