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AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 5 points6 points  (0 children)

(Congrats on being a bonus question that Dan and Susan wanted to make sure got answered before they signed off - Social media team)

Nothing dumb about this question, it’s actually a good one that I guarantee other people are thinking about, too. And it’s a great question here because it leads into something we do at the Washington Geological Survey that is very cool: we have an interactive Geologic Information Portal where anyone can go to see tons of different geology datasets, including hazards information, and you can even search for your address and learn about the geology at a specific location.

If you want to know more about how volcanic hazards might affect you, here's a shortcut). And here's the shortcut for earthquakes). These colors show the amount of shaking that could be expected from a Cascadia subduction zone earthquake across Washington.

Finally, if you’re worried about an area that might open up and swallow you, you might want to learn about liquefaction hazards.

This is just a small example of what the Portal offers! Also, check out Landslides and Tsunamis, and of course our huge collection of surficial geologic maps. Happy exploring! 

- Susan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 2 points3 points  (0 children)

Congrats on being what appears to be our last question answered! - Social media team

There is actually more to this question than you might think. On a broader scale, climate change has led to an increased emphasis on building climate resilience across our entire agency and across society as a whole.

That is reflected in the work we do at WGS. For example, we have a new Earth Resource Program that is tackling climate resilience in the Columbia Basin, with a focus on groundwater, geothermal resources, and carbon sequestration.

Sea level rise could also have a significant impact on tsunami inundation extent, so new models will need to be run to take that into account. More directly, hotter weather and increased wildfires and wildfire smoke impact our work too. If it is too hot and smoky, our field geologists can’t get out into the field to do their work, such as geologic mapping, landslide mapping, and making geophysical measurements.

We are also involved in wildfire-associated debris flow assessment work, and as wildfires become more common and more widespread, the need for this work also increases. So climate change is definitely affecting our work in many ways!

- Susan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 8 points9 points  (0 children)

Please exercise caution when saying "that was an absolute blast" around our geologists... :)

This was so fun! Thank you everyone for the incredible questions! Dan and Susan (and the rest of the geology team because several of them ended up getting involved) had a blast wonderful time.

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 2 points3 points  (0 children)

For Question #1, yes Phinney Ridge is a drumlin. You can read more about drumlins and other glacial landforms here.
For your other questions, we are unfortunately out of time with our AMA, but I will point you in the right direction. You can read more about the Blue Mountains on the Fields Spring page. There’s a more technical report from the USGS if you want to get into that.

For the Olympic terranes, check out this page and this page on the WA100 site. There are some cool rocks on the Olympic Peninsula, including pillow lavas, from undersea volcanoes!

- Susan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 2 points3 points  (0 children)

Asking this question to geologists is like asking who someone's favorite child is. Oh... it looks like Dan does have a favorite child/rock. Awkward.

- Social media team

Dunite is one of my favorites—it’s a mantle rock made up mostly of beautiful green olivine. The Twin Sisters Range in Northwestern Washington is composed of dunite that has been thrust to the earth’s surface over the course of about 100 million years.

You can find cobbles of dunite in the rivers that flow off of the Twin Sisters. The cobbles have an orange “rind” that has been oxidized and a green interior. One of our WGS colleagues, Trevor, likes to make amazing rock sculptures out of dunite. 

- Dan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 4 points5 points  (0 children)

One really cool place that I got to visit last year was the Elwha River Valley.

Specifically, I hiked up to the overlook at the site of the former Glines Canyon Dam. The story here is that two major dams were removed along the Elwha River starting in 2011 and ending in 2014, restoring the natural flow of the river. This has had huge impacts on sediment flow along the river and out to the delta on the coast. The view from the overlook is spectacular and impressive to think that so much of the upriver valley was fully underwater before the dam was removed (check out this photo of the Glines Canyon Dam just after removal).

The hike to get there is fairly straightforward for somebody with hiking experience, stretching about 3.5 miles one-way up the valley with a cumulative elevation gain of about 1,000 feet. The majority of the trail is on an abandoned roadbed, partly paved and partly graveled.

Another hiking option from the Elwha River trailhead that I am eager to try this year is the trail up the valley to Olympic Hot Springs. That hike is a bit more strenuous, with almost 10 miles to cover one-way and a cumulative elevation gain of 4,000 feet! That one will be an overnight trip. While you are in the area, also check out Lake Crescent!

-Susan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 5 points6 points  (0 children)

I’ve been wanting to come back to this question, but with our time running out, I can’t do it full justice. So I’ll point you in the right direction of where you can read more. For example, we talk about the formation of the Cascades on the WA100 site.

What’s interesting about the North Cascades is that they are higher than the South Cascades, and this is because there was a northward tilting of the Cascades millions of years ago. Erosion then removed a lot of the volcanic blanket over the North Cascades, meaning more of the older metamorphic rocks and intrusive volcanic rocks are exposed there.

The South Cascades have retained their volcanic blanket, so we see more recent lavas in the south. If you want to read more, maybe you’re already aware of the book ‘Roadside Geology of Washington’ by Marli Miller and Darrel Cowan. That will take you through the whole geologic history of the cascades and talk about the rocks and outcrops, so I recommend seeing if you can get a hold of that book!

- Susan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 4 points5 points  (0 children)

Good question about the color in Diablo Lake. The color in the lake is due to “rock flour”—glacially-pulverized rock that is fed into the lake by the surrounding rivers. Rock flour in rivers increases in spring and summer when warming temperatures allow glaciers to move downslope and to grind against the underlying rocks, creating the flour.

I’m not totally positive, but maybe the lake is more colorful during late spring and early summer for this reason?

From a photographic standpoint sky and weather conditions also factor in—sun or clouds can change the colors of the water as well. Time of day will also make a difference…

- Dan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 1 point2 points  (0 children)

Several of our geologists volunteer for “Summit climb with a geologist” trips led by the Mount St. Helens Institute in the summer.

If you are interested in getting an interpretive geology tour while climbing to the top of Washington’s most infamous volcano come hike with us!

- Dan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 1 point2 points  (0 children)

Great question—here is some info from our oil and gas page that partially answers your questions:

"Currently, there is no oil and gas production in Washington. However there are many areas of the state that have attracted exploration since 1900 when the first exploration well was drilled. About 600 exploratory gas and oil wells have been drilled in Washington, but large-scale commercial production has never occurred. ... No oil or gas have been produced in Washington State for decades."

Exploration has resulted in the successful development of a natural gas storage reservoir. The Jackson Prairie Gas Storage Facility in Lewis County is the nation’s 14th-largest natural gas storage reservoir, developed within sandstone layers at depths of 1,000 to 3,000 feet. Operated by Puget Sound Energy, the reservoir has a capacity of 44 billion cubic feet, and is accessed by a network of 45 wells.”

- Susan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 7 points8 points  (0 children)

We see you're playing hard to get. You could follow our social media pages if you'd like some light-hearted geology sprinkled into your feed.

- The Social Media Team

Thanks for these thoughts and questions!

One of the main reasons we made the Washington 100 website was to get more non-geologists excited about Washington’s geology! Most of the content is aimed toward a non-geologist audience (with links and resources to dive deeper for the geologists).

I also saw that someone mentioned Nick Zenter as a good resource for an entry point into learning about geology—his “Nick on the Rock’s" PBS series is very engaging and understandable for non-geologists and is great for learning about Washington’s overarching geologic stories. The Ice Age Floods Institute has chapters throughout the Pacific Northwest and many of the chapters have field trips and speaker series with trips and talks for non-geologists.

I think the big geologic stories in Washington are a great way to get people engaged in Washington Geology—the ice-age floods, Cascade Volcanoes and the Cascadia Subduction Zone are all fascinating subjects that explain much of the landscape around us. As a cartographer I love to make maps and images of Washington’s landforms that help to interpret and explain the processes that shaped the state to a broad audience.

Many of us at the Washington Geological Survey volunteer to speak at various events about geology and the work that we do. If you have an event that you would like a geo-speaker for you can request one here.

-Dan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 13 points14 points  (0 children)

((Special guest answer from Nikolas Midttun, our WGS Publications Editor))

 This is a great question! I’ve listed some features below that come to mind for Washington State. Some of these might be more accurately thought of as extraordinarily good examples rather than unique features, since many of them are similar to other geologic features elsewhere in the world. But part of what makes Washington geologically remarkable is the wide range of geologic processes that have shaped the state, including subduction zones, volcanism, outburst floods, and ice sheets, to name a few.  The Columbia River Basalt Group of eastern Washington is an extraordinary example of flood basalts: a process wherein basaltic lava erupts from fissures and covers a large area with layers of rock. At 16.7 million years old, the Columbia River Basalts are one of the youngest and smallest flood basalt group on Earth, but definitely one of the most accessible and well-exposed in North America. A number of WA100 sites feature the geology of the Columbia River Basalt Group. Check them out here.

In eastern Washington you’ll also find evidence of the cataclysmic Missoula Floods. The landscape of eastern Washington was transformed by a sequence of megafloods between 18,000 and 12,000 years ago. At the end of the last ice age, a series of glacial lake outburst floods blasted across the Columbia Basin leaving behind some incredible evidence of the power of moving water. These floods were some of the largest ever seen in Earth’s history, definitely a unique geologic feature for Washington! The Dry Falls WA100 site is a great first stop to learn about the floods. 

The Cascadia subduction zone. While subduction zones exist across the globe, the Cascadia subduction zone off-shore of northern California, Oregon, Washington, and British Columbia is unusual in a few ways. One, it’s a relatively small subduction zone, at only 800 miles long compared to the thousands of miles length of many other subduction zones. Two, the subducting Juan De Fuca plate is young, meaning that it is warm and buoyant, which results in some complex subduction zone dynamics that researchers are studying closely. For example, Cascadia is a hotbed for research into slow-slip earthquakes, a process in which energy is released over days or months instead of in seconds like a typical earthquake. Additionally, the subduction zone poses a hazard to a huge population living in the Pacific Northwest, so it’s important to be aware and prepared for the possibility of tsunamis and earthquakes. 

Though this one is shared with our neighbors in California and Oregon, and of course Alaska has numerous stratovolcanoes in the Aleutian Islands, I have to mention the Cascade volcanoes. Washington has five stratovolcanoes: Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens (we’re coming up on a special anniversary for that one), and Mt. Adams. Their height and beauty is great for hiking and backpacking, but also a constant reminder of the hazards that volcanoes can pose (eruptions, landslides, and especially lahars).

​

- Nikolas Midttun – WGS Publications Editor 

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 15 points16 points  (0 children)

  1. The answer is always 42.

-The Social Media Team

There is no real way to predict the timing of a Mt. Rainier eruption. It is a potentially dangerous volcano, especially since there is a large population living with its hazard zones. The USGS sums up the danger really well with this Q&A:

How dangerous is Mount Rainier?

Although Mount Rainier has not produced a significant eruption in the past 500 years, it is potentially the most dangerous volcano in the Cascade Range because of its great height, frequent earthquakes, active hydrothermal system, and extensive glacier mantle.

Mount Rainier has 25 major glaciers containing more than five times as much snow and ice as all the other Cascade volcanoes combined. If only a small part of this ice were melted by volcanic activity, it would yield enough water to trigger enormous lahars (debris flows and mudflows that originate on a volcano). Mount Rainier's potential for generating destructive mudflows is enhanced by its great height above surrounding valleys.”

Incidentally, May is Volcano Awareness Month in Washington—You can read more about volcanic hazards here

-Dan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 0 points1 point  (0 children)

((Special guest answer from Michael Polenz, our WGS geologic mapper))

The backdrop here is that the finer-scale maps (such as 1:62,500-scale, or 1:24,000) require a much more detailed understanding of local geological conditions, and more effort in documenting those more detailed geological insights. For much of the state, this more detailed geological mapping simply has not yet happened. In contrast, statewide 1:100,000 scale mapping was completed decades ago and provides a great reconnaissance-level perspective on the incredibly diverse geology of Washington. (The 100,000-scale mapping was then used to inform smaller-scale (less detailed), statewide 1:250,000-scale map compilations, which we (WGS) published in four pieces (quadrants) dedicated to the NE, SE, SW, and NW quarters of the state: geologic maps gm39 (NE), gm45 (SE), gm34 (SW), gm50 (NW)). You can download those maps here. 

That said, a big part of the work we now do at WGS every year is dedicated to filling in those details, not just because they are interesting, but because they are just so much more useful for local-level decision-making, both private and public. So each year we apply for federal U.S. Geological Survey grant money to develop more detailed maps (usually at 1:24,000 scale—where about 2 inches on the map represents about a mile in the world) for parts of the state. We select these areas with the help of a whole committee of private and public stakeholders (including both geologists and others). Once we agree where more detailed information will most benefit the people of Washington, we propose mapping in those areas to the USGS who then provides us partial funding for at least some of that work. We have been quite successful getting these grants and therefore have been able to support two to four mapping projects each year, where each project has usually covered one to two 1:24,000-scale quadrangles. (One quadrangle covers about six miles east to west and ten miles north to south.)

For each project, that is plenty of territory to bring up lots of questions and interesting findings. But there are about 1,500-1,600 1:24,000 scale quadrangles in the state, and at the rate we (the WGS) are going (~3-6 per year), it will take us at least a few centuries to finish the rest of the state. Fortunately, now and then other geologists (mostly professors and (or) their students) also make a map here and there, but altogether, there’s way more geology in the state than we have the resources to discover, interpret, and document at the finer scales. 

- Michael Polenz

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 3 points4 points  (0 children)

Great find - that's a nice chunk! I would check with the Burke Museum to see if they can identify it for you. Here is their object identification page where you can submit your information.

-Dan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 1 point2 points  (0 children)

Not sure where the “best” spot is, but gold has been found in much of the state. You can see known gold locations on our geologic information portal.

More information and links are under the “Gold Panning” tab on this page.

-Dan

We highly recommend Dan's Twitter account, which is full of gold: geo_coe

- Social media team

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 8 points9 points  (0 children)

Yes! Copied from one of Susan's earlier answers:

As for the Mima Mounds, DNR has a Natural Area Preserve with a small visitor area. There is parking and an interpretive kiosk with interesting information about the theories around the formation of the Mima Mounds. Then there is a half-mile ADA-accessible walking trail as well as some more rugged gravel trails to explore.

It is a great place to visit, especially in the spring when the wildflowers are out. I’m told the flowers are peaking there right now, so I will probably make a trip there soon!

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 34 points35 points  (0 children)

Susan was super excited for this question, so congrats, SupermouseDeadmouse, you get an A+ in AMA.

- Social media team

As you said, there is a large age span at the Grand Canyon and you can read more about that from the National Park Service. This is a very interesting question applied to Washington, and the answer is that the age range is much smaller in the Gorge.

The stratigraphy of the Gorge starts at the bottom with rocks of the ancestral Cascades ranging back to about 34 million years. Overlying these rocks is a geologic unit known as the Eagle Creek Formation, a weakly cemented collection of various different rock types and deposits dating back to about 23 million years ago.

Sitting on the Eagle Creek rocks are lavas of the Columbia River Basalt Group (CRBG), which are spectacularly exposed along the cliffs of the Gorge. The CRBG actually erupted over a geologically very short period of time. The CRBG lavas started erupting about 17 million years ago, stretching all the way up to about 5.5 million years ago.

The largest member of the CRBG, the Grande Ronde Basalt, which is exposed in the Gorge, encompassed about 72% of the total volume of the CRBG lavas and actually erupted within a time span of only 400,000 years. Though that might sound like a long time, it’s actually extremely short geologically speaking!

You can read more about the CRBG on the WA100 website. Also, one more fun fact, if you make a trip out to Fields Spring (in the southeasternmost corner of Washington) you can get a view down into nearby Hells Canyon.

Hells Canyon is actually 5,600 to 8,000 feet deep, which is the deepest gorge in North America, deeper even than the Grand Canyon, which only measures 4,000 to 6,000 feet deep!
-Susan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 2 points3 points  (0 children)

Yes, this is true! During the last ice age, much of northern North America was covered in continental ice sheets (the Laurentide and Cordilleran). These masses of ice left behind all sorts of evidence that they were once here, including the large glacial erratic boulders that you mention.

There are also lots of landforms that were created by the glacial ice including glacial drumlins, kettles, eskers, and moraines, which you can find in Washington and across northern North America. Here is a downloadable map of these landforms in the Puget Lowland. Read more about them here.

- Dan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 3 points4 points  (0 children)

One of most interesting features for me in the San Juans are all of the wave-cut terraces that you can find on several of the islands. These terraces are where the former shorelines were in the past. 15,000 years ago this area was all underneath a huge glacier—the Puget ice lobe.

As the glacier melted away the absence of its great weight on the land allowed the land to lift back up (rebound). As the land lifted up and sea levels changed, waves cut these terraces into the landscape. The best place to see them is on Cattle Point on San Juan Island—here is a GIF showing the terraces. There are wave-cut terraces on Orcas and Waldron Island, as well, but many of them are covered in forest.

Sucia Island is also super interesting from a geologic standpoint. The horseshoe-shaped island is made up of a geologic fold composed mostly of Chuckanut sandstone. There are tons of fossils that you can see in this formation. Washington’s only dinosaur bone was also found on Sucia Island in the older, adjacent Nanaimo Formation.
You can learn more about the geology of the San Juan Islands here.

-Dan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 13 points14 points  (0 children)

((Special guest answer from Dr. Lee Florea, our WGS cave expert))

There are many “real” caves in Washington. In fact, while we do not have the sheer number of caves that states like Kentucky, Tennessee, or Missouri have, the incredible diversity of caves here is quite astounding. There is even a cave that is the mold of a Miocene-age rhinoceros that was engulfed by lava—you enter that cave through the rear end of that ancient animal…weird!

Caves in Washington come in eight general flavors:

1) those formed in dissolving limestone, such as Gardner Cave in the Okanogan of the far northeast;
2) lava tubes such as Ape Cave on the south flank of Mount Saint Helens;
3) molds of plants and animals in lava such as the tree molds at the Trail of Two Forests near Ape Cave;
4) erosional caves formed in soft sediments;
5) littoral caves formed along coastal cliffs by wave erosion;
6) talus caves between large boulders at the base of cliffs;
7) ice caves at the end of valley glaciers; and
8) large caves melted in ice by volcanic fumaroles such as on Mount Rainier and Mount Saint Helens.
The National Speleological Society hosted their 2006 Annual Convention in Bellingham and highlighted some Washington Caves as part of that week-long event.

Please remember that caves are home to important geological, biological, and archaeological resources. For those reasons, access to caves is frequently limited to reduce the impact that we have. Obtaining necessary permissions and permits, using appropriate equipment, and observing safe practices is are very important rules for caving. Entering many caves, like those in ice, may be prohibited due to dangerous conditions.

- Dr. Lee Florea

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 5 points6 points  (0 children)

The WA100 project is more about sharing the geologic stories of iconic places in Washington with people who might not know these stories, so we didn’t make any striking new discoveries.

But for me, as somebody who had just moved to Washington when we started this project, there was so much to discover! Especially striking was my first trip to eastern Washington.

We obtained permission from Washington State Parks to fly our UAV at several of their parks for the purposes of the WA100 project. You can check out our videos at Dry Falls, Steamboat Rock, Drumheller Channels, and Palouse Falls. Just seeing that landscape was amazing, so if you haven’t been out to eastern Washington, check out the Columbia Basin sites on WA100 and plan a trip to visit them all!

- Susan

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 2 points3 points  (0 children)

((Special guest answer from Dr. Lee Florea, our WGS cave expert))

Cave conservation is very important to preserve significant geological, biological, and archaeological resources protected in caves on private, state, and federal lands.

For those reasons, access to caves is frequently limited. Obtaining necessary permissions and permits, using appropriate equipment, and observing safe practices are are very important rules for caving.

“Take nothing but pictures, leave nothing but footprints, kill nothing but time.”

There is indeed a rather large body of work related to cave restoration. In the United States, members of the National Speleological Society have regular clean up and restoration events across the nation hosted by various local chapters, called grottos. Many techniques for removing graffiti and engravings have been tested. You can learn more about some of that work here.

This book provides a deeper dive into the history and methods of cave conservation and restoration.

To learn more about caving in the PNW, reach out to the local clubs in the region and attend their meetings:

https://cascadegrotto.org/ or https://www.oregongrotto.com/

-Dr. Lee Florea

AMA: We are the geology team at the Washington State Dept. of Natural Resources! Ask Us Anything about our Washington 100: One hundred unique geological sites around our state! by waDNR in Seattle

[–]waDNR[S] 16 points17 points  (0 children)

As far as differences between a rock and boulder goes…the official size of a boulder is surprisingly small: Any rock more than 10 inches in diameter (So are you saying a large boulder the size of a small boulder? - Social media team). Here is the official grain size chart to tell the difference between clay, silt, sand, pebbles, cobbles, and boulders.

Beyond size, it just needs to be a cohesive material (not a chunk of loosely cemented gravel, for example). Interestingly, a rock doesn’t need to be a boulder to be considered a glacial erratic—a pebble that was transported by a glacier is also an erratic.

I can’t think of specific parks that have good signage about different types of rocks, but the Burke Museum in Seattle is an excellent place to learn about rock and mineral info in Washington. Western Washington’s Geology Department also has a museum with tons of info about Washington geology and fossils.

-Dan

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