Trellis Drainage Pattern
In an earlier laboratory, we identified two drainage patterns on the Cumberland quadrangle. We looked at the dendritic drainage pattern of Rush Run, and we identified the
drainage pattern over the southern part of the map as being trellis. We have also indicated that a trellis drainage pattern indicates structural control by differentially
eroded folded sedimentary rocks. In a humid region, resistant rocks like sandstone, form ridges, and non-resistant rocks like shale and limestone form valleys. Sometimes
the dip angles cause asymmetric ridges such as is evident on Piney Mountain on the Cumberland quadrangle.
Consider the hike from 686000 meters E, 4394000 meters N to 685000 meters E,
4395000 meters N. How would the first third of the hike compare to the first 2/3 of the return hike? Answer
You might imagine the bed of rock that makes up Piney Ridge to be like a tilted book. The top of the spine forms the top of the ridge. The binding is the first steep slope of the hike.
The front cover of the book (or back cover) forms the gentler slope. Beneath the layer of rock that forms Piney Mountain, is another thinner resistant layer. It is evident as the small, more dissected
ridge just to the east of Piney Mountain.
What is the general direction of the strike of these beds? Answer
What is the dip of this rock? Answer
The geologic map of this area shows the strike and dip of these rocks ranging in age from Silurian at Haystack Mountain and Wills Mountain to the Pennsylvanian of Piney Mountain. Remember that dip is
toward the younger rock.
On the geologic map, looking at the rocks of Piney Mountain, the geologic V is very evident. Geologic V's point to younger rock. On the topographic map, a related feature is also evident. Use the
skills that you developed in mapping the drainage divide of Rush Run and TRACE THE RIDGE CREST OF PINEY MOUNTAIN from just below the M of Mountain across Jennings Run to the E of Little in Little Allegheny Mountain.
Notice that the ridge bends before crossing Jennings Run. This V on the topographic map is called an OUTCROP V. Outcrop Vs formed by contour lines point in the direction of the dip or toward the younger bedrock. You should be
able to find several other outcrop V's on this map.
The streams in this area southeast of Piney Mountain and Little Allegheny Mountain form a trellis drainage pattern. The resistant bedrock ridges are drained by short stream and the non-resistant
bedrock units form valleys containing SUBSEQUENT streams. As is typical with the trellis drainage pattern, the location of the streams before exposure of this surface is imprinted upon the current pattern. This imprint is
evident in the way the streams have cut through the ridges. Examine the illustration below and compare it to your map.
The means by which the streams such as Braddock Run, Jennings Run, and Wills Creek have been able to flow through the mountains is that the stream is generally flowing in the same location as it has been flowing for a fair amount of geologic time.
Stand back and look at your map viewing Wills Creek, Jennings Run, and Braddock Run. Don't those three streams resemble the typical dendritic branching stream pattern? Not quite, perhaps you say. Well, what if we imagine that Braddock Run didn't follow its route
through LaVale, but rather followed the path of the interstate highway through Haystack Mountain? Now, doesn't this look like a dedritic branch? Consider this as the original stream system. Through time, erosion is wearing away the surface and
exposing the folded sedimentary rocks below. The streams are there. They continue to erode down. Where they encounter the resistant beds, they continue to wear them away. New slopes form and streams form on these slopes to become tributaries for
these main streams through the area. These main streams are said to be SUPERIMPOSED upon this current landscape. The landforms, where the streams cut through the ridges are called WATER GAPS. Water gaps are formed by RUNNING WATER and by FLUVIAL PROCESSES.
The Narrows is a very dramatic water gap. Wills Mountain, as indicated on the geologic map, is an anticlinal mountain. Do not confuse that this geologic high happens also to be a topographic high. The rocks of Piney Mountain once extended over the rocks that
form Wills Mountain. Those overlying rocks have been worn away. Given enough time, Wills Mountain may wear away to be replaced by a valley when weaker rocks are exposed here. Streams want to flow on the weakest rocks, and at one time, this stream was on non-resistant rock. However, now, it has
eroded downward and has been superimposed through this resistant rock to form the water gap. Examine the map and locate other water gaps.
Streams flow from higher elevation to lower elevation, down slope. We have calculated stream gradient, the rise over the run. Streams "want" to flow down the steepest slope or where they have the steepest gradient. We have previously discussed water landing on the surface of the
earth first collects and forms rills, that grow into gullies and that gullies may eventually grow deep enough to reach a supply of water and become streams. The process of streams extending their HEADWATERS, the tributaries near their source, is HEADWARD EROSION. Imagine two streams eroding
headward. See the illustration at the left below.
Suppose that the stream on the right has a steeper gradient than the stream on the left. At the red arrow, headward erosion is wearing into the divide separating the left stream system from the right stream system. Eventually, this stream may cut through the divide and "capture" the
headwaters, diverting them into the stream system on the right. Examine the illustration on the right. The headwaters of the stream on the left have been captured, and STREAM PIRACY has occurred. Notice how the drainage pattern does not resemble the normal joining of stream branches.
An abandoned water gap is a WIND GAP. A wind gap results from stream piracy diverting a stream so that it no longer flows through a water gap. In the past, Braddock Run did flow through Haystack Mountain. The subsequent stream to the west of Haystack Mountain had a steeper gradient, eroded through their divide, and diverted the drainage of Braddock
Run from going through the water gap. What agent and process formed the wind gap? Answer
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