GEOLOGY OF GILES COUNTY, VIRGINIA 
373 
as the main source of the material which composes it. Be- 
tween the limestones of this time and those of Beekmantown age 
in the area under discussion, the difference is very slight and it 
must be regarded that the sedimentation went on almost unin- 
terupted from the beginning of the Upper Cambrian to the close 
of the Beekmantown. We will regard the deposition of the Shen- 
andoah limestone, then, as having taken place during this inter- 
val. The close of the Beekmantown marks the beginning of a 
new arrangement in Eastern North America. 
A new fold was developed, nearly parallel with, and a little 
within the western border of the original Lower Cambrian 
trough and another fold, mentioned before as having emerged 
early in Middle Cambrian time, was accentuated. Between these 
two folds, a trough was formed which extended from Alabama 
to Quebec. It is doubtful, however, if this trough was ever again 
completely submerged after Beekmantown time. Ulrich and 
Schuchert have named the western one of these two folds, the 
Appalachian Valley Barrier, and the eastern one was named the 
Chilhowee Barrier. The Lenoir bay, which was the southern 
third of the space between the two folds, occupied a synclinorium 
containing several disconnected folds, which were high enough 
to affect the direction of currents, and consequently the char- 
acter of sedimentation. In a general way, these deposits may be 
divided into an eastern trough (Athens trough) and a western 
one (Knoxville trough). The members of each overlap or grade 
into each other on account of differential warping. 
The area under discussion lies in the Knoxville trough. The 
development of the Chickamauga limestone here indicates that 
there was at the time of deposition of its bottom layers enough 
warping to permit portions of the older Shenandoah to be sub- 
jected to erosion. The breccia which goes to make up the lower 
layers of the Chickamauga formation in this area is made up of 
angular fragments of chert which is identical with that found in 
the Shenandoah of today. That this chert in the breccia was de- 
rived from the older formation is evident and that it was carried 
but a short distance is evidenced by the fact that the fragments 
are always angular and in the upper layers of the breccia are 
seldom in contact one with the other. 
The Chickamauga of this area, that is, of the Knoxville trough, 
contains a fauna wholly distinct from that of other Chickamauga 
