Back when a large sea covered much of Ontario, the sea floor became littered with the skeletons of marine organisms. Over time, this sediment, which contains both minerals and organic material, became cemented together. That's how sedimentary rock is formed.
You will often find skeletons in sedimentary rock but there isn't anything significant in this one. Something interesting about it is, as it was formed in layers, it fractured that way. I found this rock exactly as you see it, looking like it had been neatly sliced by someone. No doubt, water seeped into the layers and a winter freeze, or maybe more, expanded the water. It's difficult to believe water can cleave rock but it does.
This specimen is ordinary limestone, a type of sedimentary rock consisting of more than 50% calcium carbonate.
Photos from the iPad of 
@kansuze