Geologic Setting and Regional Geology
 
Prologue

 

The geologic story of the rock types and their geologic history was determined through many years of research by many investigators in the Piedmont of North Carolina and the immediate area surrounding Falls Lake.  This story combines many geologic investigations conducted within the Falls Lake area and from other locations throughout the Southeast.  For a scientific review of the geologic interpretation of the various crystalline rock terranes in the Falls Lake area, see Hibbard et al. (2002) and Blake et al. (2001) and references therein.  A detailed review of the rocks of the Durham Triassic basin is provided in Clark et al. (2001).  Descriptions of the rocks in the immediate vicinity of Falls Lake can be found in Horton et al. (1994) and Stoddard et al. (1994).

 

The Lake and its Rocks

 

The water in Falls Lake begins its journey in the Eno, Flat and Little Rivers.  Geologically these rivers drain a portion of the Carolina terrane (aka Carolina slate belt) an ancient (630 to 615 million year old) volcanic island arc.  The rocks of the Carolina terrane record a long and sometimes violent geologic history of millions of years of volcanic eruptions with countless ash and lava flows.  The rocks of this portion of the Carolina terrane have been metamorphosed (changed through heat and pressure) but retain some of their original textures that allow geologists to interpret the rocks as ancient lavas, tuffs (rock formed from solidified volcanic ash) and plutonic intrusive rocks (rocks formed by the slow cooling of magma).

The Eno, Flat and Little Rivers then converge to form the Neuse River (now impounded by Falls Dam) in a portion of the North Carolina Piedmont geologically known as the Triassic basin.  The Triassic basin is underlain by two relatively younger rock types: 1) approximately 225 million-year-old sedimentary rocks; and 2) approximately 200 million-year-old igneous rocks called diabase (present as dikes and sills).  In comparison to the hard crystalline rocks of the Carolina terrane, the Triassic sediments (conglomerates, sandstone, siltstone and mudstones) are relatively easy to erode.  Before the creation of Falls Lake in 1981, the Neuse River had cut a wide flood plain across the easy-to-erode Triassic sediments.  When water from the impounded Neuse river flooded the former wide flood plain it created a relatively wide lake (compared to areas of the lake east of Highway 50). 

The sedimentary rocks of the Triassic basin tell part of the geologic story of how North Carolina sat in the center of the supercontinent Pangea at the beginning of the Triassic Period.  Great forces deep in the earth initiated the breakup (or rifting) of Pangea.  The continents did not rift apart in a simple line.  Rifting initially progressed in a piecemeal manner with a main rift and many smaller rifts.  Several rifts opened a short distance but stopped when the main rift became dominant.  The main rift later became the Atlantic Ocean.  The rifts that opened a short distance and stopped (failed rifts) became rift-valleys that quickly began to fill with sand, silt and clay.  These failed rifts are part of a system of rift basins, called the Newark Supergroup (Olson, 1978; Froelich and Olsen, 1984), that formed all along the east coast of North America.  The rift basins are similar to the modern day East African Rift system.  The city of Durham and a portion of Falls Lake sits within one of these rift basins, called the Durham Triassic basin.  The sand, silt and clay deposited within the Durham Triassic basin later turned into the red to maroon colored conglomerates, sandstones, siltstones and mudstones common along the banks of Falls Lake west of HWY 50. See Virtual Field Trip stop 10000 through 10006

The rock type diabase is present in several location along the banks of Falls Lake.  Diabase is solidified mafic magma from deep in the Earth that welled up through fractures in the crust at the beginning of the Jurassic period (approximately 195-205 million years ago).  This magma intruded the sediments of the Triassic basin and surrounding crystalline rocks.  Diabase is a mafic rock with a composition similar to ocean floor basalts.

Near where Highway 50 crosses the lake, Falls Lake crosses a major geologic boundary (an ancient fault called the Jonesboro fault) and flows back into hard crystalline rocks of another portion of the Carolina terrane.  The Carolina terrane here is characterized by igneous intrusions (bodies of solidified molten rock called magma) that may have once feed volcanoes.  See Virtual Field Trip locations 10007 through 10009.

Further east, the lake crosses another ancient fault (called the Leesville fault) and the lake crosses out of the Carolina terrane and into the Falls Lake terrane.  The Falls Lake terrane is a group of rocks geologists believe experienced a similar geologic history and is separated from adjacent rock terranes by faults.  The dominant rock type in the Falls Lake terrane is the metamorphic rock called schist (Falls Lake schist).  Other interesting rock types present in the Falls Lake terrane are metamorphosed mafic and ultramafic rocks.  Mafic and ultramafic rocks are igneous rocks that are rich in iron and magnesium.  Geologist are not sure of the origin of the Falls Lake schist or the ultramafic rocks but they speculate that the ultramafic rocks may represent ancient sea floor that was ripped up and thrust into a pile of sedimentary rocks during the subduction of an ocean plate under a volcanic island arc.  These sediments and ultramafic rocks were later metamorphosed.  The sediments (siltstones and mudstones) were changed into the schist; the mafic and ultramafic rocks were changed into metamorphosed mafic and ultramafic rocks now called amphibolite, serpentinite, chlorite-actinolite schist, and talc schist (soapstone).  Some of the soapstone present in the Falls Lake area was utilized by early settlers for carving tombstones.  See Virtual Field Trip stop 10010 through 10014, 10017 and 10019.

Within the Falls Lake terrane, the lake occupies an entrenched meander which is best described by Parker (1979): "A remarkable entrenched meander of the river occurs in New Light township a mile north of N. C. Highway 98 and 1 1/2 miles east of N. C. Highway 50. The river turns north at right angles and makes a tight hook-shaped, double loop for 3 miles before returning to within 1500 feet of the initial bend, where it resumes its southeast course. Near the middle of this loop, where the river is flowing north, a spectacular cliff and crag (Zeagle's Rock) have been formed (fig. 15). The cliffs extend along the west bank of the river for several hundred feet and the crag overhangs the middle of the river at a height of about 55 feet. The rock here and in the vicinity is garnet-biotite gneiss that dips westward at 25 degrees into the ridge. The crest of the ridge above the crag and within the meander loop at elevations of 300-310 feet is covered with iron-stained quartz gravel, deposited when the river bottom was at upland level and meandered on the then flat peneplain. As uplift of the region rejuvenated the stream, it eroded downward in its existing course and laterally on the outside of its curve, undercutting the west-dipping gneisses. The river's meandering course, then, is ancient and its steep valley sides more recent."  No longer towering over the Neuse River at a height of 55 feet, Zeagle’s Rock is still an impressive landmark on the lake. See Virtual Field Trip stop 10014.

Continuing along the lake toward Falls Dam the lake crosses another ancient fault (called the Falls Lake thrust) that separates the Falls Lake terrane from the Crabtree terrane.  The Crabtree terrane in the Falls Lake area is generally underlain by metamorphic rocks called felsic gneisses and schists.  Geologist think that the felsic gneiss may be metamorphosed sandstones and tuff with the schists representing metamorphosed mudstones and shales that were deposited off shore of an ancient volcanic island.  The schists within the Crabtree terrane have long been known by local mineral collectors as yielding nice mineral specimens.  See Virtual Field Trip stop 10018 and 10020.

The rock units in the Falls Lake area are part of a giant geologic structure called a fold.  Specifically this fold is called an antiform.  (An antiform is similar to an anticline but geologist are unsure of the ages of the rocks so it is called an antiform.)  Geologic units are repeated from one side of the anticline/antiform to the other.  Geologists call this fold the Raleigh Antiform.  Virtual Field Trip location 10018 is present near the center of the antiform in the Crabtree terrane with the schist of the Falls Lake terrane present on the east and west sides (called limbs) of the antiform.  West dipping foliations in the Falls Lake terrane rocks are displayed in Virtual Field Trip locations 10011, 10012 and 10014 were as east dipping foliations are displayed in Virtual Field Trip stop 10017.

Traveling east, the lake crosses into the east limb of the Raleigh Antiform and crosses the Falls Lake thrust again and back into the Falls Lake terrane. 

Because of complex folding, the lake once again enters the Crabtree terrane (Virtual Fieldtrip stop 10016 and 10020) and then reaches the Falls leucogneiss, part of the Raleigh terrane at Falls Dam (Virtual Field Trip stop 10015).

 

The following text provided by Skip 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 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 Falls leucogneiss  , 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 Falls leucogneiss  .  The Falls leucogneiss  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 Falls leucogneiss  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 Falls leucogneiss  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 Falls leucogneiss  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.