El Sillar (the Saddle), located 17km northwest of Tupiza, is where a road straddles a narrow ridge between two peaks and two valleys. Throughout this area, rugged amphitheaters have been gouged out of the mountainsides and eroded into spires that resemble a stone forest. The easiest way to get here is on the 3:30pm Tupiza Tours bus (B$70); it's possible to hire a bike and cycle back to Tupiza for an extra B$80.
The road continues on to San Vicente, of Butch Cassidy and Sundance fame. This entire route is part of a centuries-old trade route. From May to early July you may see a trickle of llama, alpaca and donkey trains (or nowadays more likely camiones, pickup trucks) humping salt blocks 300km from the Salar de Uyuni to trade in Tarija.
