In areas where gophers live in a thin layer of soil underlain by a more or less impenetrable layer of rock, clay or gravel, it has been suggested that they form unique structures known as Mima Mounds. The formation of these mounds has been discussed in detail elsewhere (Dalquest and Scheffer, 1942: 68-84). At least in the breeding season the gophers work mainly and make their nest where the soil is deepest. In the vicinity of this nest, considerable mounding and cultivation of soil ensues. This stimulates plant growth in the area. Much observation indicates that cultivation of earth by gophers stimulates plant growth to a greater extent than the depredations of the gopher deplete the ensuing growth. Thus the gopher, by cultivation of the soil in the area about its burrow, stimulates the growth of vegetation and so increases his own food supply. Consequently there is but little incentive for the gopher to leave the vicinity of the nest. The gopher does, however, construct lateral tunnels into surrounding areas. Earth from these lateral tunnels is, in part, thrown to the surface in mounds and in part transported back to fill the abandoned burrows near the nest. The earth from the burrows about the nest was earlier ejected on the surface. Slowly, then, earth is transported from surrounding areas to burrows in the vicinity of the original nest. Each succeeding generation finds in the vicinity of the original nest, better food and deeper soil, while areas surrounding the nest possess thinner soil and less vegetation. Over a period of thousands of generations of gophers, large mounds, known as Mima Mounds, are formed. Since the removal of earth from the surrounding areas and its accumulation in the Mima Mound are chance affairs, the contours of the mound are smooth and flowing while the contours of the intermound areas are smoothly convex.