191^^ Geology of Chapel Hill 27 



ture topography because its surface lies chiefly in slopes with 

 but little reduction of the summit level of the divides. 



There are a few hills rising from the Coastal Plain near its 

 margin almost to the elevation of the upland plateau and on this 

 plateau several hills reach an elevation of 100 to 200 feet above 

 it. Among these are Nunn's Mountain, Blackwood Mountain, 

 Mount Collier, and others. 



CYCLES OF EROSION 



Before the streams began to cut the valleys now forming, the 

 plateau was nearly continuous and unbroken save for a few 

 hills, that at Hillsboro for example, rising from it. As the pres- 

 ent stream work proceeds, the floodplains gradually work head- 

 ward along the streams thus developing a longer and broader 

 plain at the new level. On this floodplain, though sometimes 

 covered, are sand, gravel, and waterworn rocks brought by the 

 current during freshets or left when migrations of the channel 

 occurred. When the new level cut by each stream meets, in 

 places, that formed by the other streams, the result is a very 

 broad plain with hills rising from it. These hills are called 

 monadnocks and the level formed is a peneplain. A base level 

 is completed when all of the monadnocks have been removed by 

 erosion. 



The changes made in the young valleys can be observed after 

 each hard shower or from year to year. A lifetime, however, 

 reveals very little if any increase in width of the mature valleys 

 by lateral planation. The period between the time when the 

 streams begin to erode the upper plain, and the completion of the 

 one at the lower level, constitutes a cycle of erosion. The dura- 

 tion of such a period is very, very great and a cycle of erosion 

 is one of the longest definite intervals of geologic time. 



^Numerous specimens of waterworn (smooth and rounded) 

 pebbles of quartz and its varieties up to four inches in diameter 

 have been collected by the writer and others from various places 

 on the upland and hill tops both east and west of Chapel Hill. 

 These can be none other than the surviving ones from the 

 gravels of a river floodplain as it was being developed at the 



