Algal mat

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An algal mat is a layer of usually filamentous algae on marine or fresh water soft bottoms. Algae and cyanobacteria are ubiquitous, often forming within the water column and settling to the bottom. In shallow environments, they are often dessiccated and revived with the next introduction of water and sunlight. Stromatolites can form in this way (search Shark Bay for a modern example). Any puddle of water can start the process - sunlight initiates the process and photosynthesis soon creates a green layer within the puddle. That is all that is necessary. The puddle can dry up and leave a thin layer of 'algal' mat and even leave mudcracks behind. Cyclicity involves the repeated process of wetting (rain), algal production, and redrying of the new deposit. This results in layer upon layer of algal mats and intervening thin layers of mud.

In the fossil record, many examples have been uncovered including sediments with the mudcracks preserved and even raindrop impressions preserved. If some animal enters the process, even simply a fly, it has a chance of being preserved within the deposited sequence. It may be more likely, however, to find something with a more easily preserved shell, for example a snail.