Spreite

They are formed by invertebrate organisms tunneling back and forth through sediment in search of food.

[4][5] In essence, the organism "escapes" from being buried too deeply by progressively tunneling upward, thereby leaving behind it a stacked vertical succession of concave-side-up spreite.

[8] Spreiten differ from menisci, which are flat to slightly curved, back-filling structures found in some tubular burrows.

Menisci are formed when the organism packs sediment, and sometimes fecal material as well, behind itself as it moves through the burrow.

[9] For example, generally parallel-sided, elongate, horizontal burrows with menisci are typically assigned to the Muensteria ichnospecies, whereas similar burrows with spreite will generally be assigned to the Rhizocorallium ichnogenus.