Friday, November 1, 2013

Lake Johnson Geology

The Lake Johnson Greenway is divided in two by the causeway of Avent Ferry Road.  The western portion is a mostly unpaved hiking trail, while the eastern portion is paved and suitable for biking.  All rock exposures around Lake Johnson belong to various rock units within the Crabtree terrane.  The axis of a large fold, the Raleigh antiform, passes from north to south through the eastern half of the lake.

Lake Johnson West

From the parking lot at the north end of the causeway, proceed west along the northwestern edge of the lake.  The terrain is very flat here, but you should be able to see a more rugged shoreline on the other side to the south.  The western end of Lake Johnson is where Walnut Creek enters, and has deposited considerable sediment, forming a small delta that constitutes a nice wetland.  As you proceed along the unpaved path on the southwestern portion of the lake, you will encounter spots where the uphill side of the trail was cut and some low portions have been filled in, to make the trail more level.

If you look closely, you can see a small cut exposing weathered granitic gneiss, showing banding, under a pine tree.  A short distance ahead, you encounter a better outcrop of orange-colored granitic gneiss (Photo).
The banding, or foliation, of the gneiss dips moderately toward the west (in the uphill direction).

There is a short spur trail leading to a steep bluff where there is a bench and a nice overlook providing a lake vista.  Here there is an excellent outcrop of very light colored (leucocratic) gneiss (Photo).
The foliation strikes about north-south and dips about 20o toward the east.  There is also a strong lineation, plunging about 13o to the north.  This rock unit is called the Southwest Prong gneiss.

Ahead in a tributary creek, you will get a view of a big cut bank outcrop of fine-grained mica-rich gneiss or schist.

After reaching Avent Ferry Road, cross and walk north toward the causeway.  You will encounter a nice roadcut exposure at the south end of the causeway.  This rock is a mica-rich gneiss or schist that weathers into thin platy slabs (Photo).
The foliation strikes north-northwest and dips toward the southwest, and there is a lineation plunging south.

Lake Johnson East

Note:  From the parking lot along Avent Ferry Road south of the lake, this description follows the greenway trail counterclockwise.  Also, be forewarned that this section of trail has more steep hills than other trails and is usually very busy, especially if the weather is nice.

According to the geological map, the low point following the first steep hill is where the axial trace of the Raleigh antiform passes through.  This would mean that foliation (layering) in rock outcrops here should be nearly horizontal, whereas it dips away from this trace on either side.  Unfortunately, there are no outcrops visible from the trail at this point.  However, as you climb the second steep hill, you may be able to see a rock exposure in a small creek down to your left (Photo).
The foliation dips about 25 or 30 degrees toward the east.  Recall the outcrop near the south end of the causeway dips west, so we have indeed crossed the axis of the antiform (an arch-shaped fold).  A bit farther along the trail, just beyond the 2-mile marker, there is an outcrop to the right up on the hillslope and ridge. The rocks here also dip gently toward the east (Photo).
Continue along the trail until just before the dam.  There is a very large exposure of rock just below the spillway (Photo).
To see it, take the trail spur to the right toward Lake Dam Road, then walk up a small unmarked trail below the dam.  This outcrop consists of mica schist containing the metamorphic minerals garnet and staurolite. Elsewhere near Lake Johnson the mineral kyanite may also be found; together, these metamorphic minerals tell geologists that the rocks were buried to a depth of about 15 miles and reached temperatures of about 650oC.  The foliation of the schist here dips eastward very steeply, around 80o.  This rapid steepening on the east flank of the Raleigh antiform may also be seen at the greenways around Shelley Lake and along Crabtree Creek.  This indicates that the Raleigh antiform is asymmetric, more steeply dipping on the east than on the west.

You may choose to continue the Lake Johnson East Loop Trail, proceeding across the dam and then west toward the north shore of the lake.  Alternatively, the Walnut Creek Trail begins immediately across Lake Dam Road from the dam.

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