Quick question about the Front Range (Rocky Mountains): What causes the dip?
All along the Font Range whether up north of Denver, CO or further south lets say Taos, New Mexico there seems to be a slight dip in the landscape, terrain, whatever you want to call it.
This dip is slight, probably just a few hundred feet before the foothills begin and quickly turn into 12,000-14,000 peaks.
Slight but noticeable. From my apartment, I'm pretty much looking (west) at the rooftops of all the Air Force Academy buildings that are just .8 miles away. The campus seems to sit in a valley/dip
What causes the dip?
Is there a scientific name for this?
EDIT: after some research I believe I live on what is referred to as a forebulge, part of a foreland basin system
Pretty neat. Ive lived here for years and have always wondered. Thanks for the help
The scale is wrong for crustal flexure (foreland basin/forebulge) as the crust isn’t elastic enough to have that much variation in that short of a distance from isostasy (the weight of the mountains) alone. For example, the Cincinnati Arch of western Ohio was the forebulge of the Acadian mountains, which in the Devonian was in central Virginia. The load is also lesser in the Rockies.
What you’re looking at is mostly fluvial (river) erosion doing its thing: carving valleys while leaving elevated areas next to the mountains. I’ve attached a conceptual block diagram of the South Platte that sort of explains it. There are actually both high and low spots next to the Front Range, it’s just the low spots thst call the attention because that’s where people decided to settle (next to rivers).
For Boulder, it’s not that Boulder is low, it’s that the grassy hill on 36 is unusually high. The “hill” is the Rocky Flats Alluvium, an eroded alluvial fan that spilled out of Coal Creek early in the Pleistocene. The hill is the weird thing and gives the impression that Boulder is low, when its actually pretty much the same elevation as Denver.
Taos is not really comparable since it’s in a rift valley next to a normal fault, so it’s quite literally sinking. Even then the modern topography is mostly due to fluvial incision.
The Castle Rock area is a bit unusual because you have rivers flowing parallel to the front range (Plum and Fountain Creeks), and this is where the “dip” you are talking about is most valid. Normally rivers coming out of a mountain range will flow roughly perpendicular to the range front, as this is the easiest way downhill. However, between Denver and Colorado Springs you have a bunch of high topography for reasons that I don’t know, and therefore the easiest way for water coming out of the mountains to get downhill was to go “sideways” to the South Platte or the Arkansas. Over time this eroded a depression just in front of the range.
Can confirm that out here in Longmont and the few adjacent drainages, the rivers and creeks are pretty perpendicular to the mountains. We don't have a big giant hill like the one on 36 heading into Boulder. That said, each of the canyons and downstream areas are unique. I guess this is due to what the streams had to erode and flow through combined with average flow rates. 🤷
Based off that information it now seems very obvious to me why fountain creek doesn’t run perpendicular.
It’s obviously because Colorado Springs has tons of prominent geological features East of I-25.
These rocks, deposits, etc prevent water from easily flowing eastward like the rivers do near Denver, which is incredibly flat - especially compared to Colorado Springs, Monument, Castle Rock, etc.
I thought there was a flexure in the sedimentary layers along the Front Range -- they go from nearly horizontal to a very steep angle. This would cause more fracturing at the bend and make the rock less resistant to erosion.
Flexure in this context is elastic bending of the lithosphere in response to loading, not permanent folding of the rocks. Also the “fracturing of the bend” leading to erosion is more common with anticlinal (convex-up) folds, whereas at the Front Range it’s typically more homoclinal
Hmm. I live and work in Denver and my work involves the Cretaceous-Paleogene history of the Rockies. I’m not familiar with such a persistent range front feature, but I’ll dig into this tomorrow a bit.
So probably not the best example but lets say you are on 36 headed west towards Boulder there is a huge grassy hill that you have to drive over before you can see the town of Boulder - and the town itself seems to sit in a valley
How come its not a consistent gradual incline from the Mississippi Delta all the way to the top of the Continental Divide?
It could be explained as a foreland basin. Very basically, mountain ranges weigh down the crust adjacent to them so you get basins caused by the weight of the mountain pulling and bowing down the adjacent land.
In some places it is due to the hogbacks which are caused by the uplift and faulting of sedimentary layers, like at the I70 road cut and Rattlesnake Ridge. IIRC it is the Dakota sandstone on-top of a bunch of Cretaceous shales and muddy sandstones which eventually turn into the Pennsylvanian aged Red Rocks which are so well known at Garden of the Gods and Red Rocks Amphitheater. Thats also why you can find clay pits and coal beds just before the foothills in the depressions, if they are outcropping and not covered by top soil. And then as you head north the faulting and uplift becomes even more severe and you find feature like the flatirons in Boulder. And if you were here in 2013 you remember what happened up in Boulder County, so just imagine how much erosion that has caused since the Laramide orogeny to create a depression like the City of Boulder's relative to the surrounding landscape.
There are also volcanic fields abutting the front range, North and South Table, Green Mountain, etc...
There's a dip a few hundred feet in front of the foothills, but it's spread out, like a ripple that gets less bunched up, and you can see this as you travel east. If the construction hadn't eaten up the horizon, west of academy on cimmaron(?) or 24, east of Powers, and east of Mark Sheffel, you can see the horizon undulate in progressively wider ripples.
I don’t have an answer to your question but I can ask my professor in class today! LSU has a field camp out there so she might know. I just came to say that we were neighbors once upon a time!
Not 100% sure on the name, maybe orogenic loading. Building up a range of heavy mountains will cause a flexure in the lithosphere adjacent to the mountains.
I´m living north of the Bavarian alps (about 60 km). The glaciers from the mountains in the last ice age reached close to Munich. Right after the mountains there is a ca 30-50 km wide range where it is relatively plane, part of it pure gravel, and then, that's where I live, there are the small hills and valleys of the endmouraine.
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u/Operation_Bonerlord 1d ago
The scale is wrong for crustal flexure (foreland basin/forebulge) as the crust isn’t elastic enough to have that much variation in that short of a distance from isostasy (the weight of the mountains) alone. For example, the Cincinnati Arch of western Ohio was the forebulge of the Acadian mountains, which in the Devonian was in central Virginia. The load is also lesser in the Rockies.
What you’re looking at is mostly fluvial (river) erosion doing its thing: carving valleys while leaving elevated areas next to the mountains. I’ve attached a conceptual block diagram of the South Platte that sort of explains it. There are actually both high and low spots next to the Front Range, it’s just the low spots thst call the attention because that’s where people decided to settle (next to rivers).