37. Hill slope near Harpenden showing woodland at top and arable land lower down. _Phot._ Lionel Armstrong 77 38. View further along the valley; woodland and arable above, rough grassland near the river. _Phot_. Lionel Armstrong . . . . . . . . . . . . . . 79 39. Rough grass pasture near the river. Higher up is arable land. _Phot._ Lionel Armstrong . . . . . . . 81 40. After harvest the farmer breaks up his land with a plough and then leaves it alone until seed time. _Phot._ Lionel Armstrong . . . . . . . . . . . . . . 83 41. Rolling in mangold seed on the farm. _Phot._ H. B. Hutchinson . . . . . . . . . . . . . . 85 42. Soil sampler . . . . . . . . . . . . . . . . . . . . . 88 43. Cultivation and mulching reduce the loss of water from soils . . . . . . . . . . . . . . . . . . . . . 90 44 _a_ and _b_. Maize cannot compete successfully with weeds . . . . . . . . . . . . . . . . . . . . 94, 95 45. A plot of wheat left untouched since 1882 at Rothamsted has now become a dense thicket. _Phot._ Lionel Armstrong . . . . . . . . . . . . . . 97 46. A badly drained wheat field . . . . . . . . . . . . . 99 47. Highly cultivated sandy soil in Kent . . . . . . . . . 103 48. A Surrey heath . . . . . . . . . . . . . . . . . . . . 105 49. Woodland and heather on high sandy land, Wimbledon Common. _Phot._ R. H. Carter . . . . . . . . . . . 107 50. Poor sandy soil in Surrey, partly cultivated but mainly wood and waste . . . . . . . . . . . . . . . . . . . 109 51. Open chalk cultivated country, Thanet . . . . . . . . 113 52. Cliffs at the seaside, Manorbier. _Phot._ Geological Survey . . . . . . . . . . . . . 117 53. Cliffs in inland district, Arthur's Seat, Edinburgh. _Phot._ Geological Survey . . . . . . . . . . . . . 119 54. Model of a stream . . . . . . . . . . . . . . . . . . 120 55. The bend of a river . . . . . . . . . . . . . . . . . 121 56. The winding river--the Stour at Wye. _Phot._ R. H. Carter . . . . . . . . . . . . . . . . 123 57. Sketch map showing why Godmersham and Wye arose where they did on the Stour . . . . . . . . . . . . 126 58. Ford at Coldharbour near Harpenden. _Phot._ Lionel Armstrong . . . . . . . . . . . . . . 127 The photographs of the pot experiments are by Mr Lionel Armstrong. {xi} INTRODUCTION