Sylvan Lake is a small lake impounded behind an even smaller dam built between the narrow walls of granite at the top of Sunday Gulch in the Black Hills of South Dakota. Originally private land, the lake and surrounding area now make up the northwest section of Custer State Park and the western end of the Needles Highway scenic drive. The park maintains a store, hotel and campground near the shores of the lake. Several trailheads into the surrounding area start here.
Spires of granite rise above the ponderosa around the lake and nearby area. Once covered by thousands of feet of overlying sediment, the Harney Peak granite was slowly uncovered after the Black Hills were lifted during a mountain building time that started over 60 million years ago and ended around 48 million years ago. It is a very old granite dating back 1.7 billion years ago.