Essentially, the tree bark method involves finding pieces of bark that resemble rocky outcrops, cleaning them up, and blending them into the scenery on your layout. John found his bark along a riverbank, so I tried along Lake Ontario. Tree bark floats in the lake for years, eventually washing up on the shore. By then, it has taken on a grey colour and looks remarkably like rock from the Canadian Shield. I collected my specimens and brought them home, where they immediately went in the freezer.
Tree bark is organic material, and all sorts of different organisms might be living in it. Before putting it on your layout, it is best to inspect each piece for anything obvious (moss, bugs, leaves) and then to freeze it for a few days. Then, I take the additional step of baking the bark in the oven, a trick I began using years ago to sterilize soil for use on my layout. Baking bark is dangerous and needs constant supervision in case it starts to burn. For my specimens, I found 15 minutes at 200c followed by 5 minutes at 180c did not damage the bark, but sterilized it enough for use. One warning: the baking process smells awful (like campfire gone wrong). With the bark as safe as it can be, I installed it on the layout.
Tree bark rock serves as the focal point in the front yard of Dufort's Antiques in Heron. Another piece behind the barn helps disguise the supports holding the backdrop in place.
I model the Ontario Northland, which runs through the wilds of the Canadian Shield. Rocky outcrops are an integral part of the landscape, often in cuttings blasted to make way for the track. I cut holes in my plaster-bandage scenery to fit pieces of the bark, which I glued in place. After using regular scenery materials to blend the "rock" into place, the effect is amazing.
Thanks to John Longhurst for the great idea and thanks to the Canadian Model Trains Yahoo! Group for advice on killing organisms in tree bark.