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Geological cycles, that is, bicycles

July 24, 2013

I’m getting back on my touring bike these days for weight loss, fuel savings, exercise and fun, even though I sometimes need the Jaws of Life to pry me off that little bitty bicycle seat. We did 45 miles on Saturday and a “recovery ride” of about 30 on Sunday. The muscles in my legs are tired, and I’m still not much good for anything but sitting around blogging. Moving around the house is an adventure (“Daddy, do you have noodle legs?”), and I’m still trying to decide if I am simply stiff or if rigor mortis is setting in. I did the bicycle commute on Tuesday with 20 pounds of gear in the panniers, which may have been a mistake.

You get a better feel for topography on a bicycle than in a car. Geology is often expressed as topography. Same thing with geological processes like erosion. Geology is a good way to distract yourself while pedaling uphill.

When I lived in Oklahoma, we had some wonderful rides with the bicycling community there, including freezing late night Christmas rides to look at all the decorations and lights. One of the dominant features of Tulsa geology was Pennsylvanian age sandstone layers that had been tilted out of horizontal, dipping down to the northwest if my memory serves. From one direction, it was a very long uphill ride. From the other direction, a very short and very steep uphill ride, where the unwary cyclist has to either gear down quickly or else risk the embarrassment of falling over sideways while still clipped into their pedals. A group of us challenged ourselves to personal best top speeds on one of those long downhills. My cycling computer said 50 mph max when I was done. I didn’t watch too closely: it was exhilarating up to 40 mph, but after that it was just terrifying. Hitting a bump or a rock would have been catastrophic.

Around the North Carolina Piedmont, topography can be more subtle. Most of our rides leave from Cary and head west around Jordan Lake or north towards RTP. This puts us squarely in the Triassic Basin. This is mostly sedimentary rock, easy to spot because it’s usually brick red. These pull-apart fault basins formed, on both sides of the Atlantic, as Pangea tore apart in the Triassic and Jurassic. The Jonesboro Fault on the eastern margin is pretty easy to trace: the Angus Barn is built right on top of it. It crosses I-40 between Harrison Avenue and the Airport exit, at about the bridge on Reedy Creek Road, or where Crabtree Creek empties out of Lake Crabtree. In Cary, the fault lies about at the intersection of Evans Road and NW Cary Parkway. To the west of the fault lies the basin, which filled in with sediment as the bottom dropped down.

Within the Triassic Basin, topography is primarily rolling hills, sculpted by creeks and rivers. For a practical bicyclist, this means stream benches.  As you cut across a stream, you see this: \_/ with the stream at the bottom. More often you see this:

\_       _/

 \_/

As the stream cut downwards, it left banks on both sides and eroded a wide valley as it worked back and forth, then began to cut downwards again in the middle. The cyclist gets a nice downhill (woooohooo), a bridge over the creek (bump….bump) then an uphill where you lose momentum. You get to the top of the first hill (whew) which is the stream bench, a brief respite, before (no, no, no) there’s another hill to climb.

The local nemesis of bicyclists is the three-humped hill on Lystra Road in northern Chatham County.  After a nice ride in the sedimentary rocks of the Triassic Basin, you leave the western side of the basin to emerge into igneous rock of the Farrington pluton that are more resistant to weathering. You leave the flats for the hillier hills. These are Proterozoic granites and volcanic rocks of the Carolina Terrane (or as NCSU’s Prof. Jim Hibbard refers to it, Carolina Zone). There’s a geologic map of the area here. The most polite way to describe them is that they are more fun to ride down than up, but you can be distracted by the thought that these rocks formed in an island arc off of conjoined South America and Africa some 578 million years ago.

Generations of UNC Tarheels are familiar with another example of this topography, where the Triassic Basin ends and the Chapel Hill granite begins, on that long hill on Highway 54, where Raleigh Road comes up to campus from Glen Lennox. Chapel Hill really is a hill, giving UNC students the topographic advantage from which to look down on their rivals in the nearby Durham Triassic Basin below.

Geological cyclists armed with topographic maps and geological maps can pick routes according to their needs, either a nice flat spin, or a more rugged ride.  An easy ride parallels the streams, along the stream or river valleys, like the Crabtree Creek Trail or the House Creek Trail. You can also parallel the streams on the ridges between valleys, say, along Ridge Road or Dixie Trail. However, if you want a more rugged ride, ride across the stream valleys.  You can make any ride harder by heading east off Ridge Road on Churchill Road to ride the rollercoaster built by the two branches of Beaverdam Creek.

In Raleigh, I looked at the major streams, which would be Crabtree Creek, House Creek or Beaverdam Creek.  My hill climbing route in Raleigh starts at the PNC Arena, uphill on Edwards Mill Road to Glen Eden. The hills on Glen Eden are veeeeery loong, carved by House Creek. There’s more hills through the neighborhoods to Lassiter Mill Rd. /St. Mary’s Street. I ride up Lassiter Mill to North Hills Mall, then down again to climb St. Mary’s, both hills carved by Crabtree Creek. Then it’s across NCSU or Dorthea Dix to Centennial Parkway, my favorite long downhill run, but a long slow grind back up, with no shade.  Then it’s over to Beryl Road (usually a headwind there), past the Fairgrounds, then home to the Arena again.

The nice thing about in-town rides is that you are never far from a coffee shop, grocery store, or brew pub.  Enjoy the ride.

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5 Comments leave one →
  1. John Adams permalink
    July 24, 2013 4:02 pm

    Thanks Chris, for your description of how the creek valleys got so well incised. The results are particularly noticeable from a bicycle.

    • July 24, 2013 5:07 pm

      Thanks, John. I’ve always wondered about time scale on their formation. Descriptions of travel in Colonial times indicate that North Carolina was heavily deforested, gullied and eroded. I wonder how much of the landscape is “natural” and how much is “natural after human agriculture cleared everything away” to accelerate erosional processes?

  2. July 30, 2013 9:30 am

    Great blog post! Relating geology to places I see every day will definitely help me make sense of all this rock and mineral stuff. Blog also inspires me to get my lonely mountain bike back out on the trail!

    • July 30, 2013 10:02 am

      Thanks, Cynthia. You should definitely get your bike (and helmet) out for a ride soon. A great place to start is at the Art Museum or Prairie Ridge and ride the bike path down Reedy Creek Road to Umstead State Park. It’s a nice long downhill to Reedy Creek Lake and Crabtree Creek, but it’s uphill in every direction from there!

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