BASSLB&.  1 
CEMENT    MATERIALS    OE    VALLEY    OF    VIRGINIA. 
33 
GENERAL  GEOLOGY. 
The  principal  rock  formations  occurring  in  the  Valley  of  Virginia 
are  a  great  series  of  limestones  termed  the  Shenandoah  limestone 
and  a  series  of  shales  named  the  Martinsburg  shales.  In  general 
the  entire  valley  is  underlain  by  the  Shenandoah  limestone,  while 
the  shales  usually  outcrop  along  the  base  of  the  mountains  bounding 
it.  Both  of  these  formations  yield  an  abundance  of  the  raw  mate- 
rials necessarv  for  the  manufacture  of  Portland  cement. 
tr*     i— '     >=l- 
#v 
'm. 
Martinsburg  shale  and      Trenton  limestone  Crystalline 
overlying  formations        (cement  rock)  limestone 
Kittatinny  or  Knox 
•magnesian  limestone 
Fig.  24. — Map  of  the  valley  of  Virginia  from  Staunton  to  Natural  Bridge. 
Area  marked  by  diagonal  broken  lines  includes  Knox  limestone  and   underlying  forma- 
tions. 
SIIENAND0A  II    LI  M  ESTONE. 
Thickness,  character,  and  subdivisions. — This  formation  has  a  very 
great  thickness  and  is  composed  of  several  members,  all  of  which 
have  been  grouped  under  the  above  general  name.  The  chemical 
composition  of  its  several  divisions  varies  greatly,  some  of  the  lime- 
