of truncation where the formations are tilted along the granite mass. The cross 

 section shows that the limestones are of about the same thickness here as at points 

 much farther into the basin, and there are no conglomerates that would suggest 

 a near-shore sedimentation environment. The conclusion is that the higher rate of 

 thinning is caused by truncation of the upturned edges of the strata, and the close 

 spacing of the contours is, therefore, maintained parallel to the granite area. 



Around the uplift on the east side of the map area, the control points show 

 a high rate of thickening. The well samples contain large quantities of coarse 

 arkosic sands and conglomerates, and it is assumed that the granite mass was 

 the source of the sediments. With this knowledge at hand, the contours are 

 drawn so that the nature of these deposits indicates the size and shape of the 

 blank granite area within the zero line. Finer sediments in wells 190, 450, and 

 590 suggest a higher and less precipitous terrane; hence, the nose plunging to 

 the southeast corner of the area. 



It might be pointed out that the map B and section C clearly show that the 

 central uplift is older than the sediments and at the time of sedimentation was 

 higher than the granite area on the west side of the area. Conversely, the 

 western arch is probably younger than the sediments, because the flanking rocks 

 are similar to those in the central portion of the structural basin. 



A common source of error in subsurface isopach maps is the too-great 

 apparent stratigraphic interval caused by steeply dipping strata at the point 



Figure 24-15. Subsurface structural map on top of Pennsylvania?!. 



470 



