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Touring the geologic map of the United States

Published in 1974 by the US Geological Survey, the geologic map of the United States beautifully lays out our country’s geology. If you’re stuck at home these days –as most of us are—you can gaze at this masterpiece and go anywhere! USAGeolMap-allllr

S Willamette Valley

Southern Willamette Valley. Eugene’s in the center of the map

At their simplest, geologic maps read like road maps: they tell you what rock unit or recent deposit forms the ground at a given place. So right here where I am in Eugene, Oregon, I can see that I live on “Q” –which stretches north up the Willamette Valley. “Q” stands for Quaternary-age material (2.5 million years to present), which is typically alluvial material –or sediment deposited by rivers and streams.  Just to the south of me lie a variety of older volcanic and sedimentary rocks. You can look them up in the map’s legend using their symbols. Here’s Rule #0: white areas, being mostly alluvium, mark low areas; colorful areas indicate bedrock, so are typically  higher in elevation.

The beauty of this map is that you can see the whole country at once, and using just a few rules, can immediately glean the underlying structure of a region.

For any of the maps, photos, and diagrams here, you can see a larger size if you click on the image.

x-cut block diagramlr

Rule 1: Rock bodies without internal contacts appear as color swatches (or “blobs”) as seen on the block diagram below. These rocks include intrusive igneous, undifferentiated metamorphic, and flat-lying sedimentary rock. (If you’re not sure what geologic contacts are, please see my recent post on the nature of geologic contacts.)

Rule 2: Rock bodies with internal contacts appear as stripes, which you can see from the diagram above and the photos below. These rocks are typically inclined sedimentary rocks –simply because the contacts between different rock units appear as lines where they intersect the Earth’s surface.  And notice that the steeper the rocks dip (their angle of inclination), the narrower the stripe becomes. In fact, the outcrop widths of vertically dipping beds equals their thickness!

aerial dipping beds

Aerial view of tilted Paleozoic-Mesozoic sedimentary rock at the edge of the Front Range, Colorado

Rule 3. You really can read a map like a cross-section –in fact, maps are basically the same things as cross-sections except they represent horizontal slices of the crust rather than vertical ones. As an example, you get an incomplete geologic history if you only use the cross-section in the block diagram above. The map view actually has all the information because it not only depicts the fold in map view, but also the fault.

Rule 4. Folded rocks typically appear on maps as zig-zags, or in the case of the single anticline in the block diagram above, as a half zig-zag. You get a full zig-zag if you have a paired anticline and syncline, like in the diagram below. The map expression results from the hinge of the fold being inclined –or plunging—rather than simply being horizontal. plunging folds

And here’s an air photo of a plunging fold in the Montana Fold-Thrust Belt.

Plunging anticline, SW Montana

Aerial view of north-plunging anticline in the Fold-Thrust Belt of SW Montana

Time Scale2020lr

Geologic Time Scale. If you don’t know the geologic time scale, click on the image to the side to see it at full size (like all the images here). As far as reading geologic maps, it’s very helpful to know the time scale and the abbreviations of the different time periods. So, “X, W, Y, Z” are all Precambrian (X=Archean; W, Y, Z=progressively younger Proterozoic); “Pz” means Paleozoic (541-252 million years ago), “Mz” means Mesozoic (252-66 million years ago) “Cz” means Cenozoic (66 million years ago to present). Of course, the Paleozoic, Mesozoic, and Cenozoic have further subdivisions with their own abbreviations that you can see on the full-size image.

 

The map.

boxes and regionslrNow for some features on the geologic map! This post is just an introduction, so I’ll stick to the boxed areas to illustrate some geological relations –and maybe throw in a few photos along the way. Then I’ll briefly discuss the localities shown in blue –and the rest of the map is for your own entertainment!

1. SnakeRPlain, IdahoBathlrMap 1. Snake River Plain and Yellowstone. The Snake River Plain, beneath that long flat stretch of Interstate 84, is covered by Quaternary age basalt flows. The landscape is flat because the lavas are flat –and so they show up as a big swatch of pink. Compare it to the ribbon-like bands to the north and south, mostly steeply inclined, folded, and faulted sedimentary rock of the Fold-Thrust Belt. The white stripes between the ribbons mark valleys (they’re filled with alluvial deposits) so we can infer that the ribbons, which mark bedrock, are mountains–which they are.

Notice how the Snake River Plain cuts across the ribbons of the Fold-Thrust Belt. It’s much younger: the lavas all erupted over the top of the older, deformed rock. You can actually see that relation if you drive US20 along the north side of Craters of the Moon National Monument.

And just beyond the Snake River Plain to the northeast, you can see Yellowstone, marked by rhyolites that also cover the older, surrounding rocks. Yellowstone is on trend with the Snake River Plain because they are both products of the southwestern drift of the North American Plate over the Yellowstone Hot Spot. Lying beneath the Snake River Plain Basalts are former rhyolitic calderas like Yellowstone. You can see some of these rhyolites at Shoshone Falls in Twin Falls, Idaho.

Shoshone Falls, Idaho (Pan)

Shoshone Falls spills over light-colored rhyolites. Rocks on the skyline are Snake River Plain Basalt, Idaho

Then there’s the Idaho Batholith, shown as the green swatch, which formed because of subduction along the western margin of North America during the Cretaceous Period. It’s of about the same age and origin as the Sierra Nevada in California.

2. Basin and RangelrMap 2. The Basin and Range Province. Described by the geologist Clarence Dutton in 1884 as “An army of caterpillars”, the Basin and Range consists of alternating basins, shown by the white color for alluvium, and ranges, shown by the colors, indicating various types of bedrock.

Geologically, the province is varied and complex, and I can’t begin to do it justice here except to say that much of the modern day landscape is a product of crustal extension –so many of the ranges are bound by normal faults, in a manner akin to the illustrations below.  It’s quite a wonderful part of the world and includes Death Valley, my go-to place for all things geology.

Basin and Range

Aerial view across the Basin and Range Province in Nevada.

Tilted-F-blocks-and-DV

Tilted fault blocks. The photo of Death Valley region as tilted fault blocks coincides with the inset on the cross-section.

And notice how the southeastern part of Map 2 includes the ribbon-like patterns of the Fold Thrust Belt –and southeast of there, the broad swales of the Colorado Plateau! (Map 3).

Map 3. Colorado Plateau. In preparation for a field trip to the Colorado Plateau, my grad school thesis advisor described the region as “an island of tranquility surrounded by tumultuous Cenozoic deformation.” I’ll never forget that –so accurate a description and such flowery language!3. ColoPlateaulr

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Aerial view of incised meanders of the Green River cutting through flat-lying rock, Utah.

From the map, you can see that the region is marked by wide color swatches edged by narrow ribbons. The swatches mark approximately flat-lying sedimentary rocks and many of the ribbon-like edges mark monoclines, where the rock flexes from flat-lying to steeply dipping to flat-lying again, like the photo below of the San Rafael Swell. The monoclines formed in the early Cenozoic because of faulting in the underlying basement rock.

Monocline on the Colorado Plateau, Utah

Aerial view of the southern San Rafael Swell, Utah

In some places, such as Black Mesa (location “a”) in northern Arizona, you can see the ribbons surrounding the swatch. These areas typically mark individual plateaus, capped by flat-lying rocks with older rocks exposed on the slopes below. And in the Grand Canyon, the ribbon-like patterns similarly form by older rocks exposed downwards towards the river.

3a. GrandCyn+ pic

Geologic map and and photo of the Grand Canyon, AZ

And those little red dots? Those are bodies of intrusive igneous rock, including the La Sal, Abajo, and Henry mountains that intruded the Paleozoic and Mesozoic sedimentary rocks at various times during the Cenozoic.

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La Sal Mountains rise over flat-lying Paleozoic and Mesozoic rock of Canyonlands National Park, Utah

 

4. BlackHillslrMap 4. The Black Hills rise dramatically from the western plains because they’re a structural dome, a fold in which the rocks dip outwards in all directions, away from the oldest rock in the core. You can see that the rocks get younger out from the core if you click on the image to see it at full size. The core consists of bluish-gray rocks labeled “X” –which indicates an age from 2.5-1.6 billion years and the surrounding rocks are labeled “lPz” then “uPz” then “Tr”, “J”, and “K” to indicate “lower Paleozoic”, “Upper Paleozoic”, “Triassic”, “Jurassic”, and “Cretaceous” respectively. Notice how the rocks dip the steepest on the east side of the dome.

Dome

Now if you look at the state of Michigan on the USA map, you can see that it’s the same thing but in reverse. It’s a structural basin, with the youngest rock in the core!

Map 5. Transect across the Appalachians. Starting near the Ohio-Pennsylvania border, you can see that rock is pretty flat-lying because it consists of lots of irregular swatches. The irregularity mostly comes from topographic changes that expose different rocks at different elevations, in a manner similar to some of the features in Map 3. But moving eastward, you hit the Valley and Ridge Province of the Appalachians: long ridges of folded Paleozoic sedimentary rock.5. NAppalachiantransectlre

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Channel deposit in the Triassic Newark Group, Connecticut.

And then you hit the teal-colored Newark Group, a series of Triassic sedimentary rocks deposited in basins as North America rifted away from Africa. It’s part of the Piedmont, a  section of the Appalachians that’s of lower topographic relief than the Valley and Ridge Province, but much more variable in its bedrock geology. It even includes fragments of oceanic lithosphere and mantle, shown in the navy blue color, and Proterozoic metamorphic rock, labeled “Ygn”.

Finally, you reach the Cretaceous sedimentary rocks of the Coastal Plain (shown in green), deposited after the Appalachians had formed and largely eroded. These rocks, as well as the overlying Tertiary rocks are relatively undeformed. Notice how the basal contact of the green “lK” cuts off the contacts within the Piedmont!

5a. NAppalachianCloseupThis inset of a close-up of the Valley and Ridge shows a classic example of the zig-zag pattern that results from plunging folds. You can determine the direction the folds plunge by looking at the ages of the rocks: anticlines contain the oldest rock in their cores whereas synclines contain the youngest rocks in their cores. Because anticlines plunge in the direction of their “noses” (where the outcrop pattern zigs or zags), we can infer that these folds are plunging to the northeast.

Map 6. Another view of the Appalachian Coastal Plain. Here’s a great example of a regional, angular unconformity, between the undeformed Cretaceous and younger rocks of the Coastal Plain and the highly deformed rocks of the Appalachians.6. S Appalachianslr

And of course, there’s so much more!

You can download a full-sized version of this map from the United States Geological Survey at https://pubs.er.usgs.gov/publication/70136641. It comes in 3 parts: West half, east half, legend.

In the meantime, here are some of my favorite other features:

The Columbia River Basalt Group (orange swatch) that erupted between 16-6 million years ago and covers some 70,000 square miles of Washington, Oregon, and western Idaho.

Basalt flows of the Columbia River Basalt Group, Imnaha Canyon,

Lava flows of the Columbia River Basalt Group in eastern Oregon

The Lewis Thrust along the east edge Glacier National Park. The fault brings Proterozoic sedimentary rock of the “Belt Supergroup” over Cretaceous sandstone. It’s a very low angle fault so displays a very irregular trace as it winds in and out of the valleys, kind of like a contour line.

color reversal: KODAK UNIVERSAL K14. SBA settings neutral SBA off, color SBA on

Lewis Thrust, which lies just above the tree line, places Proterozoic rock over Cretaceous rock.

The Ogallala Formation (yellow swatch), only about 6-2 million years old and up to 1000 feet in thickness, covers more than 170,000 square miles of the Great Plains –and provides drinking and irrigation water to the vast majority of people in the region.

Deposits of the Mississippi River (gray). From the Mississippi delta all the way to Cairo, Illinois, the river’s deposited Quaternary alluvium on top rocks as old as Eocene.

Meander loops on Mississippi River, Louisiana

Aerial view of meander loops on Mississippi River, Louisiana

Woohoo!


Incidentally, all the photos shown here are available for free download from my geology photo website. It has a search function that accesses more than 3500 images I’ve taken over more than 35 years.

 

Oregon’s rocky headlands: geologic recycling through erosion and uplift and erosion…

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Crashing waves at Heceta Head, Oregon

You can’t avoid thinking about erosion while standing on one of Oregon’s rocky headlands. The waves keep coming, one after another, each crashing repeatedly against the same rock. Impossibly, the rock appears unmoved and unchanged. How can it not erode?

The answer, of course, is that headlands do erode, quickly, but on a geologic time scale. We just miss out because we live on the much shorter human time scale. And the erosion belongs to a cycle in which coastal uplift causes eroded and flattened headlands to rise and become headlands once again, all subject to more ongoing erosion and uplift.

Wave energy is most intense at headlands because the incoming wave typically feels the ocean bottom near the headland first, which causes the wave to refract. As shown in the aerial photo below, this refraction focuses the wave energy on the headland.

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Wave refraction causes wave energy to focus on the headland. Arrows are perpendicular to wave fronts.

As you can see in the next few images, headlands don’t erode evenly. They erode irregularly, as the waves exploit any kind of weakness in the rocks such as faults and fractures, or if they’re sedimentary, bedding surfaces. The products of this erosion are as beautiful as they are interesting: sea stacks, sea arches, sea caves… The list goes on and on.

Headland and lighthouse, Heceta Head, Oregon

Aerial view of Heceta Head, Oregon.

From the above photo, you can see that sea stacks are simply the leftover remains of a headland as it retreats from erosion. That’s a critical point, because some sea stacks, especially the one with the arch in the photo below, are a long way from today’s coastline.

Sea stacks and sea arch, southern Oregon

Sea stacks and sea arch, southern Oregon

Those rocks, 1/4 to a 1/2 mile away used to be a part of the coastline? The land used to be way out there? YES!!! For me, that’s one of the very coolest things about sea stacks –they so demonstrate the constant change taking place through erosion.

Taken to its extreme, erosion renders headlands into wave-cut platforms, such as the one below at Sunset Bay. Being in the intertidal zone, these platforms make great places for tide-pooling–and ironically, for people-watching too. Geologically, they form important markers because they’re both flat and form at sea level. When found at higher elevations, they indicate uplift.

Wave-cut bench, Sunset Bay, Oregon

Wave-cut bench at Sunset Bay, Oregon

In fact, looking carefully at the photo above, you can see a flat surface on the other side of the bay. It’s an uplifted wave-cut platform! Called a marine terrace, it’s covered by gravel and sand originally deposited in the intertidal zone. Those deposits rest on bedrock that, at an earlier time, was also flattened by the waves. The photo below shows a better view of this terrace from the other side.

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Breaking wave at Shore Acres State Park, Oregon. Tree-covered flat surface in the background is an uplifted marine terrace.

These uplifted marine terraces can be found up and down Oregon’s coastline. Researchers recognize several different levels, the oldest being those uplifted to highest elevations. The one in the photo above at Shore Acres State Park is called the Whiskey Run Terrace and formed about 80,000 years ago. You can see a similar-aged terrace below as the flat surface beneath the lighthouse at Cape Blanco, Oregon’s westernmost point. An older, higher terrace forms the grass-covered flat area on the right side of the photo.

Cape Blanco, Oregon

Cape Blanco, Oregon looking NE. The flat surface beneath the lighthouse is the ~80,000 year-old Cape Blanco Terrace, probably equivalent to the Whiskey Run Terrace at Shore Acres; the flat area on the right side of the photo is the higher Pioneer Terrace,  formed ~105,000 years ago.

Researchers take the approximate ages of the terraces and their elevations to calculate approximate rates of uplift. In this area, Kelsey (1990) estimated a rate of between 4-12 inches of uplift every 1000 years. That might seem slow, but over hundreds of thousands of years, it can accomplish a great deal.

And look! The uplifted terraces? They’re on headlands! Of course, because they’ve been uplifted! And the headlands are now eroding into sea stacks and then platforms –to be uplifted in the future and preserved as marine terraces that sit on top headlands. And on and on, as long as the coastline continues rising.

YachatsWaves1-4

Blowhole near Yachats, Oregon. Incoming wave funnels up a channel eroded along a fracture and explodes upwards on reaching the end.

Some links and references:
Kelsey, H.M., 1990, Late Quaternary deformation of marine terraces of the Cascadia Subduction Zone near Cape Blanco, Oregon: Tectonics, v. 9, p. 983-1014. (Detailed study of Cape Blanco, including uplift rates).

Miller, M., 2014, Roadside Geology of Oregon, Mountain Press, Missoula, 386p. (General reference which details the concepts and includes several of the photos used here).

Earth Science Photographs–free downloads for Instructors or anybody: my webpage!

Geologic history of the western United States in a cliff face in Death Valley National Park

Of the many geologic events that shaped the western United States since the beginning of the Paleozoic Era, five really stand out.  In approximate chronological order, these events include the accumulation of tens of thousands of feet of sedimentary rock on a passive margin, periods of compressional mountain building that folded and faulted those rocks during much of the Mesozoic–likely driven by the accretion of terranes, intrusion of subduction-related granitic rock (such as the Sierra Nevada) during the Jurassic and Cretaceous, volcanic activity during the late Cenozoic, and mountain-building by crustal extension during the late Cenozoic and continuing today.  This photo on the western edge of Panamint Valley in Death Valley National Park of California, captures all five.

View of canyon wall on west side of Panamint Valley in SE California --part of Death Valley National Park.  See photo below for interpretation.

View of canyon wall on west side of Panamint Valley in SE California –part of Death Valley National Park. See photo below for interpretation.

The photograph below shows an interpretation.  Paleozoic rock is folded because of the Late Paleozoic-early Mesozoic compressional mountain-building; it’s intruded by Jurassic age granitic rock, an early phase of Sierran magmatism that took place just to the west; the granitic rock is overlain by Late Cenozoic basalt flows, and everything is cut by a normal (extensional) fault.  And there is also a dike that cuts the Paleozoic rock –probably a feeder for the basalt flows.

Interpretation of top photo.

Interpretation of top photo.

So this is all nerdy geology cross-cutting relations talk –but here’s the point: in this one place, you can see evidence for 100s of millions of years of Earth History.  Earth is old old old!  THAT’S why I love geology!

And for those of you who crave geologic contacts?  This photo has all three: depositional, between the basalt and underlying rock; intrusive, between the Mesozoic granite and the folded Paleozoic rock; fault, the steeply dipping black line between the basalt and the Paleozoic rock.  Another reason why I love geology!


click here to see photos and explanations of geologic contacts.
or click here for a slideshow of Death Valley geology.

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