Last weekend the weather was clear, so I took photos in the cave. The first pattern happens around 5:43, when a thin line of light enters through the notch at the bottom left of the larger window.
By 5:51 this light spread to a large triangle,
that by 5:58 spread further along the wall.
The problem with taking photos is that I may be missing a pattern and introducing my own shadow. This time I tried time-lapse photography. I placed the camera on the ledge on the western wall of the cave, set to take 1 frame/second with a playback of 30 frames/second. This condensed about 50 minutes into almost 2 minutes. The video is at Youtube here.
Since the camera is looking at bright spots, they appear a little burned out. However, it is easy to see how the point of sunlight moves across the cave wall and touches the mark at 6:39. Since this is 10 days after equinox, the point may extend slightly beyond the mark.
The sun will set in the same position for August 13 at the end of April. Maybe time-lapse will reveal more patterns at Miantonomi's Cave.