Trail Section 11 - Falls Lake Trail Geologic Guide

 

FLT section 11-13 map

Geologic information by Edward F. Stoddard (in Bold Text).

Mileage and Trail section descriptions from Mark Edelstein with updates from March – April 2011.

 

Falls Lake Trail Section 11 (NC50 to Boyce Mill Rd)  (5.6 miles):

26.8 – By beginning of guardrail (On W side of NC50), turn L to reenter woods.

11-A:  Loose chunks of rock beside the trail begin to include epidote-rich varieties in addition to quartz.  Epidote is a common light green mineral that is found in metamorphic rocks that were dark igneous rocks prior to metamorphism.

11-B:  Some pieces of dark fine-grained amphibolite may be encountered.  Interestingly, these rocks probably do not represent the majority of the bedrock in this area.  They are pieces of older rock (basalt or gabbro) that was engulfed in magma of a lighter colored igneous rock.  These are called xenoliths.

27.1 – Reach a power line cut and turn R to follow this.

11-C:  In the gully at the power line, there is an outcrop of the lighter colored igneous rock, now metamorphosed.  Note the large nubbly surface created by large feldspar crystals.  This unit, the Reedy Creek metagranodiorite, is named for exposures in and around Umstead State Park, 20 miles to the south.

11-D:  As you start along the power line, look down to your left.  There is an excellent exposure of strongly foliated metamorphosed granodiorite or diorite in the creek.  If you care to investigate, you will see that the foliation surface is locally covered with long dark crystals all pointing in the same direction.  The crystals are hornblende, and their steeply plunging lineation is an important clue to the history of faulting in this region.  Also at this outcrop, it is easy to understand what strike and dip mean.  Strike is the direction of a line that is the intersection of a dipping rock layer (foliation in this case) with a horizontal plane.  When a dipping plane dips under water, the strike is the direction of the water line along the rock.  In this case the strike is about 40° to the east of north (or N40°E).  The dip is the downward angle from horizontal of the dipping layer.  In this case it is about 48°.  It is also necessary to specify the direction of the dip.  In this case, a geologist would say the foliation has a strike and dip of N40°E, 48°NW.  (Or using the right-hand rule and azimuth convention, 220, 48.)

27.2 – By a pole, turn R, back into the woods.

27.3 – Walk between 2 large, downed trees, both to sides of the trail.

27.4 – Cross a small feeder creek.

11-E:  Very shortly after the creek, you will encounter a downed tree whose root ball is full of chips of fine-grained amphibolite.  Another xenolith probably.

11-F:  When you reach a point nearest Falls Lake, an island in the lake may come into view.  If the water is not high, you may be able to observe the eroded bank of the island.  If you can, note that there is an abrupt change in the appearance of the bank:  to the right, there are no big rocks present, but to the left there are lots of very large rounded rocks along the bank.  Believe it or not, this seemingly subtle change is the location of a major fault, the Jonesboro fault, which we are approaching, but in a circuitous manner.  This fault separates the metamorphic rocks we have been seeing ever since the Falls Lake Dam, from much younger (Triassic) sedimentary rocks.

27.8 – Use a footbridge to cross a small creek.

11-G:  Just upstream from the bridge there is an outcrop of fine light-colored metaigneous rock.  The rock is strongly fractured with east-west joints.  This fracturing is most likely related to the Jonesboro fault.

28.0 – Turn R to stay on the trail and soon cross an old roadbed.

28.2 - After walking through a low area, the trail turns R.

The low area around the creek is a floodplain.  The floodplain is the flat area on both sides of the creek, formed by sediment laid down during floods.  Flooding is a very natural process, and necessary for the health of a stream and its surroundings.  The bigger the stream, the wider the floodplain.

11-H:  As the trail rises and reaches a point closest to the lake, note there is a round boulder of granite beside you.  This is significant:  you have crossed the Jonesboro fault.  There are two ways to produce round boulders:  in the bed of a fast-moving stream, or by the process of spheroidal weathering.  In this region, spheroidal weathering occurs most often with diabase, a common fine-grained mafic igneous rock type, and results in dark-colored round rocks, frequently falling in a crude line along the trace of a dike.  If you see round boulders of more than one rock type, you can be pretty sure that they were stream-rounded.

The Triassic basin, which we have just entered, differs from the metamorphic terrain in many respects.  One way it differs is there are no quartz veins, so you will see very little, if any chunks of angular quartz.  The reason for this is that quartz veins are formed when rocks are metamorphosed, as silica is precipitated into cracks in the older rocks.  Because the Triassic basin rocks have not been metamorphosed, no quartz veins were formed there.

Sedimentary rocks underlie most of the Triassic basin; these rocks include siltstone, sandstone, mudstone, and conglomerate, all clay-rich.  Nearly all produce soils that are very poor for agriculture, except for a few areas underlain by unusual clay-poor quartz sandstones.  In addition, these rocks are also very poor producers of groundwater, so water wells typically do not yield much water.  Ironically, most sandstones are good aquifers, but the North Carolina Triassic rocks are so clay-rich that water cannot move through the pores in the rock.  If you want to drill a water well in the Triassic basin, your best bet is to find a diabase dike.   Despite the fact that the diabase is hard and dense, it is typically fractured or jointed with intersecting cracks that make pathways for groundwater.

28.5 – Cross a narrow footpath.

28.6 – A view of Rolling View Marina is R, across the lake.

11-I:  In just a bit, ahead to your right you may be able to see an eroded point, on your side of the water, with lots of round boulders.

28.7 - The lake comes into full view as the trail starts to curve around a cove.

11-J:  As the trail turns eastward along the side of an inlet, there is an excellent view across the inlet.  Assuming the water is not too high, you should be able to see more large round boulders in a reddish-brown matrix (Figure 11-J).  What you see from a distance is a sedimentary rock unit referred to as boulder conglomerate.  The reddish-brown material is fine sediment – sand, silt, and clay.  It encases huge round boulders of older rocks.

The trail turns left to follow a roadbed, then right to re-enter the woods.

29.2 – Pass L of a creek with a pretty cascade and start to walk beside it.

11-K:  These outcrops lie at the foot of a steep slope that may be described as a fault-line scarp.  Looking along the trail ahead to the southwest, note the steep slope to the left and the low area to the right.  The foot of this slope runs right along the Jonesboro fault, and you can see that this is also the direction of the dipping fracture surfaces in the outcrops.  These surfaces strike N27°E and dip about 70°NW.  Look back along this trend toward the northeast and you’ll see the same thing (Figure 11-K).  If you examine the rocks, you will see that they are very fine grained, and full of veins and other accumulations of white quartz.  Such silicification is very common along faults, as a result of fluids moving through the fractured rock and depositing silica.

About 200 million years ago, the Jonesboro fault was active, and the area to the right (northwest) was moving downward relative to the area to the left.  This is termed a normal fault, because the fault dips toward the side that moved down.  This kind of fault indicates that the Earth’s crust was being pulled apart (stretched or lengthened) as a result of extensional stress.  This is the same type of force that is responsible for the East African Rift of today.  Before the time of the Jonesboro, there was no Atlantic Ocean; the supercontinent we call Pangaea included North and South America, Africa, and Europe.  When extension began, it was the Earth’s attempt to rip apart the supercontinent.  This created many tears in the crust; one of them became the Jonesboro fault and stopped moving after a few million years.  Another one kept pulling apart and resulted in the formation of the Atlantic Ocean, which even today continues to widen as extension continues. 

Even though the Jonesboro fault has been inactive for well over 150 million years, and erosion has been leveling the area, we still see an expression of the fault in the local topography here.  The reason is that the metamorphic rock types east of the fault are much harder, and resistant to erosion, than are the sedimentary rocks to the west.  Well, in all honesty, the big granite boulders in the sedimentary rock are very resistant, but the reddish silt is easily eroded.  This results in rounded boulders left behind, with no obvious source.  You can see some if you walk just a short distance down this creek.  In fact, as you explore the area on either side of the fault, it is possible to determine the fault’s location by examining the shapes of the loose chunks of rock:  round chunks on the west, blocky or jagged chunks on the east.
So, just how did these boulders get so round in the first place?  Well, back when the fault was active, a valley was created on the west, and a steep mountain range on the east.  The rocks on the mountainous side of the fault were metamorphosed igneous types dominated by granite, granodiorite, and diorite, but also including some schist and gneiss such as we see along the trail to the east of Highway 50.  Streams flowed down from the newly elevated mountain range into the new valley.  Naturally, these streams flowed rapidly, and they eroded the bedrock over which they passed.  If you examine a similar stretch of streambed in the Blue Ridge Mountains today, you would see boulders, rounded by the work of the stream, but too big to be moved significantly by it.  As the ancient streams flowed out onto the valley floor, their ability to transport material weakened, so they would deposit sediment:  first boulders, then gravel, then sand, silt and clay.  Along the foot of the mountain range, deposits of the coarser sediments spread out as they entered the valley to the west, forming sloping fan-shaped features called alluvial fans.  The general pattern preserved in the sedimentary rocks of the Durham basin reflects their alluvial fan origin:  conglomerate closest to the Jonesboro fault, then sandstone, siltstone and mudstone as we travel west toward the center of the basin.  This is definitely a trend that you may want to look for as you encounter sedimentary rocks along the trail between this point and Durham.

After leaving 11-K, you pass nearly continuous outcrop of similar silicified rock, but you may notice that the amount of fracturing decreases, while the grain size of the rock increases.  These changes may relate to the fact that our path takes us southeastward, away from the fault, and the rocks show fewer of its effects.

11-L:  Here to the left of the trail there is a small outcrop of metadiorite with visible feldspar grains.

29.6 – Cross a small creek on stepping-stones.

Then there is a nice outcrop of fine metadiorite or amphibolite in the creek.

29.7 – Cross a footpath by trail signage.

11-M:  At this point, there appears to be a fairly sudden increase in the amount of quartz along the trail.  This may be a sign that we are back along the trace of the Jonesboro fault.

30.6 – Turn R by a creek bed and quickly cross it.

30.7 – Cross an open area of woodland.

30.8 – Cross a roadbed and soon pass through the remains of a home site, with a tobacco barn still standing.

30.9 – The trail turns R.

31.1 – Cross a creek on a footbridge.

11-N:  Just upstream from the bridge, on the side of the creek you came from (north-northwest), there is a cut-bank exposure of boulderconglomerate.  If you examine it closely, you will see the reddish-brown fine matrix that is soft, together with the large rounded rocks (cobble and boulder size).  Look carefully, and you’ll see that the round rocks are not all the same kind of rock.  Just as you might see several different rock types among the boulders in a mountain stream today, you see a variety here.  All these different rocks were exposed at the Earth’s surface in those old ancient mountains that were being eroded.


If you look in the creek bed, you’ll see lots of big round boulders.  Again they are not all the same kind of rock.  Where did they come from?  They came from an outcrop of conglomerate like the one you just saw; only the fine matrix had been removed by erosion.  Do you see how these may be called second-generation stream boulders!?

31.2 – Falls Lake comes into view, R.

11-O:  If the lake level is not high, here is a beautiful view of the floodplain and delta of Lick Creek, which flows into Falls Lake here.  The delta is the flat accumulation of sediment that is deposited where the creek flows in to the lake.  As the flowing water suddenly stops, it is no longer able to move sediment, so it dumps it there.  And remember, Falls Lake did not exist before the 1980s, so this is very recent, geologically speaking.  Another thought to ponder here:  take in this entire scene, with the creek, the lake, the flat deposits of sand and silt and mud, and imagine it enlarged about a zillion times.  What do you get?  The Mississippi River, Gulf of Mexico, and Louisiana!

31.3 – Cross another creek on a footbridge.

11-P:  If you feel ambitious, leave the trail and walk upstream to the edge of the power line.  Here there is a very nice outcrop of greenish silicified fault-zone rock, called breccia.  There are jagged broken pieces of older rock that are filled with quartz veins.  Again we are right on the Jonesboro fault!

Just downstream from the bridge, there are lots of rounded boulders and cobbles of various rock types, more of the second-generation stream boulders.  The implication is that there must be bedrock of boulder conglomerate between the site of the boulders and the outcrop by the power line.

31.5 – Turn L by some trail signage.

31.6 – Turn L and walk along some old barbwire, L.

11-Q:  There is a chunk of diabase on the left.  More about this later.

31.8 - Cross a wide creek.

11-R:  As the trail turns to cross the wide creek, there is a good outcrop of highly fractured and silicified metamorphic rock that may have originated as the dark-colored volcanic rock, basalt.  Note that this creek runs toward the northeast, in contrast to most of the small creeks we have crossed, which tend to flow toward the northwest.  If the creek level is not too high, explore downstream.  You will see numerous large low outcrops in the streambed.  The rock here is riddled with fractures, running in every conceivable direction (Figure 11-R); in some cases the fractures have been filled with veins of quartz.  These outcrops constitute outstanding examples of fault breccia; they were shattered by movement along a fault, and this stretch of creek appears to run directly along the trace of the Jonesboro fault.  The cracking and shattering of rocks like this is referred to as brittle deformation.  It occurs when rocks experience strong forces at relatively shallow depths in the Earth.

If you continue to walk downstream, you will find that the outcrops of fault breccia abruptly end, and give way to large rounded and smooth light-colored boulders of metamorphosed granodiorite up to three feet in diameter.  At this point, you have passed out of the fault zone and into the Triassic basin.  The boulders are from Triassic conglomerate deposited right next to the fault.

You may be able to tell that the geological map for this spot appears to be in error; it shows the Jonesboro fault to the southeast of here.  This is not unusual.  Every geological map is an approximation, and the more detailed data that is gathered, the better the map will be.

11-S:  Another piece of diabase.

31.9 – Pass by the remains of a shed, R, and start to follow an old roadbed.

32.4 mi – Reach gated end of Boyce Mill Road (gravel). Turn R to cross a barrier and enter next section.

End of Trail Section 11

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