From the earliest days, hardpan in Valley soil has bent and broken shovels, broken picks, bent pry bars, and caused sore backs and bad moods. One of the earliest roads, the Stockton to Los Angeles Road, had a route through the valley and another in the foothills. In winter, valley soil became impassible because hardpan prevented drainage and created very boggy patches, so the foothill route was then used. Hardpan is a distinct, very dense layer of soil found 1' to 6' below the surface, and it is impervious to water and plant roots. It is caused by naturally occurring minerals in the soil that bind together with soil particles, including dissolved silica, iron oxides, and calcium carbonate. Its formation depends on how well water flows through the soil particles, and water does not flow through them well. Clay particles have a strong negative electrostatic charge that causes positively charged ions to bond with them, and the resulting combination creates strongly cemented hardpan.