Moeraki Boulders, Koekohe Beach, Otago Coast, New Zealand

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".
As determined by detailed analysis the fine-grained rock using optical mineralogy, X-ray crystallography, and electron microprobe, the boulders consist mud, fine silt and clay, cemented by calcite. The degree cementation varies from being relatively weak within the interior a boulder to quite hard within its outside rim. The outside rims the larger boulders consist much as 10 to 20% calcite, because the calcite not only tightly cements the silt and clay but has also replaced it to a significant degree.