A retired national park ranger and private pilot sent photos and a letter this week to Interior Secretary Gale Norton, detailing illegal snowmobile activity on the western boundary of Yellowstone National Park.

Bob Peterson of Jackson has 34 years of service within the National Park Service and has made frequent flights along the park's west boundary.

"The pattern of illegal snowmobile activity has become increasingly obvious," he wrote to Norton.

Adding that this trespassing is clearly neither incidental nor the result of innocent operator error, Peterson sent photos with the letter, hoping they would "energize your personal involvement in bringing this illegal activity to a halt."

Peterson noted that following Norton's inspection trip to Yellowstone last month, the Park Service apprehended 17 snowmobilers illegally entering the park's backcountry.

"The result, in our region's printed media, was that readers learned in one week about your endorsement of snowmobiling in Yellowstone, and the next week about flagrant backcountry violations of the park rules by snowmobile trespass," Peterson wrote.

He said he did not believe Norton's visit triggered the trespassing, but he worried that some might draw that connection.

On March 11, Peterson flew the southwestern border of the park, at the same time a major snowmobile event - a dealer/consumer show - was held in West Yellowstone. Concurrently, Peterson said there was additional trespassing along the park's southwest boundary.

"It has been a while since it snowed up here, but from previous flights, it was clear that there was some new tracks inside the park," Peterson said Friday.

Peterson's letter to Norton said the photos document snowmobile tracks within the park. Some record "high marking" on steep slopes, while others are in meadows, in stands of trees, as well as next to a patrol cabin. Most are a significant distance within the park, while two are adjacent to the boundary. All photos depict locations where snowmobile entry is prohibited in order to protect Yellowstone's resources.

"I have devoted 34 years to a career with the National Park Service and continue that devotion in retirement. My concern regarding the mechanical trespass in Yellowstone's backcountry is serious because it involves the nation's first national park, continues winter after winter, consumes tax dollars, and clearly involves disregard for rules that have long been in place for the public's benefit," Peterson wrote.

"In my view it is very important to end this pattern of illegal activity, and please accept my thanks in advance for whatever effort you may personally give to this issue."

John Wright, a spokesman for the Department of Interior in Washington, D.C., said Interior takes any rule violation seriously and is thankful for reports from concerned citizens such as Peterson.

Cheryl Matthews, spokeswoman for Yellowstone National Park, said park rangers worked with state, county, Forest Service and West Yellowstone law enforcement last weekend, patrolling the area, catching two snowmobilers who trespassed into the backcountry of the park.

According to park records, Yellowstone's Gallatin Sub-District rangers log thousands of miles each winter patrolling the western boundary. Over the past few years, there have been dozens of violations n many involving resource damage where backcountry trespassers have harmed young trees growing in the 1988 forest fire burn areas.

One of the most egregious cases took place last year, when five snowmobilers rode more than 45 miles into the park's backcountry across remote and pristine meadows, through steep avalanche-prone canyons, over frozen waterfalls and cascades, and off dangerous cliffs. The snowmobilers ran over and damaged numerous live trees along the way.

Under the park's temporary winter-use plan, people who snowmobile in Yellowstone must be accompanied by guides, and the number of snowmobiles is limited to a total of 720 a day at the prescribed park entrances.

"This shouldn't have anything to do with the winter-use plan," said Amy McNamara of the Greater Yellowstone Coalition. "Most people follow the rules, but when a few do not, we have to take it seriously."