316 PAST CLIMATES AND CLIMAXES. 



place whatever new soil may be formed. Being clear, the rivers and streams 

 will also be ready to become erosive agents at the first opportunity. They 

 will find their opportunity when they leave the mountains, and flow out beyond 

 the limits ordinarily reached in the preceding dry epoch. When the revived 

 stream flows in full force, its velocity will naturally be accelerated. As it is 

 not loaded to its full capacity, it will inevitably begin to erode the gravel and 

 silt of its own previous deposits. A gully will soon be formed, and will 

 rapidly work backward. In course of time the stream will once more make 

 its bed concave upward. Then it will widen the channel as well as deepen it, 

 and we shall have a flood-plain bordered on either side by a terrace." 



The terraces studied by Huntington in Asia and America form a sequence 

 reaching back from 2,000 to 30,000 years, more or less. Thus, they belong 

 essentially to the Human period, and indeed terrace-making on a small scale 

 may be seen in process in arid mountain regions to-day. In this Hes their 

 great significance, for there can be Httle question that terraces may have been 

 formed in essentially the same way just as far back as arid periods and moun- 

 tain vegetation occur. 



Fluctuations of lake-levels. — The old shore-Unes of closed lakes in arid 

 regions afford striking CAddence of former changes of climate, as has been shown 

 by Gilbert (1890) and by Russell (1885) in the case of Lake Bonneville and Lake 

 Lahontan (plate 58 b). The major shore-lines are assumed to be connected 

 with the climatic changes of the glacial period, as Huntington (1914 : 39) 

 thinks is the case also with the old strands of the Otero Soda Lake in New 

 Mexico. The latter is of special interest, moreover, on account of the record 

 which it has preserved of minor fluctuations during the present. The major 

 strands of an original lake are four, the chief of which Hes more than 200 feet 

 above the playa, and is succeeded by three others at intervals of 40 to 80 feet 

 above it. The minor strands are likewise four, at 2, 4, 20, and 60 feet above the 

 floor of the playa, though the first is insignificant and the last somewhat 

 doubtful. Huntington states that — 



"Those at elevations of 20 and 4 feet are suflBcient to show that in times 

 long after the end of the glacial period the Otero Lake has varied in size, 

 apparently because of distinct climatic fluctuations. 



"Associated with the main playa are dimes of pure white gypsum, the so- 

 called 'white sands.' These constitute a climate indicator of the greatest 

 interest, since they make clear one set of conditions under which gypsum beds 

 could have been formed in the past. During periods of the year when the 

 playas are dry, the gypsum crystals deposited are swept across them by the 

 strong southwest winds and heaped into dunes 5 to 40 feet high. The dunes 

 are driven forward by the winds, and new ones are constantly forming behind 

 them. The main body of the dunes is now flxed by vegetation, but there is 

 nowhere the typical transition from moving to stable dunes. The two 

 types exist side by side, both at the outer edge of the dune area and at the 

 iimer edge close to the playa. The only explanation seems to be either that 

 the supply of gypsum has recently increased, or that the amount of vegetation 

 has decreased so that the fixed dunes have in part become free. Either alterna- 

 tive demands a change of climate. The older phase seems to indicate a 

 period of aridity, much like the present; the fixation of the dunes apparently 

 points to a greater supply of water and a higher stand of the lake; and the free 

 dunes of the present are in motion because the climate is dry, the lake has 

 become a playa, and the amount of vegetation is limited. Here, then, we 



