I have seen that phenomenon quite a few times in the mountains both in the Cascades and in the Alaska Range. It happens when there is a layer of clouds just slightly above the summits of the peaks, and when the sun gets just low enough in the horizon to cast a shadow upwards...as the photos clearly shows Makes for a pretty neat view, but only for a short period of time. Perhaps the most dramatic example I have seen was up here last fall when a shadow just as the one shown was being cast off of the entire crest of the Alaska Range. We happened to be at a show on the deck of lodge with a perfect view of the range. The music just stopped and everyone stared until it was gone. Of course I didn't have a camera.