Housing history: located on the site of ancient Roman settlement, a new archaeology museum provides an elegant forum for the display and discovery of historic treasures.

Located on the site of an ancient Roman settlement, a new archaeology museum provides an elegant forum for the display and discovery of historic treasures.

Vienne, just south of Lyons, can trace its history back to Roman times. The river Rhone meanders through an alluvial plain framed by distant hills, inviting colonisation. The Romans first established a settlement on the west bank of the river - a splendid array of villas, baths, warehouses and markets. During the Middle Ages, however, the city moved across to the east bank of the Rhone and the Roman colony fell into ruins. It was the settlement on the east bank that eventually evolved into modern Vienne. Remnants of the Colonia lullia Viennensium were first unearthed in 1967, and at the end of the 1980s, the Rhone General Council held a design competition for a new museum of Gallo-Roman civilisation. The competition was won by Philippe Chaix and Jean-Paul Morel, who have been in partnership since 1983 and have gradually acquired a reputation for spare, elegant buildings that exploit Modernist qualities of transparency and articulation of structure. This project, together with a Civil Engineering and Geographical Sciences college at Marne-la-Vallee, near Paris, represents Chaix and Morel's ascendency into a bigger league.

The brief presented competitors with a choice of two adjoining riverside sites, both containing archaeological remains. Chaix and Morel opted to separate museological and ancillary functions into two discrete buildings occupying separate sites. Administration and research facilities, together with the entrance foyer and an adjoining space for temporary exhibitions, are housed in a linear, two-storey concrete structure that runs parallel to an existing road bridge across the Rhone. The main series of exhibition spaces occupies a lightweight, transparent pavilion hoisted above the Roman remains on a series of spindly pilotis. In this way, the exhibition building protects the remains, while a gently inclined ramp gives visitors access to the treasures below. The two parts of the museum are linked by a narrow glazed walkway.

A broad external staircase leads up from the riverside to a viewing terrace on the roof of the administration building, providing the museum with a strong monumental focus and breaking up the orthogonality of the narrow, rectangular block. Halfway up the staircase, the main entrance leads into a generous foyer, with bookshop and temporary exhibition space. To the rear of the first floor are research and restoration workshops, with additional workshops, the reserve collection and administrative offices on the ground floor. The mosaic restoration workshop - one of the most important facilities of its kind in France - is located at the west end of the ground floor. As the grand staircase incises into the building, it creates a narrow prow overlooking the river; this is occupied by the museum's cafe.

The floating box of the main exhibition pavilion is supported above the Roman remains by a grid of 24 steel columns strategically positioned between the ruins. The columns support massive beams concealed in the roof space along with services equipment. The beams in turn take the weight of hangers which support the framing structure for the two internal floor levels, so the pavilion is literally suspended over the remains. The lightness and transparency of the exhibition building (it is fully glazed on all four sides) and its modular grid of columns are strongly reminiscent of temporary structures used to protect archaeological excavations. Layers of horizontal silk-screened glass slats held in place by cable stays act as diffusing brises soleils.

Inside the building, the exhibits are organised around a linear spine, with a series of secondary spaces each devoted to a different theme, such as trade, water or domestic life. Displays combine a range of unearthed objects with mosaics and frescoes to create a coherent whole. Rather than being guided by lengthy explanatory panels, the clarity of the architecture leads visitors through the museum. A network of mezzanine walkways above affords elevated views of the mosaics, while glazed shafts in the floor permit glimpses of the remains below. And all around, the presence of the landscape, river and town viewed from the luminous interior full of precious relics, memorably connects Vienne's ancient past with the present.