Forest physiography; physiography of the United States and principles of soils in relation to forestry . Fig. 150. — Physiographic subdivisions of and eastern New Mexico. (Hill, U. S. Geol. Surv.) here and there — the products of underground solution and of unequalwind erosion. Local storm floods tend to level these irregularities byfilling the hollows and eroding the surrounding surface.^ The High Plains strata were laid down as a series of great compoundalluvial fans or a piedmont alluvial plain at the eastern base of theRocky Mountains. The eroded bedrock floor upon which the materia
Forest physiography; physiography of the United States and principles of soils in relation to forestry . Fig. 150. — Physiographic subdivisions of and eastern New Mexico. (Hill, U. S. Geol. Surv.) here and there — the products of underground solution and of unequalwind erosion. Local storm floods tend to level these irregularities byfilling the hollows and eroding the surrounding surface.^ The High Plains strata were laid down as a series of great compoundalluvial fans or a piedmont alluvial plain at the eastern base of theRocky Mountains. The eroded bedrock floor upon which the materialswere deposited and the eroded condition of the materials themselves » R. T. Hill, Physical Geography of the Texas Region, Folio U. S. Geol. Surv. No. 3, igoo,p. 6, col. 3. 4i8 FOREST PHYSIOGRAPHY imply a succession of changes in the character of the stream action fromdegradation to aggradation and back again to degradation. Thesechanges harmonize with the conception of climatic changes of known. Fig. iji. — Ideal structure of the Tertiary deposits of the High Plains. The dark band indicates theposition of a partly consolidated portion of the section known as a mortar bed. (Gould, U. ) occurrence, the dry Tertiary (aggradation) being succeeded by the wetPleistocene (degradation), which was in turn succeeded by the dry Fig. 152. — Typical view of the High Plains of western Kansas. (Gilbert, U. S. Geol. Surv.) Present in which the streams are aggrading their valley floors.^ It isalso conceivable that cutting power may have been gained or at leastincreased (i) by broad uplift on the west which would increase the 1 D. W. Johnson, The High Plains, 20th and 21st Ann. Repts. U. S. Geol. Surv., 1898-1899, 1899-1900. GREAT PLAINS 419 stream gradients and (2) by a cooling of the climate so as to reducethe evaporation. The greater part of the Tertiary deposits of the High Plains consistsof clay, sandstone, and conglomerate, with clay largely materi
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