Very large, boomerang shaped object, silently moving slowly at treetop height East to West with no wind disturbance.

I indicated that the craft sighted was "triangle" but it was really more of a boomerang shape. This thing was very large, at least a city block long from tip to tip and had dim white lights all around the edges. The color was gray/black and it blocked out the stars above as it passed over us at approximately tree top level and very slowly. There was no sound, nor was there any wind disturbance of any kind.  We kept asking each other "do you see that???" and I even pinched myself to make the point of conciousness on my part clear to myself. We did not notice it until it was almost directly above us as we sat in my '63 chev convertable with the top down (facing north) and we watched as it slowly made its way west and disappeared into the distance. The strangest part I thought was the fact that none of us were frightened by this unusual event. This was in a place that we called "space country" because there was not much of anything but space out there in the early 70's. Dirt roads and wheat and corn fields with nobody out there but us usually. We would go out late at night and hotrod our cars along the dirt roads. The area is what is now Aurora, CO. Back then it was approximately 15 minutes east of the old Stapleton International Airport via I-70. Later, a few weeks? we were out there again and it was day time and we noticed a few radar domes nearby where the sighting took place and speculated that the craft was spying on the installation which we found out was Buckley Air National Guard Base rumoured to be a testing ground for experimental aircraft. The craft we saw as related here, in my opinion, was not made by humans. It far surpassed any of the stealth type aircraft that I have seen currently deployed by our USAF. It is something I will never forget and would have reported before but, did not know that there was anyplace TO report it until finding your website tonight.
Date is approximate.  PD