Looks to me like a mule shoe that has been worn in two. The fort on the Columbia River at the Dalles was closed at the beginning of the Civil War, and lots of army supplies were packed into Washington from there on mules, which of course stopped when the war took most of the soldiers east. In normal conditions, a horse or mule will grow hoof wall at the rate of about 3/8's of an inch per month. Letting the foot grow out too long can cause all sorts of problems leading to lameness; so under ideal conditions the army shod and or reset shoes every six weeks. In the case of your find, my swag would be it was a civilian contractor that didn't shoe his stock as often as the army. I've packed mules into the Sierra Nevada high country out of Lake Tahoe. The country is all granite, and horse and mule shoes were all worn in two in less than 6 weeks. With more or less normal soil, a shoe can be pulled, the foot trimmed, and the shoe reset at least one time, then the shoe is usually so thin that it is discarded, which is before it wears in two. So there are all sorts of factors in guessing the age of horse and mule shoes, like horses have been shod in basically the same way since the days of knights in shinning armor, so if you found it in conjunction with an army post, and you know the date of the post, that would be the best guess IMHO.