Update: This is the Captain James Campbell House, and it holds an important place in St. Louis regional history. The famous story of Archer Alexander, an escaped slave whose story was later chronicled by William Greenlead Eliot, apparently helped build this house before the Civil War. To read more about Campbell and Alexander’s link to this house, see this post by Dorris Keeven-Franke at St. Charles County History. It seems to be one of the oldest houses in the region, dating to perhaps 1830.
I don’t remember exactly where this house was, but I believe it was along Boonslick Road, where there was pretty much no good place to stop and take a picture. It’s a stunning example of the Greek Revival, however, with the central portico and symmetrical design. This was is rough cut stone ashlar, too, making it all the more interesting and rare.