This is an EarthCache—there is no physical container at the site. Read the geology lesson below, observe, and Send Answers to record your “Found It” 😊
A Discover Pass or day-use fee is required to access the site. Parking is available at the Deep Lake.
⚠️ Safety & Ethics
- Stay on marked trails; cliff edges are unguarded.
- Supervise children and pets.
- Do not remove rocks, plants, or artifacts.
- Bring water in summer—temperatures often exceed 95°F (35°C).
- Respect Leave No Trace principles.
📸 Logging Tasks
To claim this EarthCache 
, you must first visit the posted coordinates and then Send Answers via the geocaching.com Message Center to the following questions based on your observations and the information provided in the short Earthcache Lesson below:
- Layer Inventory – From the coordinates, how many distinct basalt flow layers can you see in the cliffs around the lake? Describe the shape of the columns and how wide they appear.
- Plunge Pool Evidence – Name two specific features (shape of the lake, cliff walls, bottom, lip, etc.) that prove Deep Lake is a plunge pool.
- Vertical Relief – Estimate the height from the lake’s surface to the top of the cliffs. What does this suggest about the power of the flood that formed it?
- Optional: Include a photo of you, your GPS, or a personal item with the lake or cliffs in the background (no faces required).
Earthcache Lesson: Deep Lake sits in a massive, amphitheater-shaped basin, silent today but once filled with thunderous water. This was not an ordinary lakebed: it was blasted into solid rock by catastrophic Ice Age floods—specifically, the Missoula Floods—between 15,000 and 13,000 years ago. In their wake, they left behind this stunning landform, a plunge pool, created where a waterfall many times taller than Niagara once crashed down into basalt bedrock.
But this drama unfolded atop much older rock: the cliffs around you were formed over 6 to 17 million years ago by repeated lava flows that built the Columbia River Basalt Group (CRBG). These volcanic layers provide the resistant rock canvas that the Ice Age floods reshaped with violent force.
1. Columbia River Basalt Group – The Rock Beneath Your Feet
The cliffs surrounding Deep Lake are made of basalt, a dark volcanic rock. Between 6 to 17 million years ago, enormous lava flows erupted from fissures in eastern Oregon and Washington. These flows spread out across the Columbia Plateau in sheets, one atop the other, forming the Columbia River Basalt Group (CRBG).
As the lava cooled, it cracked into vertical columns, usually 5 to 8 sides—these are called columnar joints and are clearly visible before you and in the cliffs west of the lake. You may see several distinct layers, each representing a separate lava flow. Some layers have sharp, blocky ledges; others look more broken and rubbly, especially where cooling happened quickly or where groundwater interacted with the hot lava. These rubbly zones mark the tops of older flows.
2. Ice Age Megafloods – When the Ice Gave Way
At the end of the last Ice Age, a glacier blocked the Clark Fork River in Montana, creating Glacial Lake Missoula, which held more water than Lakes Erie and Ontario combined. When the ice dam broke, walls of water surged across eastern Washington. These floods happened not just once but dozens of times. Each flood carved massive features like the Grand Coulee, Dry Falls, and plunge pools such as the one now filled by Deep Lake.
The floods stripped away soil and even fractured bedrock, creating sheer cliffs and scoured lake basins. Deep Lake sits right in the path of these floods, in a spot where water poured over a cliff and plunged 120 meters into the basalt below.
3. Plunge Pool Formation – The Power of Falling Water
As the floodwater spilled over the edge of the coulee, it crashed down in a temporary waterfall onto the basalt floor. The impact eroded a deep, wide plunge pool—a semicircular pit carved out by hydraulic plucking, where water pressure breaks rock loose, and cavitation, where air bubbles violently collapse and shatter rock.
Plunge pools typically have:
- Steep walls on three sides
- A flat bottom where erosion was deepest
- An abrupt downstream edge
- Rock debris (talus) sloping away from the cliffs
These features are clearly visible from both the overlook and the lake trail. When the flooding stopped, water from precipitation and groundwater filled the basin—creating Deep Lake.
4. Basalt Durability – Why the Cliffs Still Stand
Basalt is a dense, fine-grained volcanic rock—usually gray to black in color—and resists erosion. When you examine broken basalt pieces before you, you’ll notice how hard and angular they are. That durability, combined with minimal rainfall here east of the Cascades, is why these cliffs remain tall and vertical.
You’ll also notice that the rocks don’t show signs of being rounded like river pebbles—this tells us they haven’t been tumbled far. Instead, they’ve broken off nearby and simply fallen in place.
References & Further Reading
- Washington Geological Survey. “The Missoula Floods.” Information Circular 114 (PDF). dnr.wa.gov
- U.S. Geological Survey. “Columbia River Basalt Group—Volcano Hazards Program.” usgs.gov
- U.S. Geological Survey. “Columbia River Basalt Stratigraphy in the Pacific Northwest.” usgs.gov
- Ice Age Floods Institute. “Washington’s Channeled Scablands Guide.” iafi.org
- National Park Service. “Sun Lakes–Dry Falls State Park” & “Ice Age Floods National Geologic Trail.” nps.govnps.gov
- Washington State Parks. “Sun Lakes–Dry Falls State Park History.” parks.wa.gov
- O’Connor, Baker et al. 2020. “Outburst Flood Hydrology of Glacial Lake Missoula.” (PDF). dlgeorge.github.io
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1. This geocache has an approved Permit to be placed at this location on property managed by the Washington State Parks and Recreation Commission. Visitors are responsible for acquainting themselves with policies and rules pertaining to State Parks areas.
2. The following items may not be placed in the geocache: food, illegal substances, medications, personal hygiene products, pornographic materials, hazardous materials, or weapons of any type.
3. By searching for the cache, visitors agree that they are responsible for their own actions, and acknowledge that neither the State of Washington nor the cache owner is responsible for any loss or injury that may occur in relation to such search.
4. Report any incident, problem, or violation to State Parks staff.