10015: Falls Leucogneiss Detailed geologic map 10016 Link 10015 Link
10015: Falls Leucogneiss - Large outcrop from construction of emergency spillway for Falls Dam. Falls Leucogneiss
10015: Falls Leucogneiss - Large outcrop from construction of emergency spillway for Falls Dam. Falls Leucogneiss
10015: Falls Leucogneiss - Large outcrop from construction of emergency spillway for Falls Dam. Falls Leucogneiss
100015: Composite photograph of Falls Dam. Outcrop in above photographs present in far left of photograph. Falls Dam composite

View of outcrop of Falls leucogneiss looking south on northern end of dam.

 

Falls Dam and Falls Leucogneiss

Weathered area of outcrop on northern end of dam showing prominent "pencil" structures parallel to lineation. Weathered Falls leucogneiss
Fresh surface of Falls leucogneiss looking parallel to lineation. Lineation in rock is defined by elongate feldspars, quartz, magnetite and biotite. Fresh surface Falls leucogneiss
Fresh surface of Falls leucogneiss looking perpendicular to magnetite lineation. On surfaces perpendicular to lineation, the texture of the rock appears like an undeformed granite. Fresh surface of Falls leucogneiss
Text by Dr. Edward Stoddard:

The Falls leucogneiss runs in a narrow band, typically one-half mile or less in width, from the city of Henderson in Vance County, southward through Franklin and Wake Counties, ending near Lake Wheeler.  This rock is geologically unusual and interesting, and it also has played a significant role in the history of the region.

The Falls leucogneiss (hereafter abbreviated Fln) is a very hard rock type, and is therefore more resistant to erosion than most other rocks of the region.  For this reason, wherever streams and rivers encounter the Fln, they have a difficult time cutting through.  Over time, this has resulted in the occurrence of natural rapids and waterfalls.  In addition, the stream valleys and floodplains tend to narrow at the Fln.  The Fln is therefore an example of how a Fall Line is created, with the stream too narrow, steep, and rocky, and the water flow too swift to allow boats to easily move upstream.  (In this part of the North Carolina Piedmont, we usually speak of a Fall Zone instead of a Fall Line, because the Fln is not the only hard rock type that provided an impediment to water travel, but it is an excellent example.)

Locations where sizeable streams cross the Fln typically provide outstanding sites for construction of dams because the valley is narrow, the stream gradient steep, and the bedrock hard.  For early settlers, these locations were favored for grist or saw mill sites; more recently dams have been built for water supply, flood control, and recreation.  Such sites in Wake County, from south to north, include Lake Wheeler Dam on Swift Creek, Yates Millpond on Steephill Creek, Lake Raleigh on Walnut Creek, Lassiter Mill on Crabtree Creek, and of course Falls Lake Dam on the Neuse River.

Another topographic effect of the Fln is that it tends to express its course by creating a ridge, due to its resistance to weathering.  Because it runs, in large part, along a ridge running generally north-south, it was a logical place for trails and roads to develop.  Two roads in Wake County where you can see this effect are along Lake Wheeler Road south of Tryon Road, and on Oberlin Road between Clark Avenue and Fairview Road.