Pennsylvania Topographic Map (Pittsburgh, Altoona, State College: university, vs., areas)

[PGH423]

I have a question about PA's topographic map. PA's highest elevations (and I thought steepest, most mountainous areas) are in the Laurel Highlands. So why is it the case that on the topographic map, that area doesn't stand out much but areas a little east and north of it look much more mountainous? I've attached a screen shot of the map.

[speagles84]

Originally Posted by PGH423

I have a question about PA's topographic map. PA's highest elevations (and I thought steepest, most mountainous areas) are in the Laurel Highlands. So why is it the case that on the topographic map, that area doesn't stand out much but areas a little east and north of it look much more mountainous? I've attached a screen shot of the map.

They are the highest mountains but not the steepest - almost 3000 feet tops, but raise at a constant rate.

The most relief you'll see is in the Endless Mountains of North Central PA.

[PGH423]

Originally Posted by speagles84

They are the highest mountains but not the steepest - almost 3000 feet tops, but raise at a constant rate.

The most relief you'll see is in the Endless Mountains of North Central PA.

Really? The LH mountains look pretty steep. I'm sure the people doing the maps aren't wrong. So I guess they're not as steep as other parts of PA.
Is it also the case that the central PA areas are more consistently hilly? I'm wondering if in the laurel highlands, you go up a steep mountain and then just stay at a fairly high elevation for a long time, but in other parts of PA, you're constantly going up and down?

[Craziaskowboi]

Originally Posted by PGH423

I have a question about PA's topographic map. PA's highest elevations (and I thought steepest, most mountainous areas) are in the Laurel Highlands. So why is it the case that on the topographic map, that area doesn't stand out much but areas a little east and north of it look much more mountainous? I've attached a screen shot of the map.

Because of the Allegheny Front. The Allegheny Front passes east of Johnstown, west of Altoona and State College, and just south of Lock Haven and Williamsport. In fact, the West Branch of the Susquehanna River follows the north edge of the Allegheny Front past the two towns. Everything west and north of the Allegheny Front is the Allegheny Plateau, which is an ancient plateau that eroded over hundreds of millions of years to the point that it resembles a mountain range. Take a look at a picture of downtown Pittsburgh, and you'll notice that the hills (escarpments) surrounding the downtown area are all the same height. The tops are roughly the same elevation as the ancient plateau. The erosion has cut deeper in northwestern and north-central Pennsylvania, due to the higher elevation of the plateau in those areas compared to southwestern Pennsylvania. Erosion has been so thorough because most of the bedrock is sedimentary, which is easy to erode. As a result, landslides and rock slides are common, especially in the areas with the steepest elevation changes.

To the south and east of the Allegheny Front are the Ridge-and-Valley Appalachians, which used to be a broad coastal plain until North America collided with Africa, uplifting and severely folding the earth's crust. Think of the way a rug wrinkles when an object pushes into it from the side, and you get the basic geological mechanics that produced the long ridges and valleys in south-central and east-central Pennsylvania. The reason why the Ridge-and-Valley Appalachians appear more mountainous than the Allegheny Plateau is because the ridges were uplifted and folded from a much lower elevation, so there's more prominence.

My theory on why the Laurel Highlands are higher in elevation than the Ridge-and-Valley Appalachians or the rest of the Allegheny Plateau is that there was subduction in that area for a time when North America and Africa collided. While the ridges were getting folded up in the Ridge-and-Valley region, some of the crust was just pushed underneath the Allegheny Plateau near the Allegheny Front in southwestern Pennsylvania, lifting up a portion of the plateau to an even higher elevation than before.

The geology of Pennsylvania is so unique that the Geology Department at the University of Georgia not only has relief and geologic province maps of Georgia in one of its main classrooms, but also relief and geologic province maps of Pennsylvania.

[PGH423]

Originally Posted by Craziaskowboi

Because of the Allegheny Front. The Allegheny Front passes east of Johnstown, west of Altoona and State College, and just south of Lock Haven and Williamsport. In fact, the West Branch of the Susquehanna River follows the north edge of the Allegheny Front past the two towns. Everything west and north of the Allegheny Front is the Allegheny Plateau, which is an ancient plateau that eroded over hundreds of millions of years to the point that it resembles a mountain range. Take a look at a picture of downtown Pittsburgh, and you'll notice that the hills (escarpments) surrounding the downtown area are all the same height. The tops are roughly the same elevation as the ancient plateau. The erosion has cut deeper in northwestern and north-central Pennsylvania, due to the higher elevation of the plateau in those areas compared to southwestern Pennsylvania. Erosion has been so thorough because most of the bedrock is sedimentary, which is easy to erode. As a result, landslides and rock slides are common, especially in the areas with the steepest elevation changes.

To the south and east of the Allegheny Front are the Ridge-and-Valley Appalachians, which used to be a broad coastal plain until North America collided with Africa, uplifting and severely folding the earth's crust. Think of the way a rug wrinkles when an object pushes into it from the side, and you get the basic geological mechanics that produced the long ridges and valleys in south-central and east-central Pennsylvania. The reason why the Ridge-and-Valley Appalachians appear more mountainous than the Allegheny Plateau is because the ridges were uplifted and folded from a much lower elevation, so there's more prominence.

My theory on why the Laurel Highlands are higher in elevation than the Ridge-and-Valley Appalachians or the rest of the Allegheny Plateau is that there was subduction in that area for a time when North America and Africa collided. While the ridges were getting folded up in the Ridge-and-Valley region, some of the crust was just pushed underneath the Allegheny Plateau near the Allegheny Front in southwestern Pennsylvania, lifting up a portion of the plateau to an even higher elevation than before.

The geology of Pennsylvania is so unique that the Geology Department at the University of Georgia not only has relief and geologic province maps of Georgia in one of its main classrooms, but also relief and geologic province maps of Pennsylvania.

Wow, you really know a lot about this! Thanks for all the info.

[ki0eh]

The county with the most elevation change is Bedford, between the highest point at Blue Knob (so-called "2nd highest point in PA", which seems reminiscent of Gus the groundhog), and its lowest point at Wills Creek on the MD line near Hyndman. Wills Mountain (1 ridge) and/or Martin Hill (2 ridges) just east of Hyndman dispute the highest point in PA's Valley and Ridge physiographic province.

[PGH423]

Originally Posted by ki0eh

The county with the most elevation change is Bedford, between the highest point at Blue Knob (so-called "2nd highest point in PA", which seems reminiscent of Gus the groundhog), and its lowest point at Wills Creek on the MD line near Hyndman. Wills Mountain (1 ridge) and/or Martin Hill (2 ridges) just east of Hyndman dispute the highest point in PA's Valley and Ridge physiographic province.

What does elevation change mean? I'm pretty sure the Youghiogheny River in the Laurel Highlands has the deepest gorge (with a river to mountain-top elevation change of 1700 feet at one point).

[Craziaskowboi]

Originally Posted by PGH423

What does elevation change mean? I'm pretty sure the Youghiogheny River in the Laurel Highlands has the deepest gorge (with a river to mountain-top elevation change of 1700 feet at one point).

The range in elevation above sea level. Blue Knob is more than 3,000' above sea level, but the lowest elevation in Bedford County is probably only a couple hundred feet above sea level, so the difference is likely at least 2,500'.

[PGH423]

Originally Posted by Craziaskowboi

The range in elevation above sea level. Blue Knob is more than 3,000' above sea level, but the lowest elevation in Bedford County is probably only a couple hundred feet above sea level, so the difference is likely at least 2,500'.

Oh I see. You're talking about the county's highest vs. lowest points. That makes sense.

[ki0eh]

Maximum Relief Counties by State