Answer:
They are layered mounds of deposits made by Precambrian algae.
Step-by-step explanation:
Stromatolites are layered deposits that are made in the shallow water by Cyanobacteria (blue-green algae). This algae originally existed in the Precambrian era. The layer resulted from the sediments that are deposited by the tide on the shore during storms. It is the algae that binds this sediments together, thus forming the so-called "stromatolites." They appear like sheetlike mats. These mats were able to exist without being eaten by small organisms like snails because of the high concentration of salt. This concentration resulted from evaporation, which causes the blue-green algae to survive.
Thus, this explains the answer.
Additional Information
Precambrian time- around 4.6 billion years ago. Its name was derived from the fact that it existed before the Cambrian period. Evidence of life on earth are drawn from the stromatolites that are found in rocks.