One of the most challenging natural obstacles in the path of the trans Alaska pipeline has been surmounted, highlighting the current push to complete pipe installation before the end of this year's construction season.

Workers have just put the finishing touches a four-mile stretch through Keystone Canyon, a breathtakingly beautiful but extremely rugged mountain pass near the pipeline's southern terminus at Valdez (Val-DEEZ ).

The pipeline was installed atop the east wall of the canyon. To build it in the bottom of the canyon would have required digging up the highway, with lengthy road closures and great public inconvenience. It took most of two full construction seasons to complete the effort over the top, which involved a massive airlift of equipment and materials.

Up to 35 feet of snow falls in the canyon each winter, which may come any day. Pipeline work in other
critical mountain passes also is under way now. The passes include Thompson Pass, just up the line from Keystone Canyon, and Atigun Pass, in the Brooks Mountain Range above the Arctic Circle.

According to Alyeska Pipeline Service Company, the pace has quickened along the route as workers strive to link up all 800 miles of the 48-inch diameter pipe from Prudhoe Bay to Valdez in the few weeks remaining before bitter cold and deep snows end this year's construction effort.

The company says the winter deadline will be met, and that the entire $7 billion project--including eight pump stations and the Valdez terminal facility--should be completed in time for the beginning of operations next summer.