So let it begin. Let me tell you something about central North Carolina. Outcrops are as scarce as hens teeth. But you can find them. You can find them if you look hard enough, and sometimes, when not looking at all. They certainly like to hide in plain sight.
Not to worry though. I am a trained geologist, so I’ll lead you through it all painlessly. Let’s start with Lassiter Mill Dam. Now, I’m probably going to make this a series of posts that start from the “Big Picture”, and then we can zoom into the all gory details. I figure since this forum is all about having some fun while learning something, it would just be easier on the eyes and brain. Let’s get it going.
The site of the Old Lassiter Mill is located about 3.5 miles north of downtown Raleigh, North Carolina as a crow flies. All that remains of the old grist and saw mill complex is a dam and mill race—a structure used to redirect and concentrate water flow toward a water wheel. The dam is built using outcropping bedrock on the south side of Crabtree Creek for foundation footing [Figure 1].
The bedrock geology in this area is part of the Raleigh Terrane called the Falls Leucogneiss [FL]. It’s pronounced “loo-koh-nice”. The FL is more resistant to weathering than the neighboring bedrock and has controlled stream flow direction and to a minor degree Colonial and Modern infrastructure. A generalized geologic map of Wake County is shown in the link below:
Now let me show you something cool. Upstream of Lassiter Mill Dam about a quarter mile, Crabtree Creek takes a sharp bend to the north-northeast, straightens for about two tenths of a mile, and then makes another sharp turn back toward the northwest [Figure 2].
The site of the dam is in red. The Falls Leucogneiss is in yellow. Nutbush Creek Fault is the thick black line with arrows showing relative displacement along the fault line. Note how Crabtree Creek almost follows the trace of the Nutbush Creek Fault precisely. That’s the part I’d like to focus on. When I walked up the bike path to this area, I was aware of an existing high-tension power line right-of-way. It follows the west side of Crabtree Creek and the Nutbush Creek Fault along the same north-northeast trend.
Now. Let’s talk about strike-slip faults for a moment. Don’t worry about the “strike” part if you don’t know what that is. In simple terms, a strike-slip fault is a zone where the masses of rocks on either side of it exhibit a horizontal shift along a fault plane. Movement is lateral along the fault line. There are left-lateral and right-lateral strike-slip faults. How do you tell? Take a pencil and hold it between the palms of your hands. If you move your hands in the directions of the arrows along the Nutbush Creek Fault in Fig. 2, the pencil will rotate clockwise, or to your right. This motion is called right lateral, making the Nutbush Creek Fault a right lateral strike-slip fault. If you move your hands in the other direction you have a left-lateral [counter-clockwise] strike-slip fault.
Ready for the really cool part? Look at Crabtree Creek on both sides of the Nutbush Creek Fault in Figure 2. Let’s make an assumption: Crabtree Creek used to flow straight across the fault. If at some point long ago, there was an instantaneous shift along the Nutbush Creek Fault line, and a displacement of about two tenths of a mile resulted, it would follow that Crabtree Creek would have been “broken” and shifted along the fault line. The purple side bedrock would have moved north, and the yellow bedrock of the Falls Leucogneiss would have moved south. This would be consistent with current stream geomorphology. Intuitively obvious, right?
There is one more detail that I’ve intentionally overlooked. Without additional subsurface data we can’t discern using what is presented here to determine if the total right lateral strike-slip displacement along Nutbush Creek Fault occurred instantaneously, or if it occurred incrementally over long period of time. I’ll outline one possible way we may shed light on this question, as well as present an approach to collecting additional data that may get us there.