Black disc floating towards Great Falls.

In the summer, as I was driving toward a farm in Waltham, Montana, I spied a disc-shaped object above the horizon.  I was about 1/2 mile from that horizon created by a bluff ahead and was driving my dad's car.  It looked to be about the size of a semi (sideways) just above the bluff.   
I wanted to see the object as close as I could so I accelerated around the turns toward the top of the hill.  As I did so, the object floated over the hill and I raced to the top and stopped on the side of the road, ran and jumped the fence.   
In a matter of a few minutes the object had floated a couple miles down into the valley toward Great Falls.  
This all happened in broad daylight in the early afternoon, as I recall.  
The object was pretty much black - I don't remember seeing any distinguishing features - no lights, etc.  
I watched it continue to float toward Great Falls until it disappeared in the distance.
Witness indicates that the date of the sighting is approximate.  PD