166. GEOLOGICAL SURVEY OF ALABAMA. 
valley underlaid by the Trenton or Pelham limestone, 
which is here remarkably well developed, and extensively 
quarried to supply the lime kilns at Siluria and Longview. 
This lime is the well known Shelby lime. 
Beyond this Trenton limestone valley, which is the 
Cahaba Valley proper, towards the northwest comes a high 
ridge formed of the chert of Knox Dolomite, known in its 
different parts as New Hope Mountain, Mill Ridge, Pine 
Ridge, and Anderson Mountain. 
Next follows a valley based upon calcareous parts of the 
Knox Dolomite and the variegated shales of the Cambrian. 
This has the name of ’Possum Valley, and as we approach 
its northwestern edge we find the strata gradually assuming 
a steeper dip up to the edge of the great fault spoken of 
above, and beyond this fault are the vertical measures of the 
Cahaba Field. It may be noticed here as almost every- 
where else that the dip of the strata on the southeast side 
of one of these thrust fault is usually considerably less than 
on the northwestern side, where they frequently stand 
nearly vertical. This is in conformity with the law of 
structure that prevails through the whole Appalachian 
region, viz., the steeper dip is on the northwestern side of 
the folds and faults, except where there has been an under- 
shoving of the strata, as is the case in Murphree’s Valley. 
Beyorid the fault, the measures very rapidly flatten down to 
a moderate rate of dip, which is mostly towards the south- 
east, showing that taken as a whole this upper part of the 
Cahaba Field is a synclinal basin, the axis of which is very 
elose to its southeastern boundary. The southeastern half 
of this synclinal is partly engulfed in the great fault, for 
there is usually not room enough between the axis of the 
synclinal and the boundary fault for the whole thickness of 
the Measures to come in, even in vertical position. 
Southward of the latitude of Siluria the coal-bearing 
measures of the Coosa Field give out, but the underlying 
Sub-Carboniferous strata continue as far as the limits of 
this map, and beyond even, till they are completely hidden 
below the Cretaceous beds of the Tuscaloosa formation. 
This southward prolongation of the Coosa Field is made 
chiefly by the Oxmoor shales and sandstones, which, 
