So I'm going with the old standby 'X marks the spot.'
The Poudre River I referred to yesterday is actually named the Cache la Poudre, which means 'hide the powder' in French. Legend has it that in 1836, a group of French trappers on their way from St. Louis to Green River, Wyoming, spent a few days snowed in near the present-day town of Bellevue, Colorado. Before they continued on, they buried their gunpowder to lighten their load. Later, members of the party returned and retrieved their cache. I don't know if they used a map with an X on it, but if so, it would have been about here:
For anyone wanting their own X-marks-the-spot experience, Rootles - Quests for the Curious creates personalized treasure hunts for groups large and small, casual and corporate. They're headquartered in Fort Collins, but they conduct hunts nationwide. I haven't done one yet, but it's on my list!