Thanks for the info Auspex! Neat to find areas in Ontario where the glaciers did not scrape off the Pleistocene/Holocene deposits. The fossil will eventually make it's way to the local museum.
Here's some background on the Ottawa fish fossils. At the height of the last (Wiconsian) glaciation the ice over southern Ontario and Quebec was ~3 km thick (as I recall anyway) and the weight of all that ice was able to cause the crust of the Earth to sink a substantial distance, to well below sea level. When the glaciers melted, ~12,000 years ago, the Atlantic flooded in and salt water reached far up the Ottawa Valley as well as down into New York/Vermont, forming a sea called the Champlain Sea (named after Lake Champlain). Since that time the land has been slowly rebounding, reestablishing isostatic equilibrium, with the result that the area regularly experiences small earthquakes, and occasionally ones that can be felt (such as a 3.4 magnitude quake that woke me up once). The extent of the rebound is shown by beach terraces with marine fossils (bivalves and gastropods) at levels as high as 800 feet above current sea level. The marine phase lasted only a couple of thousand years before the sea became fresh water, as inflowing fresh water floated on top of the salt water and eventually displaced it altogether. Eventually the sea drained, except for a small remnant (Lake Champlain) as the land rose. During the early days, though, the sea was fully marine with a fauna nowadays found only in the arctic Atlantic, including arctic bivalves, gastropods, sponges, barnacles, brachiopods, and I even found a starfish (really rare though, only 2 specimens known). There was an extensive fauna, including several species of seals, beluga, bowfin, and right whales, and walrus. All of those are rare, though, and are known from only a handful of specimens. In the deeper basins of the sea, fine sediment settled that formed a thick layer of bluish clay, called the Leda clay from a characteristic bivalve. In a few places, fish (and other things) would settle to the bottom, and likely the deep basin was anoxic as there is no evidence of bioturbation and many (most) of the skeletons are intact, or if partially eaten by a predator the remaining bits are articulated. All the fish are species found today in the arctic North Atlantic. Capelin (Mallotus villosus) are by far the most common; these fish today swim up estuaries and rivers to spawn, then die and was up on shore or drift back out to sea in huge numbers. All other fish are known from only one or two specimens, but they include the sculpins Myoxocephalus thompsoni* and Cottus sp, the rainbow smelt Osmerus mordax*, cisco Coregonus sp.,the lumpfish Cyclopterus, sticklebacks Gasterosteus aculeatus*, Atlantic tomcod Microgaddus*, cod Gaddus, a sucker, and lake trout. The ones I marked with an asterisk are species I collected and donated to the National Museum of Canada, most were the first record as fossil for the species, and some I published in my first paper, in (gasp! can it be 30 years ago?) 1979, when I was a sophmore (2nd year) undergraduate. When the fish settled to the bottom they were covered in fine mud, and apparently as they decayed the bacteria changed the solubility of calcium carbonate in the water (probably by removing the remaining oxygen from the water/sediment around the fish, so the carbonate would precipitate and form a concretion around the fish. Likely this happened very quickly, and helped protect the fish against disarticultion. As a result, many of the fish are in concretions that look like fish, you can see the outline of the fins and everything. Other soft-bodied organisms or bits of organic material also triggered concretion formation, as there are lots of concretions of various shapes with nothing in them. Sometimes you can find an insect, a bit of plant, even a starfish as I mentioned, and there is one specimen of a marten (carnivorous mammal) and 1/2 a seal that were found long ago. These concretions weather from the clay where it is exposed (or used to be) along the Ottawa River and area creeks; many of my specimens came from construction sites that reached into the clay when a whole subdivision was being built. Unfortunately times have changes, and most of the riverbank is "protected" against erosion by riprap now, or rich people have built big houses and docks along the river and don't appreciate you crossing "their" section of the shore. The area is all built up now, so I don't see much chance for large-scale construction. There are still exposures along the creeks and part of the river bank; they are under control of the National Capitol Commission Parks Division, and I was told that collecting is not allowed, but last time I was there I didn't see signs posted and I don't know what the rules are for certain. So Pleecan, the glaciers didn't scrape off the deposits because they date from just after the last glaciers.
I think it's just amazing that, for a thousand years or a bit more, the Atlantic came all the way to the Ottawa Valley, and whales swam where the Parliament Buildings now stand. And not that long ago; 12,000 years ago in the Fertile Crescent people were just starting to settle down in the first permanent villages.