The phrase "canyon walls" refers to the steep, vertical sides or cliffs that make up the sides of a canyon, which is a narrow gorge formed by erosion over time. Full definition
By 2002, the lake level had dropped far enough that the exposed canyon walls created a pale outline around the lake. (earthobservatory.nasa.gov)
Excellent visibility with both dramatic canyon walls and caves through the coral formations. (ambergriscaye.com)
Valley of Fire State Park See petrified logs dating from the Triassic and sandstone «beehives» resembling apartment buildings that may have provided shelter to the area's early inhabitants, who left petroglyphs on canyon walls. (discovermagazine.com)