Stromatolites are large, layered structures built up by mats of cyanobacteria. They vary in appearance, ranging from slightly wrinkled horizontal laminations in sedimentary rocks to low mounds to prominent mounds to columnar structures and other forms. Stromatolites are most common in the Proterozoic fossil record. They are scarce today, but famous modern examples occur at Shark Bay, Western Australia.
The pictures linked to below are mostly field photos, but some are museum specimens or samples in my personal geology collection. Most of these photos show stromatolites in cross-section view. The Lester Park, New York stromatolites are seen in plan view - they have been erosionally planed-off.