ONE OF THE reasons I became a geologist was the maps. I will never have enough walls in my house for all the beautiful geologic maps I'd love to hang. But these maps are also full of information about the surface of the Earth, with hints of what is below and what came before. There are colors for different ages of rocks, patterns for different kinds of rock, and lines for faults, elevation and boundaries between rock layers.

One of the best things I ever got to do in school was make geologic maps. There are compasses, hand lenses, rock hammers, measuring staffs and colored pencils involved. You get to go out, find the rocks, determine what formation they belong to, which direction those formations extend and how steeply their layers are sloping. You get to draw all that information on the map, and then color it in. It's really fun and also challenging, and very satisfying.

Some of the most memorable geologic maps I've seen have been of national parks. Maybe that's because the parks are one of the few places where you will easily be able to find, and buy, a geologic map of the place you are in at that moment. It also doesn't hurt that these maps reflect the landscapes they depict, and the landscapes are usually pretty amazing in the national parks.

In this post, I've used some of my favorite maps of national parks to explain the basics of how geologic maps work, and how to read them.

Arches National Park

Utah is lousy with beautiful national parks. Thanks to the uplift of the Colorado Plateau on which the state sits, many layers of relatively easily eroded sedimentary rocks, and sparse desert vegetation, the landscape of bare rock is beautifully layered and carved into canyons, cliffs, and many crazy shapes including natural arches. Arches National Park is in the southeastern part of Utah at the upper edge of a swath of land extending to the state's southwestern corner that also contains (from east to west) Canyonlands, Capitol Reef, Bryce Canyon and Zion National Parks.

The geologic map of the area is dominated by shades of green because most of the rocks at the surface are Triassic, which means they were formed between about 200 million and 250 million years ago and are part of the Mesozoic era. Rocks of this age are often represented by various greens.

USGS

To the left is a snippet of this map's explanation, which is a description of all the rock formations and units found on the map. Everywhere on the map that you see the colors in the explanation, you'll find the corresponding formations on the ground. Each geologic time period has its own color scheme (though there is not complete agreement among geologists about which colors belong to which period). For example, the oldest rocks on the planet were formed during the Precambrian era (from around 540 million years to more than 4 billion years ago), and are usually represented in shades of red and pink.

Each colored area on the map also has a a letter symbol, which you can see in the explanation boxes, though they'll be hard to see at the scale of the map above. Follow the link below to open a higher-resolution version, and you'll see more of these details. All the Triassic-age formations have a Tr preceding the first letter of the formation name. You may have noticed that the Navajo Sandstone has a J in front of the Tr. This because it spans the boundary between the Triassic and Jurassic time periods.

The descriptions of each formation should be enough for a geologist to be able to identify those formations in the Arches area. Many of these formations are found all over the intermountain west, and their specific characteristics – such as grain size, thickness and color – vary in other locations. The Navajo sandstone, for example, can also be found in parts of Nevada, Arizona and Colorado.

Some geologic maps include what are called cross sections, such as the one included below the Arches map at the top of the post. This is an interpretation of what a vertical slice of the crust below the surface looks like. Geologists use clues on the surface, such as the angle the rock layers are sloping (known as the dip) and thickness of the layers, to guess what lies beneath. The location of the cross section is shown on the map with a line drawn between the capital letters A and A'. (You can see the whole cross-section line on the full-resolution map.)

Yellowstone National Park

USGS/NASA/NPS

Some of the colored areas on geologic maps are overlain by patterns, including dots, stripes, triangles, grids and many more. These patterns are used to distinguish different rock types, and different rock formations of the same age.

In the examples below from Yellowstone National Park, two formations of the same age (Triassic) are the same color, but the volcanic rocks have a pattern made out of the letter V, and the Pinyon Conglomerate has dots. The Heart Lake Conglomerate also has dots, but was deposited in the most recent geologic period, known as the Quaternary (from 2.6 million years ago to the present), which is usually colored in tan and yellow shades.

Gneiss and schist are types of metamorphic rocks and have a mixed pattern, while the sedimentary formations at the far right have subtle diagonal stripes. All these patterns help geologists read the map and understand the geology.

Hawaii Volcanoes National Park

USGS

Geologic maps of volcanoes are often really beautiful because they show a bunch of different lava flows of different ages, and therefore different colors, radiating from the volcano's vent. Hawaii's Big Island has the added bonus of multiple vents.

It's easy to see in this map that geologic maps are built on top of topographic maps. The topo lines represent different elevations, and each topo line tracks a single elevation. On Hawaii, which is a mountain, most of the topo lines make parallel rings of rising elevation around the island.

USGS

The image above is a bare topo map of the southeastern part of the island. The boundary of the strangely shaped national park is outlined by a black dashed line highlighted in yellow.

Grand Canyon National Park

USGS

If you've only seen one geologic map in your life (before today), it may have been this one. The geologic map of Grand Canyon National Park is a really good map. The canyon was formed when the river carved through layers and layers of sedimentary rock that had been laid down long before. Because the layers haven't been disturbed too much by tectonic forces bending and tilting them, they are still basically horizontal. This makes for an appealing pattern of different colors tracing the same spidery pattern.

Boundaries between rock formations are marked by thin black lines (as opposed to the thicker black lines that designate faults). You can see that those boundary lines run pretty much right along the faint topographic lines on the base layer of the map. As you move through the colors from the outside of the pattern to the inside, you are moving down the sides of the canyon wall, and into older and older rock.

Wrangell-St. Elias National Park

Geologic map of the Wrangell-Saint Elias National Park and Preserve, Alaska USGS

This park covers more territory than many European countries, with 13.2 million acres of rugged, wild terrain filled with glaciers, mountains, bears, moose and eagles. Most of the park can only be reached by plane, which means it is an excellent place to get away from everyone.

Like much of Alaska, the geology of this park is a complicated mash-up of large chunks of crust known as terranes that came from different places and were thrust together. Thrust faults are the result of one chunk of crust being pushed up on top of another, and they are represented on geologic maps as a think black line with black triangular teeth on the side that has been thrust up.

Normal faults, where the ground on one side of the fault is sliding down relative to the ground on the other side, are marked with black lines that become dotted where the fault can't be seen at the surface. Strike slip faults look the same, but have arrows on either side of the line to show that the crust on one side of the fault is sliding horizontally relative to the crust on the other side.

This map's legend has some other neat symbols for dikes, calderas, ash, glaciers, and historical glacier extents.

Great Smoky Mountains National Park

Geologic map of the Great Smoky Mountains National Park region, Tennessee and North Carolina [Geologic map of the Great Smoky Mountains National Park region, Tennessee and North Carolina] USGS

The amount of detail in this map gives it a different, more dense look than the other maps in this gallery. It may look messy, but every one of those little specks and dots is a piece of geologic information. In the close-up piece of the map to the right, you can see that many of the little marks are lines with a number next to them. These are strike and dip symbols that geologists use to indicate which way a layer of rock is extending into the crust below.

Sedimentary rocks are laid down horizontally, but are often later tilted by tectonic forces. When geologists find a sedimentary layer, they use a special compass that has a leveling bubble and what's called an inclinometer, to find the maximum angle of the layer's dip. The numbers next to the little lines are dip measurements, and the dip direction is indicated by a little tick mark on the side of the line. The line represents the strike, which runs parallel to the dip direction. Using these measurements, geologists can get a picture of what is going on under the surface to help them make cross sections like the one above.

In the example to the right, you can see that most of the strikes are running diagonally up and to the right. The dip measurements in the beige area are indicating the layers are dipping down to the left of the strike. But in the blue area to the left, the beds are dipping the opposite way. In between the two areas, you'll see there's a thrust fault that has pushed the blue formation on top of the beige formation.

Bryce Canyon National Park

Geologic map of Bryce Canyon National Park and vicinity, southwestern Utah USGS/NPS

Like Arches National Park, much of the landscape in this park was carved by wind and water erosion over time. Bryce's famously crazy looking spires, known as hoodoos, were also eroded by a process known as frost wedging, where water trapped in cracks in the rock freezes, expands and pushes the crack open further, eventually breaking off chunks of rock.

In the map above you can also see another major geologic feature. The sharp boundary between the rocks colored in green and blue on the right side of the map, and the brown and pink rocks to the left is a branch of the Paunsaugunt fault. This normal fault has moved older rocks on the right side up and juxtaposed them against younger rocks on the left. This is easier to visualize with the bit of cross section expanded below.

You may notice that some of the yellow areas on the map seem to run right across the fault without being affected. These are sediments that were deposited after the most recent motion on the fault occurred.

Guadalupe Mountains National Park

Geology of the southern Guadalupe Mountains, Texas [Plate 3: Geologic Map and Sections of Southern Guadalupe Mountains, Texas] USGS

I've never been to this Texas national park, which sits just under the border with New Mexico. But this map can tell something about how it looks. The topo lines in the grey area at the top of the map show that there are some steep-sided canyons. By looking at the descriptions of those formations in the explanation (you can see this on the high-resolution map, link below), I know that those rocks are actually grey and black in person.

It's also easy to see a fault zone running through the middle of the map. All the lines running nearly parallel from the upper left down to the middle of the map are normal faults. You can see what effect these faults have had on the terrain by looking at the cross sections below.

Geology of the southern Guadalupe Mountains, Texas [Plate 3: Geologic Map and Sections of Southern Guadalupe Mountains, Texas] USGS

Olympic National Park

USGS

The Olympic peninsula is a classic example of what geologists call a subduction complex. Long ago, the ocean plate was forced under the North American plate in a process called subduction. As the plate was subducted, some of the sea floor sediments, seamounts and ocean crust were scraped off the top and shoved up against the continent.

This jumbled pile of rocks accumulated into what is known as an accretionary wedge. Eventually this wedge was pushed up onto the continent to form the peninsula. This history helps explain the somewhat chaotic look of the map. A close-up of the formations in the expanded bit of the map below looks even more messed up.

The Pacific plate is still being subducted beneath the Pacific Northwest today. Heating of the subducted plate is what fuels the region's volcanoes, including Mount St. Helens.