702 PRINCIPLES OF STRATIGRAPHY 



their upper surfaces. Successive beds may have their oblique lam- 

 inae inclined at different angles or in different directions, and hori- 

 zontal layers may rest upon the truncated edges of inclined layers, 

 thus simulating unconformity. This oblique lamination may range 

 in magnitude from beds a millimeter or less in thickness to strata 

 having a thickness of a hundred feet or more. In such a case the 

 deposit was generally formed as a delta in a standing body of w^ater, 

 w^ith strong currents flowing in and depositing the sands or pebbles 

 on the growing delta front. Oblique beds formed in this manner 

 are known as "fore-sets," and are generally truncated at the top by 

 the currents, which deposit horizontal beds or "top-sets," generally 

 of coarser material, upon the truncated upper edges. Basally the 

 fore-set beds will either rest upon or directly merge into the bot- 

 tom-set beds, which are made of finer material, mostly clay and silt. 

 The angle of the fore-set beds generally decreases outward, the 

 last fore-sets of a large delta being fine and less steeply inclined 

 than the older ones (see ante, p. 6io). Small deltas were formed 

 in numerous localities toward the end of the last glacial epoch in 

 standing bodies of water along the ice front. Their structure can 

 readily be examined in many sand pits. Several types of cross- 

 bedding may be distinguished. 



a. Delta Type. This type of cross-bedding, already noted, con- 

 sists essentially of a single bed of diagonal layers bounded below 

 and above by nearly horizontal beds. It appears to be characteristic 

 of deltas deposited in a standing body of water. Whether or not 

 this type is also characteristic of deltas formed on the sea coast 

 depends on the strength and magnitude of the tides. It is evident 

 that only one series of fore-set beds will be formed in any given 

 delta, and that relatively stationary conditions alone permit the 

 formation of the delta. If, after the building of a delta in a lake, 

 a rise in the water level should occur, a new delta might be built up 

 over the old one, and thus the resulting formation would show two 

 sets of diagonal beds, separated by horizontal beds, the top sets of 

 the first and the bottom beds of the second. Such a superimposition 

 of two deltas is, however, difficult to conceive of on a subsiding sea 

 coast where subsidence is accompanied by transgression, and con- 

 sequent transference of the zone of delta building. 



b. Cross-bedding of Torrential Deposits. Superimposition of 

 obliquely bedded strata is, however, eminently characteristic of tor- 

 rential deposits. Each succeeding deposit will be characterized by 

 fore-set and top-set beds, and the number of such superimposed 

 strata is chiefly dependent on the frequency of recurrence of the 

 torrents. The length of the fore-set bed formed by a river on dry 



