Trail Section 13 - 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.

 

Section 13 (NC-98 to Rolling View) (3.2 miles)

34.3 – Turn R to cross a culvert and enter an area cleared for wildlife. Soon turn R onto a footpath.
34.5 – Leave the cleared area to enter the woods. Cross a creek on a footbridge and soon turn R.

13-A:  If the lake level is at or below normal (250 ft)......Just downstream from the bridge, there is an outcrop of reddish pebbly sandstone in the creek.  As you look down at the rock, you are looking at a bedding surface.  Sedimentary rocks are deposited in horizontal (or nearly so) layers called beds.  This bed is actually dipping a bit, about 15°, so it must have been tilted a little after it was deposited.  When we do see bedding in outcrops in the Triassic basin, we generally expect it to be dipping no more than about 20°, and usually closer to horizontal.

34.8 – Cross an old roadbed.

13-B:  Here in the trail is an exposed piece of conglomerate with angular clasts (Figure 13-B).  This sort of conglomerate is sometimes referred to as sedimentary breccia.  Note the pieces of rock embedded are again not all the same kind; the fact they are angular says that they did not have the opportunity to be rounded by stream action.

35.0 – Go through a wet area, the result of beaver activity, on a series of boardwalks.

35.1 – Cross a feeder creek.

35.2 – Cross under power lines.

13-B2: About 50 to 100 feet down to the right off the trial is a large moss and lichen-covered outcrop of sandstone and pebbly sandstone on the slope. The flat surface of the outcrop probably represents a bedding plane, so the beds appear to dip about 15 degrees to the southeast here (The strike and dip are N55E,15SE). Although the beds were much closer to horizontal when they were originally deposited, continued downward movement on the Jonesboro fault caused them to rotate. Watch out for barbed wire.

35.4 – Cross an old roadbed.

13-C:  You cross a creek with no bridge at some point.  About 200 yards downstream along the creek, there is a creek-bed outcrop of pebbly sandstone. The rock here is somewhat finer grained than the previous exposures.  In addition, the bedding is poorly defined, which is common in an alluvial fan complex.  Also note the potholes here.

35.5 – Cross a roadbed by an open area.

13-D:  Cross a creek obliquely.  Then as you start uphill on the other side of the creek, there is a very nice series of exposures in the creek (now to your left).  They form a cascade with small waterfalls and pools (Figure 13-D).  Here you see reddish pebbly and cobbly sandstone with poorly developed bedding.  If you go upstream a bit, you may see finer material (silty sandstone) with a well developed bedding plane having a strike and dip of N45E,20SE.

35.9 – Cross a creek bed and soon junction with a wide roadbed.  Turn R to walk along it and turn L to reenter the woods.

36.1 – Use a footbridge to cross a wide creek.

13-E:  If the lake level is at or below normal (250 ft).....There are several good flat outcrops of pebbly sandstone along the creek here, including one directly under the bridge.

36.5 – After walking along an old roadbed by the lake, R, turn L off it to skirt the shoreline. Soon reach the state park boundary (no hunting).

13-F:  A point of land beyond a small cove comes in to view ahead to the right.  If the lake level is not too high, you will see big blocky outcrops of a dark rock (Figure 13-F).  This is something new, certainly not conglomerate or sandstone.  (You might even think the rocks were put there to combat wave erosion along the lakeshore – they weren’t.)

This dark-colored rock is an igneous rock we call diabase.  It is a variety of basalt, the main rock of which the crust of all the world’s oceans is composed.  Remember how Pangaea was being split apart along faults, and that one of those faults continued to widen and became the Atlantic Ocean?  That process caused the formation of molten basalt in the Earth’s upper mantle.  The magma ascended, cooled at the surface, and became the ocean floor; the process continues today at the Mid-Atlantic Ridge (think Iceland).

The same basalt magma that created the early Atlantic seafloor also was injected into thousands of fractures in the continental crust that formed as the result of the extension of Pangaea.  The result is that the Piedmont of North Carolina is cut through by hundreds of narrow bands (called dikes) of diabase, actually vertical (or nearly so) slabs of this igneous rock.  If we know how long ago this igneous rock solidified, it tells us about how long ago the Atlantic Ocean began to open.  As it turns out, the age of our diabase dikes in North Carolina is just about 200 million years – the age of the oldest Atlantic seafloor.

Diabase is a very hard rock, much more resistant to erosion than the sedimentary rocks around it here.  In fact, the diabase dike across the inlet is responsible for the existence of the peninsula along which it runs.  Most diabase dikes in this region run about north-south or a bit west of north.  If this dike runs about north-south, the trail should cross it ahead at some point.  In any case, we will want to be on the lookout for diabase.

13-G:  Here, right on the trail is a small outcrop of coarse sandstone with a most unusual feature:  in addition to sand-sized material, it contains a single rounded cobble about 3 inches across (Figure 13-G).  This is not so easy to explain.  One possible explanation is that this sedimentary rock was not deposited in a streambed by flowing water, but was deposited by moving mud.  In fact, this is a very important process along steep mountain slopes, in addition to stream activity.  These are called debris flows; you can think of them as small-to-large scale landslides with varying degrees of water saturation.  Flowing mud can carry along rocks of any size – or even cars or entire houses.

13-H:  Here is part of the diabase dike we have been expecting – excitement!

36.7 – Cross another open roadbed.

13-I:  Just as you re-enter the woods, there is a nice outcrop of coarse sandstone on the trail.  Then there is a longer (bout 20-foot) stretchof trail that is underlain by the same rock.  This outcrop is a bedding plane; you can see that it is nearly flat lying.

37.1 – Junction with a blue-blazed trail, L, to a hiker’s parking area (Soon reach a pond and turn L to curve around it  [If you take this path, here you will see some boulders of diabase on the other side of the pond near the remains of an old building.] -> at 0.2. mi. reach a road, cross it and turn L to walk along a path by the edge of the woods -> at 0.3 mi reach the lot)

37.2 – Cross a creek bed and then a footbridge.

13-J:  This creek has diabase stepping stones.

37.4 – The remains of an auto can be seen to the L.

13-K:  There are lots of boulders of diabase, and an old home site here.  You will see more diabase boulders along the paved road.  In fact, the peninsula on which Rolling View is located owes its existence to a large diabase dike, the same one we saw earlier.

37.5 mi – Reach a road by the gated entrance, R, to the Rolling View recreation area/campground.  Cross the road and turn R to walk along the grassy shoulder to enter the next section.

End of Trail Section 13

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