Moeraki Boulders, Koekohe Beach, Otago Coast, New Zealand

The most striking aspect the boulders is their unusually large size and highly spherical shape, with a distinct bimodal size distribution. About one-third the boulders range in size from about 0.5 to 1.0 metres (1.5 to 3 ft) in diameter, the other two-thirds from 1.5 to 2.2 metres (4.6 to 6.7 ft), the majority being nearly to almost perfectly spherical. A minority them are not spherical, being slightly elongated parallel to the bedding the mudstone that once enclosed them.
Neither the spherical to subspherical shape or large size the Moeraki Boulders is unique to them. Virtually identical spherical boulders, called "Koutu Boulders", are found on the beaches, in the cliffs, and beneath the surface inland the shore Hokianga Harbour, North Island, New Zealand, between Koutu and Kauwhare points. Like the Moeraki Boulders, nearly perfectly spherical Koutu Boulders are as large as 3 metres (9 ft) in diameter.
Similar boulder-size concretions, known as "Katiki Boulders", are found on the north-facing shore Shag Point some 12 miles south where the Moeraki Boulders are found. These concretions occur as both spherical cannonball concretions and flat, disk-shaped or oval concretions. Unlike the Moeraki boulders, some these concretions contain the bones mosasaurs and plesiosaurs.
Large spherical concretions, similar in size and shape to the Moeraki Boulders have been found elsewhere in the world. For example, large spherical concretions as large as 3 metres (10 feet) in diameter are along the Cannonball River within Morton and Sioux Counties, North Dakota. Large spherical concretions as much as 4 to 6 metres (12 to 18 feet) in diameter occur within sandstone outcrops the Frontier Formation in northeast Utah and central Wyoming. Similar somewhat weathered and eroded giant spheroidal concretions, as large as 6 metres (18 feet) in diameter, are at Rock City in Ottawa County, Kansas. Smaller spherical concretions are found on the shore Lake Huron near Kettle Point, Ontario, where they are known as "kettles".