Chambered cairn at Cairnholy, the area in front of the wall and Sentinel Stones is believed to have been for worship or communal purposes, near Creetown, Dumfries and Galloway, Scotland, United Kingdom, Europe

Chambered cairn at Cairnholy, the area in front of the wall and Sentinel Stones is believed to have been for worship or communal purposes, near Creetown, Dumfries and Galloway, Scotland, United Kingdom, Europe

View of the Berg Lake and Berg glacier, Rocky Mountains, Mount Robson National Park, British Columbia, Canada

Prohibition sign in a housing estate in Munich, Bavaria, Germany, Europe

A young woman trekking at Sentinel Dome Lookout in Yosemite National Park looking at El Capitan Mountain. United States

A young man on the trek sitting on the Sentinel Dome lookout in Yosemite National Park looking at El Capitan Mountain. United States

Mount Robson, Mount Robson Park, Alberta, Canada

Visitor center, Arches National Park, Moab, Utah, USA

Lettering "Keine Einfahrt", German for "No Entry", sign

Speed hump warning sign, Caution Children sign, Faroe Islands, island group in the North Atlantic, Denmark, Northern Europe

Rock formation, The Sentinel, morning light, Sunset Point, Bryce Canyon National Park, Utah, United States of America, USA

A hiker walks along rocks at Sentinel Dome in Yosemite National Park at sunset.

View of the Great Pyramid and Sphinx of Giza under a blue sky, Giza, Cairo, Egypt

Sentinel, Andy Goldsworthy, 1999, Col du Defens, Alpes de haute Provence, France "The work Sentinels, which is part of the Refuge d'art project, is a journey through the Dignois region, at the foot of the Southern Alps. Such a project associates art, nature and the territory. The three Sentinels are stone cairns, one for each of the three valleys of the Haute-Provence Geological Reserve.