A shadow moving across mountain peaks which cause could not be determined.At about 11:43 on Sunday, May 29, 2016, I observed the following. There were no clouds from horizon to horizon in any given direction. The temperature was in the low nineties. I was in a patrol vehicle at my place of employment traveling in a northern direction and drifting slightly east. To the north west of my locating about a half a mile away there are some small peaks that are maybe a thousand feet higher in elevation than my location. These peaks are strung out east to west.I glance at the peaks and saw a round dark shadow that followed the surface contour of the landscape moping from east to west. I only saw it for a couple of seconds before it passed the most western peak.What was strange about it was that the sun at that time in the morning is almost straight up and the peaks are very close to my location. I immediately stopped (less than five seconds) and got out of the vehicle looking for an object that may have caused the round shadow, but there was nothing to be seen anywhere in the sky in my general location. There was nothing between the sun and those peaks. There was nothing moving out of the area heading west as far as the eye could see. Many miles to the west (about twenty miles) I could see Sky Harbor International traffic along the I-17 corridor flying from north to south, but those shadows would be cast further to the west. I had seen shadows before across those peaks caused by aircraft but they moved slower and had the definite shape of an aircraft. This was round and moving very fast. I estimate the size of the shadow to have been about sixty to eighty feet in diameter.End of Report