The Rocky Mountain Front is a somewhat unified geologic and ecosystem area in North America where the eastern slopes of the Rocky Mountains meet the plains. In 1983, the Bureau of Land Management called the Rocky Mountain Front "a nationally significant area because of its high wildlife, recreation, and scenic values". Conservationists Gregory Neudecker, Alison Duvall, and James Stutzman have described the Rocky Mountain Front as an area that warrants "the highest of conservation priorities" because it is largely unaltered by development and contains "unparalleled" numbers of wildlife. Defining the Rocky Mountain Front Although the Rocky Mountain Front is clearly distinct from both plains and mountains, in places like the Wyoming Basin, Montana, and New Mexico it is more ambiguous. One definition of the front is that it is a "transition zone between the Rocky Mountains and the mixed grass prairie ... [that] encompasses a wide variety of wetland, riparian, grassland, and forested habitats". By one estimate there are more than of Rocky Mountain Front land in Montana and Canada. The Rocky Mountain Front is such an important geologic feature that it affects the weather in North America. Warm air masses moving from the Gulf of Mexico are blocked by the front from moving west, causing hail, thunderstorms, tornadoes, and other kinds of violent weather which then move east. "Tornado Alley", that part of the Great Plains where tornadoes are most frequent, is a direct outcome of the front's effect on weather. Areas of the Rocky Mountain Front British Columbia and Alberta In Alberta, the Rocky Mountain Front is about wide. As early as 1935, it was well-recognized that significant coal resources underlay the Rocky Mountain Front in Alberta. As of 2013, about 60 percent of all Canadian coal reserves are believed to be beneath the front in Alberta. Natural gas is also very plentiful. Royal Dutch Shell began producing natural gas there in the Pincher Creek Gas Field in the 1950s, and built a sweetening plant there in 1957. The Pincher Creek Gas Field can produce up to 150 million cubic feet of natural gas per day, and in the 1960s Shell Oil built a second sweetening plant near Waterton Lakes National Park. The Rocky Mountain Front in British Columbia and Alberta has long been inhabited by Plains Indians, and the area contains widely scattered but not uncommon Native American rock art sites. Development along the front is somewhat