THE LIGHTS were about to flash on the Washington Monument the other night when I ran into an old friend standing in Mount Vernon Place. She lives around the corner, on Park Avenue, in an old house packed from cellar to roof with the collections of a lifetime. We chatted and talked about the potent emotions that strike this time of the year.

She spoke of opening the old, ever-reused boxes where we all store our Christmas decorations and trinkets. These are not fancy containers, but plain pasteboard cartons that seem to have the right fit for all that seasonal stuff. Ornaments may get broken or replaced, but those well-worn and memory-soaked boxes and yellowed tissue papers endure.

Some of the most prosaic components of Christmas pack the most punch. In this vein, I took a call one recent morning from a friend. He asked where I got my mountain paper, truly a question only a mossback Baltimorean would pose in December.

Locally, mountain paper, on the Christmas decorating horizon, is the obscure version of, say, sauerkraut of the table, alongside a turkey. Let me explain. Mountain paper is often brown wrapping paper painted and colored to resemble mountains. It is crumbled and formed around mangers and creche scenes, or in Christmas gardens, to be anything you want - the hills of old Israel or the Rocky Mountains through which the Christmas-garden electric train runs.

It is still sold commercially, made by Baltimore's Life Like Company. In my childhood, we bought it in rolls often flecked with mica, I believe. The version I picked up came packaged in flat sheets. No mica particles, either.

A few days ago, I followed the directions and arranged it around my mother's childhood figures of the infant Jesus, Mary and Joseph.

I felt a little guilty using commercial mountain paper because I usually make my own, as my father and teachers taught me. In my many years here, I've observed some pretty fine mountain paper. My father instructed me how to make a Christmas garden out of scraps you could find in the cellar or on the street. As a South Baltimore child he scooped up sawdust from the local cigarbox factory, dyed it green and made his own Christmas garden grass. He often made his mountain paper out of old ecru-colored window blinds, the ones used in homes in the winter months. The blinds were a heavy paper that crumpled well. All you had to do was paint it.

I'll always think of the Monday morning I walked into my third-grade classroom at the old Baltimore Academy of the Visitation and saw an expanse of the most artistic mountain paper ever. Its creator was Sister Marie Therese, a cloistered nun who had no budget and never left the monastery. Yet she had gorgeous paper surrounding her bulletin board of angels and shepherds.

I gushed over its quality and asked its origin. "Oh," she said, "I made it." She confected it from brown wrapping paper and some arts and crafts tempera paint. She got the monastery caretaker to give her some old screening and the infirmarian to find some old toothbrushes. Voila, she made spatter-paint mountain paper. I thought it belonged in the Walters. Goes to show what you can do when you really want to have a good Christmas.

Now I'll tell one last mountain paper story. It comes from a woman I knew out in Ten Hills or Hunting Ridge. She had set up her children's Christmas garden with train, streets, houses and mountains, of course. It was a really fine mountain, beautifully formed with a tunnel portal. When it came time to run the train for her children, she turned on the electric current and sent the locomotive and cars down the track. When it got to the mountain, it entered the portal, but then the mountain exploded and took off.

Her big old fat house cat had taken up a slumbering residence within the warm confines of her rock paper tent and took the quickest escape route he could find when so rudely awakened by the speeding Lionel.