Can you provide some references to that. Also I've never before heard about Christian crusades earlier than the late 11th century one started by pope Urban II. Perhaps you are thinking about peoples like the Wends in Arkona (Slavic "pagan" tribe in northeastern Germany), who were force converted into Christianity in the 12th century. If they somehow were worshiping the stones in question, then that sounds more like retrofitting existing ancient monuments into their own religion, rather than keeping some ancient knowledge alive.