The picture below is from Lester Park, a small roadside rock outcrop a few miles west of Saratoga Springs, NY (here's a brochure from the NYS Museum with directions). Note the Swiss army knife for scale.
The fossils visible here on this bedding plane of 500 million-year-old limestone (the Cambrian Hoyt Limestone, to geologists) are called stromatolites. In life, these fossils were made by mats of colonial cyanobacteria - they're sometimes called blue-green algae but they're not algae.
Here's a picture of modern cyanobacteria from Wikipedia.
Looks like pond scum. Even though it's bacteria, it photosynthesizes - water, carbon dioxide gas, and sunlight are used to create sugars and release oxygen gas as a waste product.
Some 500 million years ago, this area of New York was submerged under warm, shallow water (New York was around 30 degrees south of the equator at this time). On the seafloor, mats of cyanobacteria happily photosynthesized. With ocean currents sweeping seafloor sediments around, sediment grains stuck to the bacterial mats. The bacteria grew up to the top of the sediment layer. They were buried. They moved up. This occurred over and over again to form mounds of finely-laminated structures which eventually became fossilzed as stromatolites.
The first picture showed an eroded rock layer which cut a horizontal plane through the stromatolites. Below is a picture showing a cross-sectional view as well.
Given their shape, these fossils are sometimes called "cabbage heads."
Besides being neat fossils, what's the significance of stromatolites? Well, they date back to almost 3.5 billion years - the oldest fossils on Earth - and are still around today. They were the dominant reef-forming organisms for a couple of billion years until the evolution of multicellular life (while colonial, cyanobacteria are single-celled without a nucleus). While rare today, they still exist in a few environments - most famously in a location called Shark Bay in western Australia (picture from Wikipedia).
Even more importantly, at least for us, is that cyanobacteria are mainly responsible for Earth's atmosphere changing from one dominated by carbon dioxide gas (like modern-day Venus and Mars), to one with 21% or so molecular oxygen. We can actually see oxygen levels rising during the reign of cyanobacteria in Earth's ancient oceans since oxygen is so reactive and leaves evidence in the minerals and rocks preserved from those times.
If it wasn't for that ancient "pond scum," we wouldn't exist.