|Since these are domes, way-up is toward the top of the photo.
You'll see what looks like numerous parallel series of stacked parentheses. Let's zoom in on them:
|Note that the column near the center splits into two smaller columns going up.
|There's been just enough weathering to make the layers stand out nicely.
The stacks aren't necessarily separate columns, because if you look closely you can see that layers can continue from stack to stack, but "column" is a handy quick descriptor. In life, the colony would have had a lumpy upper surface composed of a number of distinct but connected domes. We can see this in upper and lower surfaces of stromatolitic blocks.
|In this case, weathering has partially removed some of the uppermost layers, "dissecting" the tops of the domes.
|This photo, taken along the edge between an upper surface and a side, shows how the stacks translate into domes.
I am completely comfortable interpreting these lumpy stacked colonies as Stauffer's Cryptozoon rosemontensis, which he named from the Shakopee Formation not too far away. Almost all of the stromatolitic blocks I saw fall under this category, but there were a couple of examples that did not have stacks, instead having large hemispherical structures. These are much more typical of a variety named Cryptozoon minnesotensis.
|These two photos show a much different flavor of microbial colony immortalized as stromatolites. A shows multiple lobes, while most of B is one big dome. These are in the vein of Cryptozoon minnesotensis.
Some of the blocks are well-preserved, even though they are now exposed to the elements. Others are now weathering, breaking down either by spalling at the tops of the dome stacks or by the stacks themselves beginning to fall out. The difference may have to do with how much sand was incorporated into the colony.
The Prairie du Chien Group is not noted for its non-stromatolitic fossils, although I have seen some snails in the Shakopee elsewhere. These blocks did not change that overall impression. There were a couple of cases of burrows or things that look very much like burrows, though.
|A couple of examples of burrows or burrow-like features. A has a long feature from the lower left to near the center, plus other shorter similar features. B has what looks like a web of burrows.