Monday, August 5, 2019

Falls Leucogneiss – Median of Centennial Parkway – Raleigh, NC


Rock Type:  Lineated granitic gneiss

Geologic terrane or major geologic element:  Raleigh terrane

Age:  Late Proterozoic  – approximately 550 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Raleigh West

Site Access:  Centennial Parkway runs in a pair of large arcs between Avent Ferry Road and Lake Wheeler Road.  It borders the Centennial Campus of NC State University and the State Farmers Market.  Traffic on Centennial Parkway may be heavy at times and moves quickly.  This site should be approached on foot and safely.  Parking may be found in the nearby Mission Valley Shopping Center.  Large fresh blocks of Falls leucogneiss, that were excavated during construction of the Parkway, have been placed in the median (Figure 1).



Figure 1.  Median of Centennial Parkway with blocks of leucogneiss.
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Technical Information:  Blake and others, 2001, A Temporal view of terranes and structures in the eastern North Carolina Piedmont, in Geological Society of America, Southeastern Section, Field Trip Guide for 2001.  See description for Stop 2 on p. 164-166.
Stoddard, E. F., and Blake, D. E., 1994, Carolina Geological Society Field Trip Guide, 1994.  See the descriptions for Stop 9 and 9A on pages 101-103.
Caslin, L. A., 2001, Age and significance of the Falls leucogneiss, Wake County, North Carolina:  M.S. thesis, NC State University, 39 p.
Farrar, S. S., and Owen, B. E., 2001, A north-south transect of the Goochland terrane and associated A-type granites – Virginia and North Carolina, in Geological Society of America, Southeastern Section, Field Trip Guide for 2001.
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Introduction
       The Falls leucogneiss is a very distinctive and unusual rock unit that runs beneath downtown Raleigh.  It is important in the geological history of the region, and it has also exerted significant influence on the region’s human history.  Because it is unusually hard and resistant to weathering and erosion, there are many natural exposures of Falls leucogneiss in Wake County.

Nature of the rock
       The prefix “leuco” means light-colored, as in leucocytes (white blood cells).  So this rock unit is a light-colored gneiss, meaning that it contains less than 10% dark minerals.  In fact, most samples of Falls leucogneiss have less than 5% dark minerals.  The remainder consists of quartz and two varieties of feldspar, and their relative percent classifies the igneous precursor of the leucogneiss as granite.  The sparse dark minerals are biotite (black mica) and magnetite.  All the mineral grains are small in size.  Gneiss is a metamorphic rock that is characterized by alternating darker and lighter layers.  The Falls leucogneiss then is a granitic gneiss.  However, the distinction between this rock and other varieties of granitic gneiss is its strong lineation.  In most gneisses, the layering is prominent, but in Falls leucogneiss it is difficult to discern.  Instead, this rock is characterized by very thin parallel lines of dark minerals that run through (Figure 2).



Figure 2.  Fresh block of Falls leucogneiss showing strong lineation.  Note how the thin dark lines are visible only on the side of the rock, not on the end.  When the leucogneiss decays by weathering, thin “pencils” of rock material are formed that accumulate on the ground.

Geometry and magnetic expression
       The Falls leucogneiss runs in a narrow (typically about one km wide) band for about 80 km (50 miles) from near Lake Wheeler north to Henderson, NC.  The direction of its trace correlates with the trend of the lineation seen in outcrops of the leucogneiss.  Furthermore, the lineation is nearly horizontal in most exposures, or else it plunges gently toward the north or south.  Because of the lineated magnetite grains in the rock, the leucogneiss is relatively strongly magnetic.  (Most magnetic rocks contain mostly dark minerals; light-colored rocks are almost always non-magnetic.)  A sensitive magnet (for example, a small bar magnet tied to a string) will be attracted to a fresh piece of Falls leucogneiss.  The magnetism is strong enough that this rock unit shows up clearly on aeromagnetic maps of the region (Figure 3).



Figure 3.  Aeromagnetic map of the Wake County area.  The various patterns and trends are the effect of varying magnetism of the rocks.  The location of the Falls leucogneiss is indicated by the arrow.  Its NNE trend is clearly displayed.

Geological significance
       Most of the Raleigh terrane is made up of Raleigh gneiss, which is typically dark gneiss or well-layered gneiss with some dark layers; most likely it was originally igneous rock.  The Falls leucogneiss is thought to represent granite that intruded into this igneous precursor rock of the Raleigh gneiss, then much later they were both deformed and metamorphosed, during the formation of the Appalachian Mountain belt.  One of the blocks of leucogneiss at the Centennial Parkway site contains some Raleigh gneiss (Figure 4).


Figure 4.  Block of Falls leucogneiss in the median of Centennial Parkway.  The dark stripe on the left side is thought to belong to another rock unit, Raleigh gneiss.  This is taken as evidence that the igneous precursor of the leucogneiss intruded the precursor of the Raleigh gneiss.

       There is a major fault zone that runs through the eastern Piedmont of North Carolina, parallel to and just west of the Falls leucogneiss.  It is called the Nutbush Creek fault.  Geologists know that the Nutbush Creek fault was active about 300 million years ago, and that it was a right-lateral strike slip fault.  In this type of fault, the two sides move sideways along the fault (not up and down), with the opposite side moving toward the right when viewed from across the fault.  (This is the same type of fault as the San Andreas fault in California.)  The Nutbush Creek fault (and other similar Piedmont faults) were formed during the collision of continental plates that created the Appalachians.  The colliding plates did not meet exactly head-on, but obliquely, forming right-lateral strike-slip faults like the Nutbush Creek.

Influence on streams and topography

       Streams in the central and eastern Piedmont flow generally east and southeast, across the north-northeast trend of the Falls leucogneiss.  Because of its resistance to erosion, the leucogneiss presents a major barrier.  Consequently, streams may be diverted where they meet the rock unit (Figure 5), and where they do flow through it, the streambed is very rocky, with rapids and waterfalls prevalent.  In fact, this is how the old community of Falls, in northern Wake County, go its name (and of course how the leucogneiss got its name).  Naturally, the places where streams cross the leucogneiss made terrific locations for dams and mills.  Lake Wheeler, Lake Raleigh, and Falls Lake all have dams built across streams where the leucogneiss is located; Lassiter Mill, Yates Mill, and the old Falls Mill were also constructed there.

Figure 5.  Topographic map of a portion of Crabtree Creek, showing the right turn made by the creek as it encounters Falls leucogneiss (orange arrow), and the location of the Lassiter Mill dam (black arrow).

       The leucogneiss also has produced north-northeast trending ridges in the local topography in a few places.  Two good examples are Lake Wheeler Road from Tryon Road south to Yates Mill, and Oberlin Road between Hillsborough Street and Glenwood Avenue.  Alas, neither Ridge Road nor Blue Ridge Road follow the leucogneiss.

Use as a building stone
       Falls leucogneiss was a favored building stone during Raleigh’s history, and numerous small quarries produced blocks of stone that were used to construct many historic buildings, walls, and steps in the area.  The location of Glenwood Village Shopping Center, at the intersection of Oberlin Road and Glenwood Avenue, was one such quarry.  A small section of the wall of the former quarry may be seen behind the Harris Teeter grocery store there.  Broughton High School and several of the older churches in downtown Raleigh, as well as many older homes inside the beltline, and commercial buildings (for example, Mitch’s Tavern on Hillsborough Street) are all constructed from Falls leucogneiss.

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