As to the origin of the sands and gravels which compose the 'terraces' we know that the glacial stages were periods of the wearing away of vast materials from the summits and sides of the mountains, which were transported by the rivers to the valleys and plains. These vast deposits of glacial times spread out over the very broad surfaces of the pristine river-bottoms, which in many valleys it is important to note were from 100 to 150 feet above the present levels. The diminished and contracted streams of interglacial times cut into these ancient river beds, forming narrower channels into which they transported their own materials. Thus, as the successive 'river terraces' were formed, a descending series of steps was created along the sides of the valleys. In many valleys there are four of these 'terraces,' which may correspond with several glacial stages; in other valleys there are only three; in others, again, like the valley of the River Inn which flows past Innsbruck in the Tyrol (Fig. 6), there are five 'terraces,' while in the valley of the Rhine above Basle there are six, corresponding, it is believed, with the materials brought down by the four great glaciations and with the river levels of Postglacial times. In general, therefore, the 'high terraces' are the oldest ones, that is, they are composed of materials brought down during the pluvial periods of the First, Second, and Third Glacial Stages, while the 'lower terraces' and the 'lowest terraces' in the alpine regions are composed of materials borne by the great rivers of the Fourth Glacial and Postglacial Stages. In the region around the Alps the 'higher terraces' are products chiefly of the third glaciation; in the valley of the Rhine they are visible near Basle. On the upper Rhine the 'low terraces' are products of the fourth glaciation; they cover vast surfaces and contain remains of the woolly mammoth (_E. primigenius_), an animal distinctive of Fourth Glacial and Postglacial times. [Illustration: FIG. 7. Cross-section through the terraced Pleistocene formations of the Rhine valley above Basle, Switzerland. After Penck. _Ib._ Outwash of the First Glaciation--Günz--Deposits on the first erosion level. _IIb._ Outwash of the Second Glaciation--Mindel--Deposits on the second erosion level. _IIIb._ Outwash of the Third Glaciation--Riss--Deposits on the third erosion level. _IVb._ Outwash of the Fourth Glaciation--Würm--Deposits on the fourth erosion level. _Va._ Erosion terrace, Achen oscillation--fifth erosion level. _VIa._ } Post-Bühl erosion--sixth and seventh erosion levels. _VIIa._ } _IIIc._ Moraine of the Third Glaciation--Riss. The section of the Rheinfelder Hill lies 3 km. west from the Möliner Field.]