View Full Version : Geology - prehistoric lakes

12-18-2011, 03:44 AM
Back in high school, I remember my geography teacher telling us that the land we were living on (Southwestern Ontario, nestled in between three Great Lakes) used to be underwater. And we also, of course, learned that the area had been glaciated.

I swear that the teacher told us that the prehistoric lake was called Lake London, but I can find no reference to that online, so possibly either the teacher or I (or both) are crazy. I also can't find any real reference to the fact that the area used to be underwater.

But I really hope that it was! I have a character who's trying to make an impassioned argument for the sanctity of the area's farmland in the face of an oncoming gravel pit. I want him to say something about how the area used to be a lake bed, and that's why it's so flat and fertile. But for that to make sense, the lake would have had to come AFTER glaciation, right? Because the glaciers would have dumped the gravel that the quarry guys are after, and then the lake silt would have built up on top of that to be farmland?

Am I even close to anything that makes sense, here? I'm not being too specific about geography in the story, so this doesn't have to relate to the mythical Lake London. I think basically what I need to know is the likely geological history of an area that has a layer of gravel covered by a layer of rich, flat farmland. Geological events, approximate timelines... whatever you've got!

Thanks for any help.

12-18-2011, 05:33 AM
Glacial lake - here's a link that references it. Puma


Even better - look under Warbler Woods in this one


12-18-2011, 06:10 AM
If i remember right, this is an old theory from the 1700 and 1800 hundreds in which the Blue Ridge and Appalachian Mountains formed a barrier from Nova Scotia to Alabama that trapped an inland sea in the Ohio, Indiana, Illinois area up into Canada. This was later discounted.

Lake London might be a Canadian thing referring to this great inland sea of North America. Not sure...

12-18-2011, 06:37 AM
Excellent sources, Puma, and I'm relieved to see mentions of Lake London! But the first one was a bit geology-dense for me. I got the lake part, but I was a bit confused by the relationship to the gravel deposits. It sounds like the gravel left by the glaciers was 'fluvial gravel' while the best building gravel is 'deltaic gravel'? I assume deltaic gravel is from river deltas, so is it a subcategory of fluvial gravel? Or is the gravel that would have been left by the glaciers not actually all that good for building?

Or, perhaps I could just impose on you to fact-check what I've written in my story? My character is a local farmer/carpenter, so he isn't expected to be a geology expert, but I don't want him lying, either. I've got him saying, “There used to be a lake here. You know… prehistorically. Just after the glaciers dumped all that gravel you guys are after. But the lake covered it all, and over the years, the sediment built up as the lake turned into a swamp, and then dry land. Flat, fertile land.”
Is there anything there that's obviously wrong?

Thanks again!

Histry Nerd
12-19-2011, 07:04 AM
Captcha -

I'm no geologist, but I don't see anything wrong with what your guy says above, especially if that's the extent of it.

And on the subject of "deltaic" vs. "fluvial" gravel: if deltaic gravel is found in river deltas (which seems reasonable), it would be rounded and smoothed from traveling down the river bed. Fluvial gravel, having been crushed, picked up, and transported by a glacier, would be rough and sharp-edged, and therefore likely to cause stress points in any concrete it was mixed with.

Just a SWAG* from a guy who had one course in geography (which included a few geologic excursions) and five in civil engineering almost twenty years ago. Take it for what it's worth.


*Scientific Wild-Ass Guess

12-19-2011, 06:09 PM
Deltaic gravel is gravel washed down rivers and deposited in the deltas - along with all the other nutrients washed out of the land upstream. Nutients would include fertilizers, plant material, dead fish and other wildlife, etc. Fluvial gravel is also river gravel, but it would not have as much silt and nutrients deposited with it. Think about what the bottom of a river would look like if you drained off the water.

As far as the type of stone in the gravels - around here (Ohio) glacial gravel is very nicely rounded and smoothed from all the abrading it received while it was being transported by the glacier. There are deposits that are quarried for concrete plants and even highway sub-surface. The river gravels here have a mix of some glacial gravel that has washed out of the fields along the rivers - and - a large percentage of broken bedrock from the rivers - in our case, broken up chunks of limestone that are angular and frequently flat slabs instead of rounded. It's fresh rock in comparison to the glacial debris.

So, you'd have to look at what your "bedrock" is around London, Ontario to determine what the fluvial gravel would be made up of - it could be granite. Then you need to look at what glaciated rock looks like around there - if you walk out into a cultivated field - are there rounded rock pieces lying around? And, you probably ought to also look at the source / course of the river to get an idea how rich the deltaic deposits might be.

Hope that gives you some ideas of where to go to substantiate your idea. Puma