An algal mat is a layer of usually filamentous algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 on marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 or fresh water

Fresh Water

Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

 soft bottoms. It may be considered one of many types of microbial mats

Microbial mat

A microbial mat is a multi-layered sheet of micro-organisms, mainly bacteria and archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces but a few survive in deserts. They colonize environments ranging in temperature from –40°C to +120°C...

. 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

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and sunlight

Sunlight

Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

. Stromatolites can form in this way (see Shark Bay

Shark Bay

Shark Bay is a World Heritage listed bay in Western Australia. The term may also refer to:* the locality of Shark Bay, now known as Denham* Shark Bay Marine Park* Shark Bay , a shark exhibit at Sea World, Gold Coast, Australia* Shire of Shark Bay...

 for a modern example). Any puddle of water can start the process - sunlight initiates the process and photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 soon creates a green layer within the puddle. The puddle can dry up and leave a thin layer of 'algal' mat and even leave mudcrack

Mudcrack

Mudcracks are sedimentary structures formed as muddy sediment dries and contracts.- Formation :...

s 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.