Dam good.

This visitor and research centre lies perhaps at one end of a spectrum of educational buildings. It is intended to provide facilities for exploration and research into the petroglyphs made by ancient peoples in the Arizona desert.

The Deer Valley Rock Art Center by Will Bruder is exceptional not because it is a building looking feta site, but because the compound peculiarities of site - where ancient artifacts that have rested for thousands of years meet the lip of a massive contemporary earthwork - generated the $1.5 million building and drove its programme. The Rock Art Center is a tiny 70 [m.sup.2]/700[ft.sup.2] building, a kinked line low in the land.

The process began when the US Army Corps of Engineers built a two-mile-long dam in 1982 on the northern edge of Phoenix, one of three such dams enlarging the parameters for the city's growth. At one end, the dam terminates at a sharp angle to the Hedgpeth Hills jutting into the valley below, increasingly frenzied with bulldozers clearing ever more desert land for housing. Some 1500 petroglyphs - odd, compelling figures pecked into age-blackened rock by the Hohokam Native Americans - lie on these hills.

This is precisely where an outlet channel was cut at the dam's base. Dry and apparently benign most of the time, this concrete chute comes into its own during violent moments, directing roaring storm water away from the valley below; in effect, Bruder says, castrating the power of the water for all time. Thus, the chute links the ominous pressure from the waterless dam with the pressure of the oncoming glut of newcomers to Phoenix, on a site itself already long charged with atavistic tension and memory.

To the initial consternation of the Corps, the administrators of the project, Bruder decided his building would straddle the chute, creating an asymmetric boomerang in plan with virtually no parallel or right angles. The longer arm (153ft/46 m) continues the line of the dam behind it, the shorter arm (35ft/11 m) is abruptly bent where the hills collide with the dam. The building acts as a hinge linking the dam to the hills, covered with black rock and the brilliant yellow flowers of the brittlebush and palo verde; hinging present to past.

Bruder underscores the latent power of the channel, a deep, ugly cut in the land, by using it as a datum line, overlaying the geometry of the boomerang with a series of lines parallel to the chute. This determines how the building terminates and locates major elements, such as beams and columns, and minor ones such as the concrete expansion joints in the entry ramps bookending the centre, setting up an alternating cadence of strong and muted notes throughout the work. In elevation, the importance of the channel cut is also emphasised in the high point of the building, where a thin slit of light penetrates the roof.

The site also propelled the programme. The Corps was mandated to house artifacts recovered from the excavation of the dam. Simon Bruder, Will's wife, wrote a report in 1976, and based on that, the Corps decided to make a building which would obviously oversee the petroglyph site, now increasingly vulnerable to vandalism. 'You can't hide petroglyphs', says Peter Welsh, the centre's director, 'some of the boulders already have bullet holes, so we really needed some sort of presence here ... Because the dam is here, we can say this is not only an interpretative centre for petroglyphs but a centre for interpreting how people modify the land in meaningful ways, in the present as well as the past, and pretty substantially'. The local flood district donated 47 acres for the $1.5 million federally funded building.

The centre, operated by Arizona State University, had to accommodate two very different programmes, one, a warehouse to safely store artifacts and support archival preservation; the other - now becoming the more important - introduces the contemporary visitor to rock art and behaviour at such fragile sites.

On the interior, Bruder has carefully choreographed the visitor's experience of the interior as though one were a particle of water rushing through the channel. He effects a swift transition from the all-encompassing bright desert light, bringing the visitor over a steel and concrete ramp stretching out to the parking lot into the dark funnel of the interior. A curved reception desk parts the lobby area into two separate but parallel sequences. The curatorial and docent side to the north houses storerooms, classroom, offices, and gift shop (the latter two with glass walls). The public side on the south is a long unbroken space - really a wide corridor - with a narrative sequence of educational events flanking the space. The roof slopes sharply from 18ft 6ins (5.5 m) to about 12ft (4 m), bringing the scale down from the public side to the more intimate curatorial side on the north.

There are few openings in this high-ceilinged public area, ensuring museum-quality light levels but also to 'take away the city and allow someone to concentrate', said Bruder associate Wendell Blumette. Unadorned mechanical equipment and lighting tubes overhead accentuate the sense of longitudinal procession. The width of the tunnel decreases, as in a single perspective drawing, pulling the visitor to a window at the end framing a rock outcrop before an abrupt turn blasts the person out into the light again, this time to walk along a path to see the actual petroglyphs. Here, an overhang, similar to the entrance one, is reversed to deflect the eye from the city and reassert the past.

Bruder uses materials to tell stories about Arizona and he explores tectonics as sculpture to articulate assemblies. In one striking detail, Bruder replaces the top flange of a standard I-beam with an extruded cylinder designed so it barely appears to touch the purlin it supports; the move also maintains the horizontal plane of the bottom flange of the beam across the building's changing span. Steel, whether used for the decorative entry ramps outdoors or the structural system, inside, has been sandblasted and covered in a rich copper rust, the colour of much of the state's bedrock, sustaining the sense of the material throughout the project. Perforated pig stall floor steel is used for sun shade in some places and security in others.

The site-cast panels forming the walls respond to the site. Their exteriors, embedded with rough copper slag from a mine 70 miles away, echo the patina known as desert varnish on the petroglyphs. These forbidding panels are offset occasionally to create slits to let the sun mark time on the walls.

In the Rock Art Center, Bruder negotiated a thicket of bureaucracy over a six-year period, even wrangling for permission to be on site once construction began on the building that embodies his growing passion for 'architecture as rune, as a metaphor of the landscape rather than a projection of architecture'. On this strange, remarkable site, he has imbued his thesis with dignity.