Cache is not at posted coordinates, but in order to find it, you need to go there.
It was 1898, and while out searching for silver in New Mexico and obtaining a baby from a basket, Daniel Plainview struck oil in Minnesota of all places. He left behind an artifact near his oil claim and headed off to the Pacific Ocean. At the Pacific, he treaded water with a scary look on his face while swimming with some guy who claimed he was Daniel's brother. Plainview made it big, and bought out Standard Oil. His brother turned out to be a fraud and met a bad fate.
Flash-forward to 1927. After drinking his milkshake, Plainview remembered where the artifact and the oil claim was, sort of. He remembered that if he put himself parallel to the park rules trail sign, he could walk 40 paces to the north-northeast and run into a lone stump on the east side of the trail. From there he would walk an additional 18 paces to the east until it brought him to the edge of a strange water structure. He was close. This place looked different from last time. Trees had been felled by loggers looking for Plainview's black gold. That or they were jerks who took his cache of goodies. Where could they have moved it?
Will you find Plainview's artifact based on his memories? If so, you'll get a smiley face. If not, watch out for bowling pins, because Plainview is fond of those. Remember, paces for him might be different for you. Someone taller might have a larger stride. If you hear water running, you're close.
The stamp is NOT a trade item. It stays in the cache.