CLIMATE AND TERRESTRIAL DEPOSITS 373 



complication, however, is that in many regions profound crustal move- 

 ments are also known to have occurred. This correlation of cHmate 

 with erosion and aggradation has been made by W. D. Johnson to 

 explain the oscillations between stream-cutting and stream-building 

 of the Great Plains region during the Quaternary. He notes that: 



There are two possible disturbing influences which may result in transforma- 

 tion of the gradation plane — deformation and change of climate. But it is not 

 necessary, in order to account for change in behavior of the traversing streams, 

 to appeal to deformation. A sufficient cause may be looked for in change of 

 climate. There is record of erosion, with reversal to deposition and rebuilding, 

 and reversal again finally to erosion, and there is reason for believing that this 

 series of interruptions of the gradation cycle was an effect of climatic oscillation 

 rather than of earth movement. The date of the building of the great debris 

 sheet, so far as included fossil remains would seem to determine it, might range 

 anywhere from middle Tertiary to early Pleistocene. However, the beginning 

 of the final and present degradation stage, during which the smoothness of the 

 Great Plains has been in large part destroyed and their surface lowered, with 

 exception of the sod-covered plateaus of the central zone, doubtless dates from 

 the opening of that period of climatic oscillations in the Pleistocene which, in 

 the Great Basin region of Utah and Nevada, gave rise to repeated fioodings of 

 large areas and the creation of lakes. Indeed,' it is not unlikely that the grading 

 of certain of the minor plateau surfaces of the High Plains, which stand appre- 

 ciably below the general level, is to be correlated with the several returns in the 

 Great Basin to severe desert conditions during this period, which also are plainly 

 recorded among the old lake evidences.^ 



In investigations of Pleistocene geology great attention is prop- 

 erly paid to the surface form, and in fluviatile and pluvial work erosion 

 is more studied than sedimentation. In more distant geological 

 times, however, it is the record of the deposits which must be almost 

 entirely the basis of study. For such purposes, therefore, the rela- 

 tions of erosion and aggradation within the High Plains are not of 

 so much final importance as the question, Wliat was the nature of 

 the deposit made by the erosion of the sands and gravels of the High 

 Plains during times of wetter and colder climate? The suggested 

 but not demonstrated answer is, that the landward edge of the 

 corresponding deposit appears to be the Orange Sand Delta of Hilgard, 

 now known as a Mississippian portion of the Lafayette formation. 



I The High Plains and Their Utilization. 21st Ann. Rept., U. S. Geol. Survey, 

 Pt. IV, 1901, p. 626, 628-30. 



