SEDIMENTARY FORMATIONS 307 



posed of anticlines and synclines and secondary and tertiary anticlines 

 and synclines. The axes of the major folds strike northeast-southwest 

 and the axes of the minor folds northwest-southeast. 



The crystallines of the southeastern belt emerge from beneath the 

 Trias and on the southeast flank of the great anticlinorium to form the 

 Chester Valley synclinorium. The limbs of the synclinorium, sepa- 

 rating it from the bounding anticlinoria, are formed by the Chickies 

 quartzite. This formation, resisting erosion more successfully than the 

 limestone, gives rise to the hills which bound Chester valley in its ex- 

 tension to the northeast. These hills are thus monoclinal in structure. 



To the southwest the southeastern limb is first partially faulted out 

 and then concealed by a younger formation, which, appearing first at 

 Conshohocken, expands southwestward and forms the south Chester 

 Valley hill. This expansion is due to a general southwest pitch of the 

 synclinorium, caused by minor folding. It is this southwest pitch that 

 brings the nose of the synclinorium to the surface in the northeast end 

 of Chester valley and causes the expansion northwestward of the oldest 

 formation, the Baltimore gneiss. The character of the synclines and 

 anticlines which compose the synclinorium is inferred from that of the 

 secondary folding on the limbs of anticlines and synclines. These sec- 

 ondary folds (see plate 51) in the Chester Valley limestone are iso- 

 clinal in character, the isoclinal dips varying from 45 to 80 degrees 

 southeast. The more pliant mica-schist shows closer folding and a 

 crumpling absent in the limestone and quartzite. 



The synclinorium is bounded on the southeast by a fan-shaped anti- 

 clinorium, which brings to the surface the Baltimore gneiss. The south- 

 west limb of this anticlinorium is faulted, and west of the Schuylkill the 

 northwest limb is also faulted. This fault, called the Cream Valley 

 fault from the local name of the valley which it marks, brings about the 

 almost complete disappearance of the limestone and quartzite of the 

 northwest limb of the anticlinorium. Limestone appears in Mechanics- 

 ville, west of the Schuylkill, on the crest of a subordinate anticline, and 

 is then cut out by a thrust fault, which has brought the Baltimore gneiss 

 on a level with it. 



The anticlinal character of the folding of the Baltimore gneiss is shown 

 in the Schuylkill river section. The secondary folding is steep on either 

 limb of the anticlinorium and gentle in the center. The dips vary 

 from verticality to a steep southeast dip on the northwest limb and are 

 predominatingly northwest on the southeast limb, thus producing a 

 somewhat fan-shaped anticlinorium. There is a very gentle northeast 

 pitch indicating a subordinate minor fold. The only other locality 

 where the structure of the Baltimore gneiss can be seen is in a cut of the 



