6io PRINCIPLES OF STRATIGRAPHY 



delta in Lake Geneva has a thickness of i8o to 275 meters and a 

 length of nearly two English miles. 



Delta Slopes. The surface of the delta has always a gentle 

 slope, this being steeper in the smaller deltas than in the larger ones, 

 and steeper also in the deltas of coarse material than in those of fine 

 silts. The frontal slope of the delta is, as a rule, much steeper, 

 being sometimes as high as 25° or 30°, or in some cases even 35° 

 in the small deltas of Pleistocenic and modern lakes. In the 

 larger deltas, especially those on the sea coast, the frontal slope 

 is much gentler. The strata of the Rhone delta in Lake Geneva 

 are so slightly inclined that they almost take on a horizontal atti- 

 tude. The total thickness of this delta, 180 to 275 meters, is dis- 

 tributed over nearly 2 miles. The frontal slope is i to 18. A 

 much gentler slope is seen in the delta of the same stream in the 

 Mediterranean, where the rate is i in 160, the depth increasing 

 from 4 to 40 fathoms in the distance of 6 or 7 miles from the mouth 

 of the stream. 



Deltas of small lakes often show a steeper inclination for the 

 older coarser beds than for the finer younger ones. Thus the delta 

 of the Aar in the Lake of Brienz shows near the shore an inclina- 

 tion of the beds amounting to 30 degrees. About 300 meters from 

 the shore the grade has decreased to 20 degrees, while at the ex- 

 treme margin of the present delta, 1,100 to 1,200 meters from the 

 shore, the beds are nearly horizontal. The delta of the Dundelbach 

 in the southwest angle of the little Lake of Lungern in Switzer- 

 land shows coarse beds near the margin, sloping at an angle of 35 

 degrees, while the younger layers have a very gentle slope only. 



The Bird-foot Delta of the Mississippi. The lower part of 

 the Mississippi delta has a remarkable form, distinguishing it 

 from all other modern deltas. From Forts Jackson and St. Philip 

 onward for a distance of nearly 25 miles the river is confined in a 

 narrow channel or "neck" which finally divides at the "Head of the 

 Passes" into three divergent channels, or passes, each bordered 

 by low banks of stifif clay and forming a structure resembling a 

 bird's foot. One of these passes, the Pass a I'Outre, divides again 

 into the North Pass and the Northeast Pass. The other two, the 

 South Pass and the Southwest Pass, continue, as single narrow fin- 

 gers, the latter for nearly 20 miles. Some distance above the head 

 of the passes a similar channel, the Main Pass, extends northward, 

 and still farther up a group of small channels diverges from the 

 neck. The material composing the banks of the neck and the 

 passes is wholly unlike ordinary river silt, though in general a thin 

 superficial layer of this occurs. Primarily, however, the banks 



