Niagara Gorge (the area above the Falls) is around 12,000 years old, cut into Ordovician and Silurian rocks (mainly dolomites, shales, sandstones, and limestones) as well as (in the Upper Gorge) Wisconsin glacial deposits. The Falls have eroded back 7 miles toward Lake Erie in the last 12,500 years, although because of decreased flow the erosion rate is now only about 1 ft/year. On a normal day, the power plants in the Gorge above the Falls restrict the flow of water through the Gorge to about 75% during the day in peak tourist season, and 25% at night and off-peak. (This actually reminded me of something that I commented on when I first visited Niagara Falls at about age 10 - I told my parents that it seemed strange that the Falls were "on all the time" and didn't get turned off at night, like other tourist attractions I'd been to. Ironically enough, the Falls can be turned off at night - and almost are!)
Our field trip began at Whirlpool State Park on the American side, overlooking Niagara Glen (a great place for bouldering, if you're into that and have a passport). We spent a little time discussing the history of the Falls (including how the bend in the river occurs because the river hit the buried gorge and started cutting into glacial deposits that had filled a paleo-Niagara River system, formed >45,000 years ago). Then we climbed down a lot of stairs and switchbacks to get to the trail, which runs along the river to the Whirlpool. Along the trail, you can see lots of talus from the cliffs, the jetboats on the river (which are the only boats allowed in that part of the Gorge), and a place where the water is flowing in two directions at once, thanks to the Whirlpool. (Here's some more cool trivia: under natural conditions, the Whirlpool circulates counterclockwise, but when the power plants take the river down to 1/4 flow rate, the Whirlpool reverses and flows clockwise.) The river is about 160 feet deep in that part of the Gorge, possibly 200 feet deep in the Whirlpool (although this is unconfirmed as of yet).
Now for the photos! I was using my cheap, "I won't be devastated if this accidentally gets dropped in the dangerous and unswimmable river" digital camera, so they're not as good a quality as I'd like, but they shouldn't be too bad.
Here's the group at the first overlook, looking roughly to the north at the Niagara Glen and one of the two power plants.
Looking south along the river at the Whirlpool.
The spot on the river where the (surface, at least) water appears to be flowing in two directions at once. It's hard to see in this photo, but I believe the water on the near side of the river is flowing to the left (downstream), and the water on the far side is flowing to the right (upstream).
Looking out over the Whirlpool, with the lines of the Canadian gondola service overhead.
Silurian shales and limestones in the cliffs above the bend in the river.
The rapids leading into the Whirlpool. These are Class VI - the most dangerous kind. More than 2800 cubic m of water/second go through these at peak flow, compared with 1700 cubic m/second on the Colorado River's Lava Falls Rapids. You'd have to be pretty stupid to try and navigate these, because not only is it incredibly dangerous, it's illegal for any boat to be on this part of the river. There was, at one time, a rafting company that planned on taking people down this stretch of the river; on their maiden voyage, their boats capsized and three people were killed. This is also the location of North America's largest series of standing waves. This is an awesome place to be in, but also quite humbling - one misstep on a slippery rock, and you're toast, because there are no rescue boats on this part of the Niagara River.
A close-up of the wave. (Standing waves are constantly in motion - in this case, always crashing in the same spot. )
Here's a video of the waves crashing.
There is more to look at than just the water, however. The bench we stood on is made up of Whirlpool Sandstone, which is possibly Ordovician or Lower Silurian in age (there are no fossils in the sandstone to date it with, although the rocks above are Middle Silurian and the rocks below are Ordovician). It does, however, contain some neat cross-stratification, shown below (hand lens for scale). The cross-stratification is small in scale (a few cm or tens of cm) and unidirectional, and in some places ripple marks are also preserved.
Looking downstream at more cliffs and rapids (and the first of the oversized hotels). These are perhaps slightly more navigable, although again very stupid and illegal. (I don't really know which country would send someone down there to arrest you, though - maybe they fish you out later, if you survive.)
And that's it for now. Next weekend I'll be TAing a trip to the Falls themselves, where we'll likely get to see some very cool fossils. (I'm starting to turn into a paleontologist up here - limestone everywhere!)