The clearest examples of the evolution of the seven or eight implements of the Chellean culture from the five or six rudimentary types of the Pre-Chellean have been found at St. Acheul by Commont. The abundance and variety of flint at this great station on the Somme made it a centre of industry from the dawn of the Old Stone Age to its very close. It was probably a region favorable to all kinds of large and small game. The researches of Commont show that with the exception of Castillo in northern Spain no other station in all Europe was so continuously occupied. From Pre-Chellean to Neolithic times the men of every culture stage except the Magdalenian and Azilian-Tardenoisian found their way here, and thus the site of St. Acheul presents an epitome of the entire prehistoric industry. Even during the colder periods of climate this region continued to be visited--possibly during the warm weather of the summer seasons. At Montières, along the Somme, we find deposits of Mousterian culture which is generally characteristic of the cold climatic period but is here associated with a temperate fauna, including the hippopotamus, Merck's rhinoceros, and the straight-tusked elephant. Great geographic and climatic changes took place in the valley of the Somme during this long period of human evolution. The Pre-Chellean workers first established their industry on the middle and high 'terraces' at the time when the Somme was visited by the straight-tusked elephant and other much more primitive mammals of the warm Asiatic fauna. The early Acheulean camps on the same terraces were pitched in the gravels below layers of 'loess' which betoken an entire climatic change. The fourth glaciation passed by, and the Upper Palæolithic flint workers again returned and left the débris of their industry in the layers of loam which swept down the slopes of the valley from the surrounding hills. This succession will be studied more in detail in connection with the industry. [Illustration: FIG. 75. Excavation on the 'high terrace' at St. Acheul, known as the _ancienne carrière Dupont_ and more recently as the _carrière Bultel_, showing eight geologic layers from the Upper Palæolithic deposits of brick-earth at the top(9) down to the sub-Chellean yellow gravels(2) overlying the chalk terrace at the bottom.]