CROSS-BEDDING 703 



land is of course much less than that of a delta fore-set. Most 

 likely a greater length than six feet is rare, while probably by far 

 the greater number fall below a foot in length. The angle which 

 the fore-set beds make with the horizontal varies proportionately 

 to the coarseness of the material, but in any given case it is ap- 

 proximately uniform. Moreover, the angle of the fore-sets of suc- 

 cessive strata is as a rule similar and in the same direction, while the 

 dividing top-set beds are parallel in the successive strata. Thus a 

 section of a torrential deposit will show a succession of obliquely 

 bedded strata, separated and bounded above and below by strata, 

 which make a high angle with them, are parallel to each other, and 

 originally represented the surface slope of the deposit. The inclina- 

 tions of the laminae of the successive cross-bedded strata are uni- 

 form, and the laminae all slope in the same direction. This is the 

 type of cross-bedding found in many ancient sandstones, and it 

 seems to be highly improbable that any such structure could be 

 produced by agents working on the sea coast. (Fig. 123.) 



c. Cross-bedding of Eolian Deposits. Such regularity of cross- 

 bedding as is found in both lacustrine and torrential deposits is, 

 however, not characteristic of eolian deposits. In these the laminae 

 when oblique show no uniformity of slope or direction within either 

 the same stratum or successive strata. Nor are the dividing laminae 

 parallel to each other. Cross-bedding of eolian deposits is brought 

 about in the following manner : A sand dune in its structure shows 

 a series of concentric shells of sand consisting of alternating coarse 

 and fine layers. This is a feature characteristic, according to 

 Forchhammer (9:7), of every dune of the Jutland coast, and in- 

 ferentially of the majority if not of all sand dunes. Toward the 

 side of the wind the dune layers have an angle of 5°, while on the 

 lee side the angle is as high as 30°. The stratification is shown 

 chiefly by the varying coarseness of the grains composing succes- 

 sive layers, this being determined by the variable strength of the 

 winds, to which the dune owes its origin. If through a change of 

 the conditions which built up the dune, i. e., change in the force or 

 the direction of the wind or in the amount of the sand supplied, 

 or through other causes, the dune begins to migrate, a part of 

 its basal portion may remain behind, as the truncated base of the 

 dune, while upon this truncated surface a new dune may accumu- 

 late, which in turn will meet with the same fate, leaving its basal 

 portion behind. Successive portions of this kind will eventually 

 produce a bed of sand in which the cross-bedding is of extreme 

 irregularity and inconstancy. (Fig. 137.) (\\'alther-22 : 775 

 [5^9] •) Huntington (15) has shown that a characteristic feature 



