Forest physiography; physiography of the United States and principles of soils in relation to forestry . use ever-increasingdanger from forest fires, floods, and summer droughts.^ Soils of the Pacific Coast Valleys The soils of the Pacific coast valleys range from residual and colluvialsoils of the mountain foothills to deep and extensive river flood-plainand delta sediments and ancient and modern marine and lacustrineshore deposits. The wide range in value of these soils and their adapta- 1 J. W. Toumey, The Relation of Forests to Stream Flow, Yearbook, Dept. of Agri., 1903,pp. 286-287. 2 Van
Forest physiography; physiography of the United States and principles of soils in relation to forestry . use ever-increasingdanger from forest fires, floods, and summer droughts.^ Soils of the Pacific Coast Valleys The soils of the Pacific coast valleys range from residual and colluvialsoils of the mountain foothills to deep and extensive river flood-plainand delta sediments and ancient and modern marine and lacustrineshore deposits. The wide range in value of these soils and their adapta- 1 J. W. Toumey, The Relation of Forests to Stream Flow, Yearbook, Dept. of Agri., 1903,pp. 286-287. 2 Van Winkle and Eaton, Quality of the Surface Waters of California, Water-Supply PaperU. S. Geol. Surv. No. 237, 1910, p. 17. PACIFIC COAST VALLEYS 189 tion to crops is dependent largely upon the possibilities of irrigation andupon local climatic conditions of rainfall and temperature. The soils of the alluvial fan deposits, colluvial and alluvial washfrom foothills and higher adjacent soil bodies, and occasional small areasof residual material are derived mainly from sandstones, shaly sandstones, 120°. Fig. 4S. — Irrigation map of the West. Irrigated areas solid black, irrigable areas dotted.(Newell, Irrigation in the U. S.) and shales (Cretaceous and Tertiary), and occur upon rolling marginalhills, sloping, elevated, and dissected mesa or bench lands, and in someplaces on the margins of lower nearly level valley plains. The soils composed of recent alluvial materials derived from a greatvariety of rocks and deposited as river and delta plains generally occupy IQO FOREST PHYSIOGRAPHY a lower topographic position, are of more recent origin, are subject tomore frequent overflow than other soils of the region, and often supporta growth of swamp vegetation, brush and willow thickets, and timberin the river bottoms and lower valley plains. The surface is generallylevel, shghtly sloping or sometimes uneven, and is frequently markedby sloughs or the interlacing channels of streams man
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