Ribbon-like veins and nodules of calcite
Molar tooth structures are ribbon-like veins and nodules of calcite that are found widespread in Precambrian carbonate sedimentary rocks between approximately 2600 to 570 million years ago. Their mechanism for formation remains debated, with hypothesis including the generation of methane gas within sediments, pumping of water through sediment by wave action, tsunamis, and bacterial processes.
History and etymology.
The first documented observation of molar tooth structures was by Hilary Bauerman in 1884, during mapping of the Rocky Mountains for the Canada-United States border and the Geological Survey of Canada. Their name originates their similarity to the markings of elephants' molar teeth:
 ;templatestyles src="Template:Blockquote/styles.css" / ;"... there are associated with these beds some laminated white and black shales with small concretionary points of carbonate of lime, which pass into an impure limestone, in which the carbonate of lime is intermingled with argillaceous patches resembling the markings in the molar tooth of an elephant."
Physical characteristics.
Molar tooth structures are millimeter- to centimeter-scale microcrystalline ribbons and 'blobs' of calcite within argillaceous carbonate sedimentary rocks, sometimes reaching tens of centimeters in size. The ribbons can be oriented both vertically and horizontally. The sediment matrix that molar tooth structures occur in is generally composed of finely crystalline calcite and dolomite, and fine-grained detrital quartz, feldspar, and clay minerals. The depositional environments that molar tooth structures are found in span from deep waters near storm wave base, to shallow intertidal. 
These structures are known to have formed during very early diagenesis while the host sediment was unlithified (i.e., still soft sediment) because bedding is deformed around molar tooth structures, indicating they formed prior to compaction of the sediment. This is further supported by deformation or fracturing of the molar tooth structures during deformation. Finally, fragments of molar tooth structures are observed as 'rip up clasts' in storm deposits, further supporting an early formation.
Molar tooth structures have been observed in Precambrian strata on all continents except Antarctica. 
Mechanisms of formation.
A range of mechanisms have been proposed for the formation of molar tooth structures.
References.
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