Stay on Highway 61 and you'll see the modern version of Cottage Grove. Drive off the beaten path a bit and you'll find the original: Old Cottage Grove. The posted coordinates will bring you to a big rock which contains some information. Use the only number on the sign to come up with the coordinates for the final.
The final cache is located at
N44 50.AAA W92 52.ZZZ
AAA = The only number on the rock minus 6224
ZZZ = The only number on the rock minus 6263