Multnomah Falls, the nation's second largest year-round waterfall, is located east of Portland, Oregon. The two-tiered waterfall tumbles down a total of 620 feet, cascading over Grande Ronde Basalt, one of the basalt formations in the Columbia River Basalt Group.
The Columbia River Basalt is a collection of massive fissure lava flows which covered quite a bit of Idaho, Washington State, and Oregon. Formally called the Columbia River Basalt Group (CRBG), the flows are divided into five formations - the Saddle Mountains, Wanapum, Grande Ronde, Imnaha, and Picture Gorge Basalts. The majority of the CRBG flows are Early Miocene, dated between 17 and 5.5 million years old, and were erupted from north-south fissures near the present-day Washington-Idaho border. The CRBG consists of approximately 300 thick sequences of flood basalt flows, each flow from 10 to over 100 feet in thickness, with an estimated eruptive volume of at least 700 cubic miles, making them the largest documented individual lava flows on Earth. The flows reached maximum thickness of 16,000 feet in the Pasco Basin, and in the Columbia River Gorge, 21 flows poured through, forming layers of rock up to 2,000 feet thick. 
After the basalt was laid down, the Missoula Floods of the last Ice Age scoured along the cliff, removing looser rock, and leaving the falls to "hang" high above. This has turned Multnomah Falls into one of the best places in the Columbia River Gorge National Scenic Area to study geology exposed by floods. Five separate basalt flows are visible in the fall's cliff face.
Even today, water relentlessly erodes the walls of the Columbia River Gorge. You can see the evidence of this at nearly any waterfall. Look for curving cliffs which form amphitheater-like basins. As water flows into tiny cracks in cliff walls, it freezes and thaws, wedging rocks apart. Over time, large chunks of the cliff come hurtling to the ground below. Falling rocks are a common occurrence at Multnomah Falls. On Labor Day, 1995, a bus-sized piece of basalt fell from behind the waterfall into the pool below, and on January 9, 2014, a small boulder crashed through the Benson Bridge, forcing access to the bridge and upper trails to be closed for several months while repairs were made.
You should be able to spot these three types of basalt here at the falls:
Entablature basalt - relatively fast-cooling lava can fracture into irregular patterns and joints, forming entablature basalt.
Pillow basalt - when basalt flows encounter water, they solidify almost immediately, causing round "bubbles" of various-sized rocks, known as pillow basalt.
Columnar basalt - usually found underneath entablature basalt - columnar basalt forms when lava cools slowly and fractures to form 5- or 6-sided crystals.
**Please note the following so you can better define the five layers of basalt that you see: columnar basalt and the entablature basalt it is found beneath are usually part of the same basalt flow. For instance, the top layer at the lip of the falls is an entablature-and-columnar basalt layer.
To ensure your safety while visiting Multnomah Falls, the following regulations are in effect in the area:
Hiking is allowed only on paved trails.
The base of the Upper Falls beyond the wall is closed.
The trail is closed every day from sunset to sunrise.
Also, please note that while the Multnomah Falls area is open year-round, the short trail you will need to take in order to complete this Earthcache is only maintained from April through November, meaning that if you visit during the off-months, you may find the trails scattered with tree debris from the latest rain- or windstorm, or other debris, so be forewarned and watch your step. In the event of an ice storm or heavy snowfall, or similar event which would make ascending the trail to and beyond the Benson Bridge dangerous in any way, the gate at the beginning of the trail will be closed until the situation has cleared. This means there will actually be times when completing this Earthcache will be impossible, but please try again if you encounter this rare occurrence; the views are spectacular and you won't want to miss it!
To complete this Earthcache, please do the following:
1. Visit the first waypoint and look for the small stone plaque telling you the height of the upper and lower falls, in feet. Remember the number for the upper falls.
2. Proceed to the second waypoint: N 45 34.593, W 122.06.935 (it was very difficult to get this reading due to the cliff. If you can get a better reading, please share in your email along with your answers) This waypoint will be at the first hairpin turn past the Benson Bridge.
3. Look to the right (your right) of the falls and determine to the best of your ability where the boundaries are between the five basalt flows (the layers are more easily defined on the right side).
4. Estimate the thickness of the third (middle) layer of the five, in feet; also estimate the approximate distance from the top of this third basalt layer to the top of the falls, in feet, using the number you got from the first waypoint to help with the scale of the view.
5. Email me these two numbers, as well as what kind or kinds of basalt appear to make up this third layer of basalt in the cliff face. Also, please list the number of people in your group who got to experience this with you. No pictures are required, but feel free to share one if you took one (and why wouldn't you take one?)
Please remember to email your answers when you log your find. If I don't receive an email from you within one week of receiving your log, I'll delete your find.
Multnomah Falls does not require a Northwest Forest Pass.
Traveling Westbound on Interstate 84, you may access Multnomah Falls via the Historic Columbia River Highway (Highway 30) at Exit 35 or you may use the freeway parking lot by taking Exit 31. If you are traveling Eastbound on Interstate 84, you may access Multnomah Falls via the Historic Columbia River Highway (Highway 30) at Exit 28 or you may use the freeway parking lot by taking Exit 31. Please take note, both Eastbound and Westbound exits into the parking area from I-84 are left lane exits, as the parking lot is tucked between the Eastbound and Westbound lanes.
It is recommended that people with disabilities use Exit 28 (traveling Eastbound) or Exit 35 (East- and Westbound) for direct access to the lodge.
: Swanson, D.A., and Wright, T.L., 1981, Guide to Geologic Field Trip Between Lewiston, Idaho, and Kimberly, Oregon, Emphasizing the Columbia River Basalt Group
Frumious Jane is the author of the Caching Out geocaching mystery series, written under the name Morgan C. Talbot.