Salem, N.H. — At this leafless and gloomy time of year I traveled, in the spirit of the symbologist Robert Langdon of “The Da Vinci Code,” to America’s Stonehenge, in this town five miles from the Massachusetts border. Scholars have debated whether the stone cairns and chambers here were built by early American Indians, enterprising colonial settlers or, more controversially, a migrant European culture that visited these woods nearly 4,000 years ago. Determined to plumb these mysteries, I arrived at a rustic information center and gift shop on a cold and gray Sunday morning. Inside I was greeted by the...