744 ARTHUR C. TROWBRIDGE 



roughly into different textural divisions. These strips diverge 

 from the axis of the fan. 



No straight section can be cut in such a deposit without cutting 

 the filled channels. If the cut is longitudinal, practically all the 

 channels will be cut obliquely and at low angles; a few might be 

 cut at right angles. If the fan is dissected by streams cutting down 

 in the old channels, buried channels will still be cut obliquely, as 

 distribution does not take place along lines exactly parallel with 

 previous distributaries. 



The filling of a buried channel, when cut along a straight line 

 oblique to the original channel, is exposed as a lens whose length 

 and degree of pinching out depends primarily on the obliquity of 

 the line of cut. A channel filled, buried, and then cut at right 

 angles reveals itself as a pocket in section, the size and shape of 

 which depends on the size and shape of the channel. Continuous 

 layers, uniformly thick can occur only where the depositing dis- 

 tributary was long, straight, and contained uniform material, and 

 where the filling was cut along a straight line exactly parallel to 

 itself. Obviously where exposures are along longitudinal cuts, 

 the result is many lenses, a few pockets, and practically no con- 

 tinuous layers. 



That this is the correct explanation of the lenses and pockets of 

 the fans of the region is shown by a correspondence in size between 

 lenses and present surficial channels. On the Sierra bajada, the 

 channels are about 8-10 ft. deep on the average, and the lenses and 

 pockets are about 8-10 ft. thick at their thickest parts. The present 

 flood surface in Mazourka Canyon has a relief of about a foot; the 

 lenses in the older alluvium near by have just about that thickness. 



It is understood that any deposit from distributing or anasta- 

 mosing streams will reveal a lens or pocket structure in straight 

 cuts. The principle probably applies to all alluvial fans, pied- 

 mont alluvial plains, flood-plain deposits, glacial valley trains and 

 outwash plains, and deposits on tidal deltas. 



THE DISSECTION OF THE SIERRA BAJADA 



A variety of events might bear causal relations to the dissection 

 of alluvial fans. Among them are changes in climate, uplift of the 



