That "extra hour fighting the flow" hit home, we had the same mess on a side walkway last summer and ended up hand bucketing the last few yards. Funny how the pump always fits fine until you actually need it to.

Did the washout cut clean enough to show you the actual foundation line, or were you just matching up rocks that happened to fall in a row? Because I've had false starts with old maps before, and the difference between a real buried wall and a natural stone alignment can be really hard to tell, even with coordinates. The pottery sherds are a solid clue though, since that kind of stuff doesn't usually end up in a random line by accident. What kind of pottery was it, plain utility ware or something decorated? That might tell you if the wall was part of a living area or more of a work space. Also, any chance you marked that spot before the next storm washes it all away again?

That 20 minute hunt hit me hard, I've been there too.