Irregularly-shaped large elongated triangle with blue after-glow seen soundlessly moving over Cape Cod.

My friend and I, who were 14 and 15 years old respectively, were walking northwest on route 130 in Sandwich, Ma. through a residential neighborhood at around 20:30 hours in the evening when we both saw the following elongated triangular object.
 The neighborhood was very quiet with no cars passing by on the road. My friend and I were walking somewhere to join other friends to "hang out". Though we did at times use alcohol and marijuana at that age, we were both very sober and "straight" at the time of this occurrence.
 We both looked up when we noticed the very large, silent, slow-moving object passing over our heads above the trees at a slow speed. It was about the size of a football field, shaped like an elongated, irregularly shaped triangle. Its appearance was most like that of a gray rock though it did have the shape I have noted. It was silent and had a bluish glow from the back as if it were emitting some sort of flame or exhaust. This was the only light visible and we only noticed that after it passed us.
 When we saw it we were both amazed, exclaiming to each other, "what the (expletive) is that?!!" We began to run after it, in the direction it was moving slowly and soundlessly (North by Northwest) until we realized that there was no way we were going to keep up with it.
 We then stood watching it fly on, as it continued to head out toward Cape Cod Bay. (though we could not see the water from where we were. It apparently continued on out over Cape Cod Bay.
 Amazed at what we had seen, we returned to my house to tell our skeptical families and friends what we had seen. Later, my sister informed me that she had heard on the radio that others had seen the object including, possibly, police officers.
 Over the years I have told many people about this experience. I have never doubted the validity of it but have also never been able to explain it or hear from others who have seen it or any similar object.
 I would not be considered a person who believes in intelligent beings on other planets or other solar systems and I question the validity of many supposed UFO sightings but I do know that I did see what I have described above.
Witness indicates that the date of the incident is approximate.  PD