Notice: "Cache seekers assume all risks and responsibilities involved in seeking this earthcache." “Just because it is there, doesn’t mean you have to seek the cache!” Don’t let earthcaching control you, but rather you control earthcaching.
Note: The formation is located 8 miles south of Ellensburg, Washington on Interstate 82 south at the Manastash Vista point. All observation may be completed at view point. Even though the side of the formation is stable and to help preserve the structure, please don't climb.
Manastash Ridge Formation
Lava Pillows within the Manastash Ridge Formation
Bottom Layer (see diagram)
15 million years ago lava flowed west from Idaho out of great fissures or vents to the Pacific Ocean as part of the Columbia River Basalt Group. These flows laid down basalt 13,000 feet thick. Basalt is made up of two minerals: plagioclase feldspar and pyroxene. This basalt is aphanitic which means the crystals are very fine grained and invisible to the eye. Some of this basalt has holes or a porphyritic texture. These holes were created from escaping gas like a soda pop as the lava slowly cooled.
Middle Layer (see diagram)
The horizontal arkosic sandstone cross beds were laid down next. An ancient river flowed over the basalt from north to south bringing in sand. These sands layers are horizontal now, but in the past, the river ran down hill. These sandstone layers contain similar sized quartz, feldspar, biotite and hornblende. The inline layered pebbles were created from a high energy short event.
Upper Layer (see diagram)
As the basalt lava flowed once again across this area and over the sandstone, the lava came into contact with water to create lava pillows. These pillows can be seen directly above the sandstone layer and is the product of subaqueous extrusion. Example of this extrusion is lava flowing into the ocean from the Hawaii Kilauea Volcano. The yellow green substance around the pillows is the breaking down of obsidian call plogogornite.
From Horizontal to Anticline
Lastly, the Manastash Ridge was created by the squeezing upward from its original horizontal plane to an anticline. The Sierra Nevada block/plate pushes northwards against the stationary Canadian Coast Mountains Buttress and the ridge had nowhere to go except up.
To log this Earthcache, please send a private message to me with the answers to the following questions.
1. How did the formation become incline from its original horizontal plane?
2. What created the lava pillows?
3. What high energy short event do you think created the inline layered of pebbles?
4. Discribe the sand found in the sandstone layer?
Optional: You may upload a photo to the page of any local wildlife, unique vegetation or geology in the area.
If you would like to learn more about Manastash Ridge Formation and its geology view:
'Nick On The Fly' #20 - Manastash Ridge Field Trip