As the worst drought in a hundred years shriveled crops in Northern France last summer, small airplanes searched the parched earth for tell-tale patterns that were to produce a startling picture of life in Northern France 2,000 years ago.

To French farmers - and taxpayers, who had to pay a "drought tax" - the lack of rain was an economic blow. For archeologist Roger Agache, who has pioneered aerial archeology in France for the past 17 years, it was a blessing.

Acting like heat on invisible ink, the drought brought out traces of more than a thousand ancient structures never before detected. Among them are huge pre-Roman farms belonging to the Gallic nobility, with buildings frequently more than 400 or 500 feet long. Although Julius Caesar wrote about such farms, which he called aedificiae, traces of them had never before been found.

"It's by far the most important discovery of the year; we knew almost nothing about these big farms before the Roman conquest," said Agache, director of the government-funded Office of Prehistoric Antiquities of Picardy, a region of Northern France.

The discovery of the farms - far bigger than all but the most modern Picardy farms today - shows that 2,000 years ago agriculture was big business in this then "barbaric" country.

Coupled with the earlier discovery of similar Roman farms, it completely reverses the traditional view of Northern Gaul as a few villages or towns surrounded by vast woods.

"In the big fertile plains of the north of France, aerial searches have shown just about the opposite," said Agache, who lives and works in the Picardy town of Abbeville. "These rich wheat fields were dotted with isolated estates, but villages were very few and very small.

An archeological map he recently drew shows that one area 16 miles long and 12 miles wide contained 31 Roman estates, five with buildings more than 600 feet long. There were also many smaller structures - but no villages.

In another area he found traces of an outdoor theater half the size of a football stadium, built in the open countryside. An abortive attempt to establish a town? He wonders. Or entertainment for the populations of neighbouring estates?

With few exceptions the Roman estates appear to have faithfully followed the precepts of Latin agronomists - including advice not to build too near roads because of the tradition of hospitality, which could be ruinous.

Latin documents and archeological "digs" have yielded more information about these farms, which the Romans called villas. Equipped with baths and central heating, they were far more comfortable than the 17th-and 18th-century chateaux of the French nobility. But above all they were agricultural centers built to exploit the rich produce of the conquered country.

Slaves, including Germanic prisoners of war, harvested wheat and sheared wool to be sent by boat and horse-drawn carts to Rome a thousand miles away.How these slaves were kept in submission is puzzle, Agache said. One theory is that they lives in a hierarchy in which more privileged slaves despised and mistreated the others.

When there were not enough slaves to bring in the harvest, a kind of mechanical reaper called the vallus was sometimes used. It disappeared during the Middle Ages and mechanical farm equipment did not reappear in France until modern times.

Many of the Roman villas were built on the sites of pre-Roman farms, which made these earlier estates harder to detect. In any case they were built not of stone but of wood, which decayed long ago.

But the drought enabled Agache to detect and photograph traces of some of these estates: Differences in soil where the wood decayed can sometimes be seen from the air. The aerial findings have been confirmed on the ground; holes for beams of a pre-Roman estate have been uncovered. But many questions remain about these huge farms. What was their social structure? And who bought the produce?

Although aerial photography is cheaper and quicker than excavations, finding the sites has taken hundreds of hours of flights. And interpreting the patterns seen from the air is not easy.

Agache uses small airplanes that belong to flying clubs, piloted by students who need to accumulate flying time. When these pilots first take part, they often see archeological patterns even where they don't exist.

Agache has learned to distinguish between the myriad patterns: present-day paths, mushroom circles and differences in soil caused by fertilizers as well as Bronze-Age funeral circles, Roman and pre-Roman estates and the more irregular medieval structures. He also learned that he must fly over the same ground in different seasons and different weather.

To take advantage of the unequalled opportunity last summer, Agache, who has only one assistant, flew more than 10 hours a day. The abrupt changes of altitude have taken their toll; for the past several months he has been unable to fly because of heart trouble.

But the sense of urgency he felt then was real.Some of the photographs he took last summer may soon be the only record of the past: Modern construction, chemical fertilizers and deep plowing are destroying the patterns traced by the Gauls. And it could be another hundred years before the weather is equally as bad for farmers - and equally as good for aerial archeologists.