Picasso invented the beach. Well, maybe not single-handedly. But if French 19th-century artists such as Degas defined the traditional seaside, it was Picasso in the 1920s who gave a visual form to the modern hedonism of sand and sensuality sur la plage.

It was in that decade that the French Riviera became the image of sultry decadence, a mythic status it would keep through the 20th century until global resorts offered still steamier thrills. Picasso had money by then, and decamped to the south of France every summer. The 20s Riviera inspired one of the century's definitive novels, Tender is the Night by F Scott Fitzgerald. It also inspired great paintings, as Picasso mythologised the new beach culture in works that go far beyond the elegance of impressionist marine painting.

Two Women Running On the Beach, painted in the summer of 1922, is a monument to the new freedoms that swept the world after the first world war. At the time, everywhere from Hollywood to the high street, the stiff conventions of the Victorian age were thrown off. In Picasso's joyous and powerful painting, in the classical style he was then enjoying, women who resemble Greek mythological maenads run in loose Grecian dresses that hang down to reveal big, round breasts; their hair flows free, they hold hands in pure abandon. It must have been a good summer. The sea and sky are slightly different shades of Mediterranean blue: a brilliant cartoon of the seashore.