It's known as Ten Box Shelter. Built by the WPA in 1940, it served as the location of the last of a line of ten locked phone boxes used by park guards.
In August of 1986, two of my best friends from high school, Chris Brown and Brian Bowers, visited me in Philadelphia. I was living in the Art Museum area then, but had already fallen in love with the Wissahickon. I thought it would be a great idea for us to take the R8 train line all the way out to Chestnut Hill, hike the mile or so to the northern most entrance to the park, and then walk the ten or twelve miles back to my apartment.
It was a typical day in August for Pennsylvania, hot and humid, cranking up to the inevitable climax, a horrendous thunderstorm, which hit us exactly as we rounded the bend toward Ten Box Shelter. We dove inside to wait it out. As soon as we got inside the rain came down in torrents. Gusts of wind bent the surrounding trees. At the height of it all we couldn't see more than fifteen or twenty feet beyond the shelter. Then it all broke as suddenly as it had started. The sun was out. Birds sang. We kept going.