168 GEOLOGICAL SURVEY OF ALABAMA. 
lowed for a considerable distance, gradually merging into a 
fault which cuts into it in the direction of Montevallo. 
One of the most important results of this disposition of 
the rocks is to bring to the surface a great area of Trenton 
limestone with very gentle dip, except at the southeastern 
edge of the basin, all along the river for several miles each 
way from Pratt’s Ferry. Much of this limestone is of very 
great purity, and is capable of receiving a fine polish, and it 
will undoubtedly very soon come into use for building and 
ornamental purposes. 
That part of the valiey below Montevallo differs slightly 
in structure from the upper part. Starting at the Sub- 
Carboniferous measures which here form its southeastern 
border we pass over a valley of Trenton limestone, then over 
a wide area of Knox Dolomite, three or four miles, chiefly 
cherty lands, into a belt about a mile wide of the Cambrian 
shales of the Montevallo type. Then comesa fault by which. 
the Knox Dolomite is again brought to the surface. This nar- 
row belt of the Knox Dolomite runs out entirely in township 
24, range 11, east, but at the base of the map in township 24, 
range 10, it is perhaps half a mile wide. The southeastern 
edge of this belt of Knox Dolomite has a nearly vertical 
position, and, together with part of the Trenton, forms the 
edge ofthe synclinal above named. 
In the upper part of the valley down to about the upper 
line of nship 22, the edge of the coal field runs approxi- 
mately parallel to the strike of the rocks exposed in the 
valley, bu ow the point named this is not the case, for 
the strata of the Montevallo shales that are in contact with 
the vertical measures of the Cahaba Field through township 
22, and the upper part of township 24, have a strike nearly 
northeast and southwest while the edge of the coal field runs 
nearly north and south; the Cambrian strata appear to run 
up against the Coal Measures at an acute angle; and while 
the border of the Cahaba Field above Montevallo runs nearly 
north and south, changing abruptly at Montevallo to nearly 
east and west, the strike of the older rocks remains ap- 
proximately the same, i. e., nearly northeast and southwest. 
At the apex of the right angle formed in this change of di- 
rection, a little southwest of Montevallo, near Thompson’s 
