558 CENOZOIC ERA— AGE OF MAMMALS. 



These jDhenomena are shown in all the river-beds of the Drift region, 

 but especially by those of the Mississippi basin. Sometimes there is 

 only one terrace or bluff ; sometimes there are several, on each side. 

 The Connecticut Eiver is a good example of the latter, the Mississippi 

 Eiver of the former. 



The Connecticut Eiver is bordered on each side by a succession of 

 terraces rising one above and beyond the other, composed wholly of 

 old river-deposit. Beyond this, of course, is the country rock of Jura- 

 Trias sandstone, covered more or less with drift. 



The Mississippi River is bordered on each side by its present flood- 

 plain deposit, or river-swamps. This, as already said (p. 25), extends 

 from the mouth of the Ohio Eiver to the head of the delta, a distance 

 of 500 miles, and has an average width of 30 miles. This, its present 

 flood-plain deposit, is limited on the eastern side by bluffs in some 

 places 200 to 400 feet high, composed of Tertiary strata, capped with 

 an old river-silt, or Loess, 50 to 70 feet thick, and this, again, covered 

 by a yellow loam, which extends beyond the limits of the Loess. A 

 layer of Orange sand separates the Loess from the Tertiary. Patches 

 of the Loess or Muff -deposit are found also on the western side, show- 

 ing that the old flood-plain extended beyond the present flood-plain 

 on both sides ; but on the west side it has been mostly removed by sub- 

 sequent erosion. Also similar deposits, often of great extent, form 

 banks on each side of all the great tributaries of the Mississippi. Be- 

 neath the present river swamp-deposit is found, by borings, a deposit 

 belonging, like the Loess, to the Champlain epoch, but to an earlier 

 period, probably an estuary deposit, and called by Hilgard " Port Hud- 

 son" varying in thickness from thirty feet at Memphis to several hun- 

 dred feet in the delta. Beneath this is first the Orange sand and then 

 the Tertiary. 



All these facts are represented in the ideal section of the river and 

 the strata in its vicinity, given below, constructed from the investiga- 



[\A/^^ ... _ --*_*?***= 



Fig. 933.— Ideal Section across Mississippi below Vicksburg: OS, Orange sand; PR, Port Hudson, 

 estuary deposit, Champlain; Is, Loess or old flood-plain deposit, Champlain; /, loam covering 

 the Loess, but more extensive; rs, river-swamp deposit, moderate. 



tions of Prof. Hilgard. It is evident that a great trough was hollowed 

 out in the Tertiary strata during the late Tertiary and early Glacial 

 epoch, filled with deposit to the level I I during the Champlain, and 

 again partly cut out during the Terrace. 



