28 JOUENAL OF THE MiTCHELL SoCIETY [Juue 



temporary base level of that time. Similarly rounded smooth 

 pebbles in similar positions are found throughout the Piedmont 

 Belt. Also the folded layers of stratified and metamorphic 

 rocks of this region are truncated at the elevation of the upland. 

 The plateau level of the area about Chapel Hill and elsewhere in 

 the Piedmont is therefore a peneplain. 



On this peneplain several monadnocks occur, the hill at Hills- 

 boro for example, also numerous hills of igneous origin that are 

 in all probability monadnocks. Among these are Nunn's 

 Mountain and Blackwood Mountain, each about five miles north 

 of Chapel Hill, and Mount Collier^ about the same distance to 

 the west. 



Two cycles of erosion are therefore evident in this vicinity, 

 one nearly completed, during which the peneplain was 

 formed, the other now in progress, during which the valleys are 

 being excavated and the new temporary base level developed. 

 When the Piedmont as a whole is considered, strong evidence of 

 earlier cycles of erosion is found. 



AKCHEOZOIC EKA 



The rocks of this system consist of gneisses, schists, slates, 

 etc., with granites and other igneous rocks intruded into them. 

 The metamorphic rocks are complexly folded and have been 

 truncated by erosion. All are disintegrated to a depth of 20 

 to 50 feet. They occur throughout the vicinity except in a 

 circular area near its center and are closely related in age with 

 the oldest rocks of the American continent.^ 



PKOTEKOZOIC ERA 



The rocks of this system are conglomerates, both basal and 

 intraformational, sandstone, porcellanite, slates, schist, etc., 

 interstratified with numerous acidic lava flows, chiefly trachytes 

 and rhy elites. The outcrops are along the right (south) valley 

 slope of Morgan's Creek, and are best exposed at the old site of 



'Mount Collier is so called in honor of Professor Collier Cobb, Head of the 

 Department of Geologj' in the University of North Carolina, who in 1892 was the 

 first to recognize its volcanic origin. 



- Pre-Cambrian Geology of North America, 1909, Van Hise and Leith. Bui. 

 360, U. S. Geol. Survey, p. 677 ff. and 695 ff. Gives a summary of Pre-Cam- 

 brian Geology of North Carolina and lists references for the State. 



