Monday, November 25, 2013

Snelling Branch Trail

The Snelling Branch Trail greenway begins at the northeast corner of the Shelley Lake Loop, follows the creek valley eastward across North Hills Drive, and ends at Optimist Park and the athletic fields of Sanderson High School.

Just uphill from its junction with the Shelley Lake Loop, the trail parallels a steep stretch of Snelling Branch, and there is nearly continuous rock exposure in the creek.  These rocks lie on the eastern limb of the Raleigh antiform, a large arch-shaped fold.  A long rock outcrop features cascades and small waterfalls formed by Crabtree terrane gneiss dipping steeply toward the east (photos).
View from the foot of a long stretch of rock outcrop,
just uphill from the Shelley Lake Loop trail.

Detail of previous photo.
These rock layers dip east (right) at about 65 degrees.

A bit farther uphill, within sight of North Hills Drive, the creek is much closer to the trail.  Here, a very nice pavement exposure, and fin-shaped outcrops can be seen (photo).  These rocks are thinly banded fine-grained dark biotite gneiss.  The banding dips very steeply toward the east.

Another rocky stretch of creek.  Trail is just to the right.

The trail crosses North Hills Drive, then continues to follow the creek.  Here, there is lots of rock exposed along a steep cut bank; there are some small cascades as well (photo).  This section of the trail is within the Southwest Prong gneiss, a subunit within the Crabtree terrane.  This rock is rich in quartz and feldspar, and is more like granite than most other parts of the Crabtree terrane.  In some places the rock is highly weathered
to form saprolite, intermediate between hard rock and soil.

Hard Southwest Prong gneiss exposure above North Hills Drive.

Banding in the tan to orange gneiss here dips almost vertically, and the rock has a strong horizontal lineation, as if stretched like taffy (photo).
Partly weathered block of gneiss showing lineation.

After climbing this steep stretch, the terrain flattens out a bit.  The trail passes between athletic facilities of Sanderson High School and terminates at the parking area for Optimist Park.

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