Quern stones were used in China at least 10,000 years ago to grind wheat into flour. The production of flour by rubbing wheat by hand took several hours. Due to their form, dimensions, and the nature of the treatment of the surfaces, they reproduce precisely the most ancient implements used for grinding cereal grain into flour. Saddle querns were known in China during the Neolithic Age but rotary stone mills did not appear until the Warring States Period. A prehistoric quern dating back to 23,000 BCE was found at the Longwangchan archaeological site, in Hukou, Shaanxi in 2007. The site is located in the heartland of the northern Chinese loess plateau near the Yellow River.