Finding a Mongolian section of the Genghis Khan Wall made of many saksaul branches presented an unexpected opportunity.

By radiocarbon dating, we could discover when the saksaul was cut and used.

I collected three samples from the same location, where the Wall stood about 2.5 meters high.

Sample A was a fragment from a thick saksaul branch from the Wall's base. Sample B was a fragment of rope, made of braided hemp and exposed on the mid-level surface of the Wall. Sample C was a fragment of a thin branch, retaining copper colored bark from the Wall's top.

According to the laboratory report from Beta Analytical in Miami, the samples contained plenty of carbon for assessment, and the dating tests proceeded normally. Evaluated ages of the samples were found to be: Sample A = 970 years old; Sample B = 890 years old; Sample C = 860 years old.

Subtracting these age figures from our current year, 2012, tells us when the branches and hemp (from which the rope was made) were cut, and presumably used fairly soon afterwards in the course of constructing the Wall.

Specifically, Sample A was made in 1042 AD. Sample B was created in 1131 AD. And Sample C was fashioned in 1151 AD.

My original view of the origins of the region's Wall - the "jigsaw theory" - that it constitutes a missing link (in Mongolia) of the Western Han Dynasty Great Wall, remain unchanged in the light of these radiocarbon dating results.

It is clear from conventional maps produced in both China and Mongolia, and from Google Earth, that a very long, ancient barrier, unlimited by the contemporary border position, was built through the region.

The range of these dates, from 1042-1151, do however reveal the "recycling" of the Han Wall approximately 1,000 years after its original construction and that this second construction period at this location was 109 years in duration.

Three political regimes co-existed during this period: the Western Xia (1032-1227), the Liao (946-1125), the Jin (1115-1215).

Within the heart of Mongolia, we are talking about the pre-Genghis Khan or Proto-Mongol period, about half a century before Temujin united the steppe tribes into the federation of the Mongols and became Genghis Khan.

Ogodei Khan, who ruled 1229 to 1241, had confessed in The Secret History of the Mongols that his building of walls to restrict the free movement of gazelles was a sin. Given that the test results indicate construction and use from 1042-1151, we can rule out the Wall being for such a purpose.

Historical geographical sources suggest that the northern limit of Western Xia territory was indeed in the south Omnogovi region - precisely along the line of the extant Wall I investigated.

Genghis Khan's campaign against the Western Xia began in 1209 and was sustained until 1227.

However, prior to Genghis Khan's unification of steppe tribes in 1206, individual tribes - Proto-Mongols - would surely have been perceived as a problem by the Western Xia. It seems that for this reason the Wall may have been rebuilt and maintained as a defense.

The Western Xia Tanguts were originally a semi-nomadic people and built their capital on the site of today's Yinchuan, capital of the Ningxia Hui autonomous region.

Their second most important city was Khara-Khoto, known as Heicheng in Chinese, located where the Ejina River made habitation possible in the desert. Remains of this city lie just 150 km southwest of the Genghis Wall in Omnogovi, which may have been a reason for the Wall's construction.

It would seem logical to use the existing line of the Han Wall and benefit in a small way from the foundations it provided, as well as its exploitation of the best lie of the land.

It took the Mongols almost 20 years to defeat the Western Xia, and Genghis Khan died during the concluding 1227 campaign.

The anger that it took so long was vented on the people, places and records of the defeated Western Xia. Almost everything was thought destroyed.

Khara-Khoto was rediscovered by a procession of foreign archaeologists in the early 20th century, including the Russian Kovlov, Aurel Stein from Britain in 1917 and American Langdon Warner in 1925.

The only extant Western Xia architecture is the remains of Kharao-Khoto city, and the tombs of the Western Xia kings, backing up against the Helanshan Mountain. Enclosing walls several meters high and thick surround both sites.

It was not previously thought that the Western Xia had built any long defensive Wall.