_C._ _Transition Period_, resembling _B_ in most respects. [_A_, _B_, and _C_ make up the Old Stone Age, before the use of metals.] _D._ _Neolithic Period._ Some stone implements polished. No metal except that copper is introduced toward the end of the period. Agriculture with domestic plants and animals. Pottery but no potter's wheel. Dawn of Civilization. _E._ _Bronze Period._ Bronze implements or utensils. Dawn of History. Begins about 2500 B. C. in northern Europe. _F._ _Iron Period._ Iron introduced. Historic Times. Begins about 1000 B. C. in northern Europe. CHAPTER II THE PERIOD OF TRANSITION. SHELL-HEAPS During the last great advance of the ice in the earlier Magdalenian epoch the Scandinavian peninsula had been buried beneath a great mass of ice, and resembled the central portion of Greenland to-day. A great glacier extended southward, obliterating the Baltic Sea and crowding into northern Germany. As the glaciers withdrew, North Germany became a vast tundra, across which we may imagine the reindeer and other Arctic and subarctic mammals retreating northeastward before the milder forest and meadow conditions already prevailing in France and Russia.[27] The low temperature of the water of the emerging Baltic is shown by the presence of an arctic bivalve, _Yoldia arctica_, which has given its name to the epoch. A few scattered bone implements show the presence of reindeer hunters in Germany at this time. Before the close of the Yoldia period Germany began to pass from tundra to forest--a transformation which was also now progressing in Denmark. The temperature moderated slowly. The land rose in such a way that it separated the Baltic from the North Sea and the Arctic Ocean, with which it had been connected, and made of it a great fresh-water lake. The characteristic animal of this lake was a small pond animal, _ancylus_, which has given its name to both lake and epoch. The next epoch--the Litorina (or Tapes) depression--was characterized by a sinking of the land in which the barrier between the Baltic and the North Seas gave place to a wide communication. The Baltic became more salt than at present, and the oyster-banks became abundant. It was during this epoch that the shell-heaps were accumulated. The following chart gives a condensed view of the succession of events (in reverse order):[28]