Friday, November 1, 2013

Centennial Bikeway Connector

This short greenway segment extends from Western Boulevard, where it departs from the Rocky Branch Trail, crosses south to Centennial Parkway, then follows the sidewalk on the southwest side of the Parkway, almost to Lake Wheeler Road, where it follows a cutover downhill to the right, joining the Walnut Creek Trail.  Once arriving at the Walnut Creek Trail, go west (right) through the NCSU Centennial Campus and on the Lake Johnson, or east (left) toward southeast Raleigh and the Neuse River Trail.

After leaving the Rocky Branch Trail at Western Boulevard, you go uphill and to the left.  This long hill you climb, toward Centennial Parkway, owes its existence to the hardness and resistance to erosion of the underlying rock unit, Falls leucogneiss.

The Centennial Bikeway Connector contains some of the best examples of Falls leucogneiss in the greenway system.  After reaching Centennial Parkway, the bikeway continues on the other side of the road and heads downhill to the left.  If you want to see the best Falls leucogneiss, you my want to (carefully) visit the median, where huge blocks of leucogneiss were placed near where they were excavated for construction of Centennial Parkway (Photo).
If you examine some of these blocks, you can see the light pink or tan color of the fresh unweathered rock, and its other distinguishing characteristic, a strong lineation, composed of tiny dark minerals strung out in lines (not layers) through the rock (Photo).
The dark streaks contain the mineral magnetite, and as a result this rock will attract a sensitive magnet, such as a pivoting magnetic stud-finder.

The Falls leucogneiss is part of the Raleigh terrane, along with the Raleigh gneiss.  The leucogneiss is thought to be a body of granite that intruded into the Raleigh gneiss, and they were both later squeezed, stretched and heated together.  Some of the blocks on the Centennial Parkway median contain portions of Raleigh gneiss that were engulfed by the granite magma (Photo).  The dark portions of this rock are Raleigh gneiss, similar to an outcrop that may be seen on the Rocky Branch Trail greenway.
The Falls leucogneiss unit has been dated as about 540 million years old.  This would be the time when the granite intruded as a magma; the Raleigh gneiss must therefore be somewhat older than that.

As you continue along the Centennial Bikeway Connector sidewalk, after crossing the intersection with Blair Drive, you will see a green chain link fence to the right.  Beyond that, under the electrical transmission lines, is a series of excellent natural ledge outcrops of Falls leucogneiss (Photo).
If you go for a closer look, you will see that the rock is largely covered with lichen, but you can still see the lineation, which runs a little east of north.

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