San Juan Mountains, Colorado. Autumn aspen line the Sneffels Highline Trail above the town of Telluride in the San Juan Mountains of southwest Colorado. The mountains around the town of Telluride provide ideal habitat for extensive groves of these trees which reach their peak of color in late September. From the town of Telluride, this 13-mile long loop trail gains 3,600 feet in elevation as it ascends into a wildflower-filled basin below13,809-foot Dallas Peak.