Concretions "kettles", sedimentary rock, Kettle Point, Ontario. Paleozoic Upper Devonian, 370 million years ago, Kettle Point Formation. The concretions are known as kettles and are found in shale along the shore of Lake Huron at Kettle Point. The kettles were formed while the mud which formed them was still soft. Bacteria in the mud caused sulphate reduction and the precipitation of concretions. The crystals formed in a radiating pattern, from the centre out, creating a ball these can range in size from 30 cm to 1.5 m across. The pieces in the geological garden are fragments from a ball.
Marcasite/iron sulphide concretions also formed in the same brown shale. They have a rusty brown surface and when broken, the pale white-gold metallic interior is seen. The marcasite nodules grow to about 3 cm, in diameter. As the mud, under pressure from overlying rocks lost water, the rock hardened and reduced in volume. When seen at Kettle Point the concretions look as though the layers of shale have wrapped around them. When the kettles are studied the original layering in the mud may be seen.
