In the twentieth century, a series of mining companies reprocessed the tailings, extracting gold that was increasingly difficult to separate from the gravel. Even though the ore had already been processed, it was the principal source of gold in California for some time. Two mining towns, Hammonton and Marigold, were founded to house the gold miners and their families, but have been largely abandoned since 1957. By the 1970s, it was economically impossible to retrieve any more gold, and the debris became a source of aggregate, an essential ingredient of concrete. It has been estimated that up to $15 billion worth of aggregate lies within the goldfields, although the mining industry disputes this figure.