NH Notes: Bent Rock on Display – The Sideling Hill Road Cut

The road cut through Sideling Hill in Maryland on I-68 is one of the best displays of roadside geology east of the Mississippi River.  Having spent considerable time in the western US it doesn’t seem that impressive to me but since I’ve been deprived of seeing geological strata for the past year I took a little detour on our family trip to North Carolina to check this road cut out.  With the rest of the family sound asleep in a small hotel in Hancock MD I slipped out at 5 am on a brisk June morning to catch the sunrise at Sideling Hill on Interstate 68.

Yes, rock does bend.  There are hundreds of layers of rocks here that must have originally been the result of horizontal deposition.  The layers of coal, sandstone and shale rocks here would have become solid while still horizontal and them were slowly warped by constant pressure and heat over long periods of time.   On display here are over 800 feet of strata in a folded syncline.  The long mountain ridge that is Sidling Hill runs North and South and is the result of an erosion resistant sandstone layer at the top of the ridge.  Below is a diagram of various ways that rock can be folded.  Sideline Hill is most like the synclinal ridge.

Here the sunrise is just hitting the top of the south  side of the road cut.  I took the picture with the truck for scale.

The bend in this rocks would have formed from millions of years of compression while there were still hundreds or thousands of feet of rock still on top of these layers.  As the horizontal layers folded they also were eroding. Eventually the entire region saw thousands of feet of rock erode leaving many ridges like this one and hundreds of others in the MD/PA/WV region.  It might be tempting to think that these layers bent very soon after they were formed and were still soft but as intuitive as that may feel there is no evidence to support that.   Soft layers would have mixed rather than bent.  If all these layers were somewhat soft when they bent why would they have suddenly become rock shortly thereafter?   The erosion that occurred to this area to produce these ridges makes sense only if all the layers were solid rock as the layers were bent.    You might think that rock would shatter rather than bend.  These rocks are full of billions of very tiny stress fractures but much of the rock is truly bent due to the enormous pressure placed on them over time (see this article on plastic deformation for an explanation).  However had they been bent quickly – say in a hundred to 100,000 years – then those cracks would be far more significant and complete rupture of the layers would have occurred.

For an excellent discussion of folded rock and its implications read this post from Questioning Answers in Genesis: Rock Layers Folded not Fractured, or are they?

There is pedestrian bridge that crosses the interstate on the east side of the road cut. There is an opening in the fence so you can take a picture here.  You can very easily see the U-shape bend to the rock layers here.  The very top layer in the middle is a hard sandstone that is resistant to erosion contributing to the formation of the long ridge.

 

Above and below show the alternating bands of shales and sandstones.  The darkest bands are organic rich coal seams.

On display in these layers or rock are a record of a changing environment from wetland to shallow seabed and back over long periods of time.