Gravel Wave

Exposure structure refers to the structure formed in the condition that the surface of deposited sediments is intermittently exposed to atmosphere. The exposure structures frequently seen in the Dunhuang Geological Park are mainly mud crack and rain mark.

Mud crack

It develops on the argillaceous surface of Yardang body. The surface of deposited argillaceous sediment is exposed to the atmosphere, on which regular crack develops due to the dry condition. Then the crack is filled with overlying sediment filling.

The shapes of mud cracks (Figure 4-22) on the plane are mainly polygons, which show a "V" shape on the flat surface. Mud cracks generally reflects the environment transferring from water to exposure, which corresponds to the environment in which water level of lake and flood land is easy to change.

Figure 4-22 mud cracks

Rain mark 

Rain mark is one kind of planar exposure structure in the sedimentary rocks, which refers to the round or oval recesses produced in the condition that rain falls on the surface of unconsolidated fine grained sediments. The depth of recesses varies from 1 mm to 3 mm, with the diameter varying from 1 to 5 mm. Rain mark occurs at the top of the muddy layer, through which the stratigraphic sequence can be determined. It is of the same sedimentology significance of rain mark as that of mud cracks.