Panama land bridge as it is today (left) and the same region as it may have appeared during the general submergence of the middle Tertiary (right). When the isthmus is above the water, it forms a migration route for the land animals and is a barrier to the sea animals. When the area is submerged is becomes a barrier to land animals but provides an open route for exchange of Atlantic and Pacific sea life. (Stokes, 1960, p. 403).
The Alaskan-Siberian land bridge wich functioned as a migration route along which an interchange of animals took place between Asia and North America during the Pleistocene. (Zumberge, 1958, p. 360).
Uncovering a mammoth tusk in preparation of a plaster block in the field: (a) discovery, (b) outlining the block, (c) application of shellack, (d) first plaster strips over wet newspaper, (e) reinforcing block, (f) plaster strips covering side of block. (Stirton, 1959, p. 35).
Evolution of the Canadian Rockies. The upper diagram shows the Rocky Mountain trough filling with sediments during the late Mesozoic. The middle diagram shows the compressional deformationof the Rocky Mountain trough to form the the Canadian Rockies. The lower diagram shows erosion ofrming the present topgraphy. The numbers show the corresponding beds in successive stages. (Stokes, 1960, p. 164).
