Western Pennsylvania full of valleys, not hills

Awhile ago I wrote an article that explained that most of the western Pennsylvania terrain doesn't have hills. My point was that in the true meaning of a hill -- a part of the landscape raised above another part of the landscape -- western Pennsylvania is lacking.

You may recall that my explanation of this seemingly outrageous statement was that much of our western Pennsylvania landscape began as a broad flat plain laced with slowly meandering rivers. Geologic forces uplifted the flat land to an elevated plateau. Once raised above sea level, the rivers became more energetic and eroded into the layers of earth and rock. The results were, and still are, a dendritic pattern of various sized water courses in various sized valleys. The uplands surrounding the valleys are all the same elevation -- the elevation of the former plateau.

Thus, western Pennsylvania doesn't have any hills, but it does have lots and lots of valleys. Instead of going up and down hills we are going into and out of valleys.

I began my explanation with the hedge that "most" of western Pennsylvania doesn't have any hills. Not all, just most.

To the south and east of Pittsburgh is a region we call the Laurel Highlands. The Laurel Highlands, with Chestnut and Laurel Ridge and Allegheny and Negro Mountains, are far from flat; they are indeed hills, or even mountains.

The story of the Laurel Highlands is not just about western Pennsylvania, but is linked to a much larger chain of mountains, the Appalachians. These are some of the oldest mountains in the world. They arise in northern Georgia and sweep in a 1,600-mile arc to the north and east and end in Quebec, Canada.

The Appalachians are the backbone of eastern North America. They hold some of the most notable topographic features with which Easterners are familiar, such as the White Mountains of New Hampshire, Green Mountains of Vermont, the Catskills of New York, and the Smokey Mountains of southern United States.

The highest elevations found in eastern United States are in this chain of peaks. Mount Washington in New Hampshire rises to 6,288 feet above sea level and Mount Mitchell in North Carolina pushes up a couple hundred feet more, to 6,684 feet, the highest point east of the Mississippi River.

The Laurel Mountains are on the western edge on this vast physiographic region. In Pennsylvania this province is a quartet of parallel ridges that stretch from the southern boundary of the state into Indiana and Blair counties. The principal mountains of the chain are Chestnut Ridge on the west, Laurel Ridge in the middle, and Allegheny Mountain on the east. Negro Mountain only extends into Pennsylvania a short distance in Somerset County and is between Laurel Ridge and Allegheny Mountain.

What's the difference between a "ridge" and a "mountain" as the terms are used here in western Pennsylvania• I honestly don't know. I guess that mystery is another along with the inconsistent way we name our waterways as runs, creeks, streams and rivers. To avoid extra words, I'll stick with mountains for these elevated formations of landscape.

The Laurel Mountains have the same foundation as the plateau to the west where Pittsburgh is located. However, rather than being flat layers like a multistoried cake, the foundation rocks of the mountains were deformed by extraordinary geologic forces.

Picture the process this way.

You have a pile of paper on a table in front of you. Each sheet represents a layer of rock. You can push the pile along the surface of the table and it stays flat. However, if one end of the pile hits something immovable -- say a brick sitting on the table -- the pile of paper stops at one end and, if you keep pushing from the other end, the pile bends in a smooth wave. If the sheets of paper are long enough you get several folds.

Now imagine the sheets of paper are layers of rock. The push is the slow drift of the continent. The brick is a point where the rock layers get caught on deep geologic strata. The folds or waves in the pile are what geologists call synclines and anticlines. The synclines are the bottom of the wave and the anticlines the top.

As mountains go, we know the anticlines -- or tops of the rock waves as Chestnut, Laurel, Allegheny and Negro mountains. The synclines are the valleys between and we know them as the Ligonier or Indian Creek valleys, or the Johnstown or Somerset valleys as just a few examples.

You can see the folded rocks of the Laurel Mountains for yourself in a couple of places.

Drive the Pennsylvania Turnpike east from Monroeville. The highway descends into the Turtle Creek valley and after crossing the stream there are rock outcrops on the right. The rock layers there are flat since you are still in the plateau region.

Drive farther, beyond New Stanton, to east of Mt. Pleasant and just north of Laurelville. There is a major road cut exposing the layers of foundation rock. Here, the western edge of the first fold is visible in rock layers tilted to the east. This is the beginning of Chestnut Ridge. Shortly the turnpike begins to climb the mountain following the Jacobs Creek valley.

By the time you get to Donegal you have crossed the summit of Chestnut Ridge -- the anticline -- and have begun to descend into the Indian Creek valley -- the syncline. Ahead of you is the second fold, Laurel Mountain.

For a closer look, get out of your car and onto your bike for a ride on the Great Allegheny Passage.

Starting on the eastern side of Laurel Mountain at Ramcat Hollow in Ohiopyle State Park the trail is in the syncline that holds Confluence. As you ride toward Ohiopyle, following the Youghiogheny River as it cuts through the heart of Laurel Mountain you come to a cut made by the Western Maryland Railroad that once followed this path. The rock on both sides is Pottsville sandstone. Look closely and you can see just a bit of a tilt with the low side to the east and the high side to the west. At the top of the rock walls are groves of rhododendron. This plant needs acidic soils, and the Pottsville sandstone provides the acidity.

Since the rock layers are folded, and the bike path relatively flat, you are passing through the layers of the bent cake.

Farther along is another steep rock wall on the left side of the trail. This layer is the Loyalhanna limestone and is a level below the sandstone you saw before. The acid loving rhododendrons are gone and plants needing limey soils like red columbine can be found hanging on the cliffs in the spring.

Keep going, getting within a couple miles of Ohiopyle and the limestone decorated with columbine appears again as that layer is bend downward on the west side of the mountain. Likewise a little farther and rhododendrons cover cliffs of sandstone indicating the next sheet of the rock cake.

Finally, if you want to visit the top of the highest anticline in Pennsylvania drive a little more to the east and Negro Mountain. Here the top of the ridge isn't very prominent but more like a flat-topped bump in the terrain. This rock cap is the Pottsville sandstone again, but more importantly it is the highest point in Pennsylvania -- Mount Davis at 3,213 feet above sea level.

The whole story is a bit more complex than I have explained. The Laurel Mountains were once a mighty mountain range that rivaled the Rockies. Given time, water and gravity they were eroded down to sea level. With the uplift of the rest of the region that gave the Pittsburgh area its valleys, the roots of the old mountains were exposed and gave us a true mountain terrain in western Pennsylvania.