A french drain will be around the footing of your house, so 6-8 feet down makes sense, and if it is 12-18 inches, you are seeing the gravel rocks that are part of it. It's possible it was put in for surface water drainage due to no where to channel rain water? Mine had a downspout directly into the french drain, which was a BAD IDEA. Some times these old houses just have weird things like that.
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