A sighting of something in the sky

Two days ago I was a passenger in a van that was headed to the Denver Airport.  When the van was a few miles away from the airport, I saw something for several minutes that I find somewhat difficult to describe.  Of course, now I wish that I had taken pictures, but I did not think of that at the time. 

It was around 3pm in the afternoon on a relatively clear day.  I don't know exactly what it was, and it could have been something conventional, and perhaps some of you will have some ideas or possibly have seen something similar.

It was at least several miles away and it was composed of what looked like hundreds, or at least dozens of dark objects.  We were too far away for specific details, but from what I could see they looked round or oval.  They were arranged in what looked like an overall triangle pattern.  My first thought was that it was a very large flock of dark birds, with a leader bird and the others trailing in a V pattern.  However, as we drove along at around 60mph these objects stayed in the same place. 

As I said, I was able to look for several minutes and the person next to me was looking as well and we were both saying something like: "What the heck is that?"

Another thought I had was that it looked like very dark smoke, but it did not have the right formation for typical smoke and it did not disperse.

With regard to size, I was able to compare these objects to a plane that was flying by and the plane was probably farther away than these objects and the plane was about twice as big as one of the objects.  I understand that distance can distort the perception of size, but by a rough guess, I thought that birds would be too small for that sort of size comparison.  That makes me think that it was more likely some sort of atmospheric  event. 

I know this may all sound a bit vague but I am doing my best to be specific and then as now, I was unclear about what I was looking at.  Any ideas?