In regard to their origin, Peter Comestor asserts that the mountains may not have been as high at the time of the Flood as they now are,[970] and Gervase of Tilbury cites the opinion of some that there were no mountains at all on the face of the earth before the Deluge.[971] Bartholomew Anglicus[972] conjectured that in the very beginning the earth was a plain covered with waters, the movements of which produced the valleys, while the heights were the ridges that remained separating the valleys; many mountains also were the result of great telluric convulsions and were full of caverns that give forth immense volumes of water and form the sources of rivers. In a translation by Alfred of Sareshel of an Arabic work,[973] perhaps that of Avicenna, we have a strikingly modern description of the geological processes resulting in the production of mountains by the forces of erosion and by the accumulation of soil and earth. “Mountains may arise from two causes, either from uplifting of the ground, such as takes place in earthquakes or from the effects of running water and wind in hollowing out valleys in soft rocks and leaving the hard rocks prominent, which has been the effective process in the case of most hills. Such changes must have taken long periods of time, and possibly the mountains are now diminishing in size. What proves that water has been the main agent in bringing about these transformations of the surface is the occurrence in many rocks of the impressions of aquatic and other animals. The yellow earth that clothes the surface of the mountains is not of the same origin as the framework of the ground underneath it but arises from the decay of organic remains, mingled with earthy materials transported by the water. Perhaps these materials were originally in the sea which once overspread all the land” (Geikie’s translation).[974] If, in reading the above passage, we feel that we are dealing with ideas that could well stand in the light of modern science and that in this passage at least geomorphology has emerged from its infancy, we are brought back to the Middle Ages when we turn to Rupert of Deutz’s teleological explanation of the reasons why God created deep valleys and high mountains on the land. According to this mystic, these features were made to serve as a protection to the human wanderer upon the surface of the earth from the violence of the winds which would otherwise have unlimited power over all things, as they do on the Libyan desert or on the ocean.[975] THEIR SIZE AND HEIGHT