Cohesive sediments behave quite differently from cohesionless sediments, and a unique set of bedforms are created in cohesive sediments, as discussed below.

Why are cohesive sediments cohesive?

Fine-grained sediment (clay and fine silt) behaves cohesively because unsatisfied electrostatic charges on the surface of clay and silt grains create attractive forces among particles. These forces also exist for larger particles (e.g., sand, gravel), but in the case of the larger particles the momentum of grains during movement far outstrips the electrostatic charges among particles. Because clay and fine silt grains are so small, they have little momentum during transport (momentum is mass-dependent), and the electrostatic attractions among particles cause them to behave cohesively.

In addition to the importance of electrostatic charges, fine-grained sediments may resist erosion because the size of the particles is such that they may reside entirely within the viscous sublayer, providing an additional means of "protection" against erosion.

I) Bedforms formed by a moving fluid over a cohesive substrate:

Two broad classes of bedforms are created by a moving fluid over a cohesive substrate: 1) bedforms created by the fluid itself (flutes, gutters); and 2) bedforms created by the impact of some object (a "tool") against the substrate.

a) erosional bedforms created by the fluid: These include flutes and gutters. Flutes are erosive structures that are produced by movement of a fluid over a cohesive sustrate. Flutes typically have an elongate tapered shape with a higher relief upstream "head" and a downstream "tail" that has less relief. The characteristic shape of a flute is caused by flow separation at the upstream lip of the flute and flow reattachment downstream at the lowest point in the flute. Flutes come in a very wide variety of shapes, probably because they typically form from erosional modification of some "imperfection" on the substrate. Gutters are created by the interaction of a fluid vortex along the substrate and are typically associated with highly turbulent flow during storm events. Gutters are elongate "gutter-shaped) features that, unlike flutes, are not usually strongly asymmetric when viewed in cross-section parallel to flow direction.

This shot depicts a set of flute casts on the sole of a bed of Cretaceous sandstone in northwestern China. Notice the well developed "heads" of the flutes. Flow was from upper right to lower left. Lens cap for scale.
 
 

These samples show a variety of flute cast morphologies, all from the same bed. Each of the samples is about 10-15 cm across and each is from a Cretaceous lake deposit in Mongolia. Each of the samples is oriented so the flute casts show flow from top to bottom of the photograph.
 
 

The yellow field book in this shot is resting on the sole of a bed of Carboniferous sandstone in northwestern China. The elongate features (oriented approximately horizontally) are a series of flute casts. Flow orientation is difficult to determine from this photograph, but is probably from right to left.
 
 

This unusual set of sole markings occurs in a Carboniferous sandstone in northwestern China and may represent the erosion created by interaction of a helical vortex and the muddy substrate.
 

Here's a spectacular shot of some flute casts on the sole of a Pliocene turbidite in the upper part of Split Mountain Gorge, Salton Trough, southern California.  Notice the very well developed upstream 'heads' of the flutes and the downstream tails of the flutes.  The erosive current that produced these flutes flowed from upper right to lower left.  Photograph by Art Sylvester (UC, Santa Barbara).
 
 

These sole markings are found in the Cretaceous Sites Formation of the central California Coast Ranges. Both groove casts and flute casts appear to be present. Flow was from bottom of the photo to top of the photo. Hammer on the left hand side for scale.
 
 

This is an outcrop of the Jurassic Carmel Formation in southern Utah. The lighter tan colored rock is sandstone and the red rock is shale. Shown in the lower central part of the photo is a Swiss army knife for scale. The knife is resting on a gutter cast that protrudes out of the outcrop and is made of sandstone. Though it is difficult to see in this photograph, the prominent bed of sandstone at the top of the outcrop is hummocky cross-stratified; the association between gutter casts and HCS is fairly common on storm-dominated shelf environments, as this is interpreted to be.
 
 

Close up shot of a single gutter casts in cross section. The gutter projects into the outcrop. There is a lens cap on the upper left portion of the gutter cast for scale. Notice that the gutter clearly cuts into the underlying mud and that the sandstone forming the gutter cast itself is completely encased in shale.
 

b) Tool Marks: These include a wide class of erosional features that involve the interaction with some object (a "tool") on the substrate. Tools may include sticks, clasts, skeletal material, etc. Tool marks may include skip or bounce marks (formed by an object coming into repeated contact with the substrate), groove marks (formed by an object carving out an elongate groove from the bottom), and prod marks (formed by an object prodding the substrate). In many instances, tool marks are further modified by erosion by the moving fluid.

Many of the erosional bedforms discussed above occur on muddy shelf environments during storm events. For a good discussion of erosion and deposition during storms, check out the following article, posted on the UM G432 ERES site:

Myrow, P.M., and Southard, J.B., 1996, Tempestite deposition: Journal of Sedimentary Research, v. 66, p. 875-887.
 

II) Bedforms created by shrinking or volumetric modification of the cohesive sediment:

In addition to bedforms created by a moving fluid over a cohesive substrate, the expandable nature of the clay minerals in a cohesive sediment may create mudcracks. A variety of crack morphologies are possible, depending on the means by which the crack developed. It’s important to remember that post-depositional compaction of all crack types can modify their original morphology.

Most common are desiccation cracks, formed by drying of wet mud and associated shrinkage as the individual clay plates lose volume. Desiccation cracks typically taper downward in cross-section and are commonly filled with coarser-grained sediment. The desiccation polygons that result may curl up at the edges with sufficient drying and may subsequently be reworked as mudclasts.

Here are a few photos of modern and ancient desiccation cracks:

These modern mud cracks occurred in a backwater portion of the San Juan River in southeastern Utah. Most prominent are the wide desiccation cracks that taper downward. In addition, there are some thin, straight cracks that don’t seem to be forming desiccation polygons. These were probably formed by freezing of the pore fluid in the mud and subsequent melting. This photo was shot early in the morning after a cold night in which the air temperature dropped well below freezing.
 
 

Modern desiccation cracks and polygons ona dry playa lake deposit.
 
 

Modern desiccation cracks and polygons on an exposed playa lake surface in southern Mongolia. Notice that the edges of the polygons are upturned. Polygons such as these are commonly reworked in the rock record as mudclasts.
 
 
 

Some ancient desiccation cracks in Permian strata of northwestern China. This bedding plane exposure also has at least three joint sets that can be confused with the desiccation cracks. The joint sets are straight and parallel.
 
 

This photo depicts a section of red shale and tan sandstone from the Jurassic Carmel Formation in southern Utah. The continuous sandstone beds have symmetricallly rippled tops, suggesting deposition in shallow water. The red shale contains an abundance of mudcracks, shown here in cross-section. Notice that the cracks are filled with the coarser tan sandstone and taper downward.
 
 

In addition to cracks that form by desiccation, cracks can form by subaqueous expulsion of water (synaresis cracks) and movement of gas bubbles through fine-grained sediment as a result of decay of organic matter. Synaresis cracks typically consist of a single crack or sets of three radiating cracks that do not form polygons as with desiccation cracks. Gas expansion cracks may have very complex morphologies in cross-section, as gas bubbles work their way to the sediment-water interface. For a good discussion of gas bubble and expansion cracks in the Middle Proterozoic Helena Formation in western Montana, check out the following articles are now posted on the UM G432 ERES site:

Furniss-George; Rittel-John-F; Winston-Don, 1998, Gas bubble and expansion crack origin of "molar-tooth" structures in the middle Proterozoic Belt Supergroup, western Montana: Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes. 68; 1, Pages 104-114.

Pratt-Brian-R; Winston-Don; Rittel-John-F; Furniss-George, 1999, Gas bubble and expansion crack origin of molar-tooth calcite structures in the middle Proterozoic Belt Supergroup, western Montana; discussion and reply: Journal of Sedimentary Research. 69; 5, Pages 1136-1145.