
They are the peculiar concrete markings that have been spotted all across America, prompting hikers and travellers to wonder what on Earth they could mean and where they are pointing to.

But these giant arrows, which can measure up to 70ft in length, are actually a forgotten remnant of a bygone age.

Dotted among scrubland, placed in seemingly random spots, these huge arrows were once markers for early airmail flights across the US - forming the first land-based navigation system in the world.

A huge concrete arrow is found in perfect condition in Bloomington, Washington County, Utah

The Bloomnigton arrow was one of 102 found by retired couple Brian and Charlotte Smith, who have travelled across the US to photograph the monuments

The concrete arrows were placed at the base of lit beacons near airways, showing pilots the direction they needed to fly in to reach the next stop-off to deliver mail.

Airmail in the US began as early as 1911, with the first official flight departing Petaluma, California, and arriving in Santa Rosa, California. As the flights got longer, with more frequent stop-offs, so the beacons and arrows were placed to help pilots on their way.

Following World War One, the US Postal Service began to use surplus war planes for mail delivery and many were flown by former army pilots.

The arrows define a moment in history at the beginning of the 21st century when the US Postal service began delivering air mail

Perfectly set against the snow-capped mountains of Nevada, this arrow was photographed in Golconda, Humboldt County

Captured near the tiny town of Medicine Bow in Wyoming, this beacon was on the famous New York to San Francisco route

One of the most important flight routes and the first to be completed, stretched across the US from New York to San Francisco

The giant arrows started to be placed across the country, sometimes at 10-mile intervals, from 1924. Painted bright yellow and placed alongside beacon with a gas light at the top, the idea was that the markers could be seen for a distance of up to 10 miles so pilots could find their way.

In the summer of 1924, they stretched from Wyoming to Ohio and by the following year, the arrows had reached New York. By 1929, the arrows could direct flights all the way across the width of America.

The main New York to San Francisco air route stretched from New York to San Francisco, following the yellow concrete trail.

Medicine Bow town in Carbon County has a population of just 284 and was originally established due to the construction of a transcontinental railroad

Not all the arrows are in good condition, with each passing winter more cracks appear in the concrete, such as with this arrow in Mote, Nevada

Pumpernickle n Nevada has its own concrete arrow too, which one again is overlooked by the state's beautiful mountains

In the days before radio and satellite communications, the arrows were a way to ensure pilots could find their way, even in poor weather.

While the arrows are now long-forgotten, with many lost forever, there are fans who, having stumbled upon an arrow or two in the countryside, have started mapping those concrete markers that have been left behind.

Retired couple Brian and Charlotte Smith were sent an email which piqued their interest and have been hunting down the arrows ever since.

The email playfully explained: 'Every so often, usually in the vast deserts of the American Southwest, a hiker or a backpacker will run across something puzzling: a large concrete arrow, as much as seventy feet in length, sitting in the middle of scrub-covered nowhere.'

The couple say they have found 102 arrows so far and have set up their website Arrows Across America as part of their photography site dreamsmithphotos.com so fellow fans can share their own pictures and help map where the remaining arrows are.

The Smiths told MailOnline Travel: 'We have located 102 arrows or portions of arrows, some are badly deteriorated and not much is left.

Arrowheads and ancient earth mounds have been discovered at Poverty Point, in Utah, but it also has a more recent concrete arrow

A beacon has survived in good condition in New Mexico (left) and at Cottage Grove in Oregon (right) - many of these were lost as the metal was used in the war effort during the Second World War

At Thayer Juntion , in Wyoming, there are boards that supply information about the arrows and their use to passers-by

'Our interest grew out of an email circulating in August 2013, we wanted to know where they were, and set out to find them.

'We have managed to take photos of approximately 40, and have a trip planned this fall to take more photos of some that we were unable to get to last spring.

'Eventually we hope to have photos of all the arrows that we have located, but time and money will dictate how long that will take us.