The result of these investments was a massive irrigation system covering some 57,000 hectares between the Shatt al-Arab and the now-dry western channel of the Tigris. This system was first reported in 1962, when just 8,000 hectares of it remained in use, for the cultivation of date palms, while the rest had become desert. This system consists of a regular pattern of two-meter-high ridges in straight lines, separated by old canal beds. The ridges are extremely saline, with salt deposits up to 20 centimeters thick, and are completely barren. The former canal beds are less salty and can support a small population of salt-resistant plants. Contemporary authors recorded how the Zanj slaves were put to work clearing the fields of salty topsoil and putting them into piles; the result was the ridges that remain today. This represents an enormous amount of work: H.S. Nelson calculated that 45 million tons of earth were moved in total, and with his extremely high estimate of one man moving two tons of soil per day, this would have taken a decade of strenuous work by 25,000 men.