For three months, workers carved Marys Rock Tunnel through six hundred feet of solid granite (granodiorite), the greatest construction challenge on Skyline Drive. Twice each day they drilled forty holes, each twelve feet deep, into the tunnel's rock face and filled them with five hundred pounds of dynamite before detonation. After each blast, crews spent hours clearing loose boulders and stone and rolling them over the side. Three eight-hour shifts of about fifteen men each kept the machinery working continuously except on Sunday, and every day the blasts removed fifteen or more feet of solid rock. In January 1932, the tunnel broke through to daylight, and almost immediately venturesome sightseers drove through. Although some questioned whether the tunnel was really needed or built partly for show, area historian Darwin Lambert said it seemed justified because it eliminated extensive scars and expensive rock retaining walls.
