From this he deduced that the darker regions are flat, low-lying areas, and the brighter regions rough and mountainous.[1] Basing his estimate on the distance of sunlit mountaintops from the terminator, he judged, quite accurately, that the lunar mountains were at least four miles high. Galileo's engravings of the lunar surface provided a new form of visual representation, besides shaping the field of selenography, the study of physical features on the Moon.[2]