See the context of this sign.

Aerial Tramway from Mexico to Texas

From 1915-1919, an aerial tramway spanned the desert landscape. Carrying zinc,
silver, and lead ore, the tramway started at the old Puerto Rico Mine above
Boquillas, Mexico, and terminated in the Ernst Valley, a six-mile-long journey.
Four 6-mile-long cables were strung between massive wooden towers and
could carry 90 buckets at a time, delivering 7 tons of ore per hour to the
discharge terminal in the Ernst Valley.

From the Ernst Valley terminus, it was an 85-mile wagon or
truck trip to the railhead in Marathon, Texas. Cables are still
visible on the ground across the road. A short trail behind
this exhibit leads to the foundations of the first tower on the
north side of the Rio Grande.

Don't miss the rest of our virtual tour of Big Bend National Park in 1083 images.