This cache can be done from your car. It is best to drive towards Salmon la Sac, then find a safe place to turn around and return to GZ. The space for parking here is generous.
There are two types of rock exposed in the roadcut on the east side of the road. These rocks are typical of the the area. The first is the Swauk Formation, a sedimentary formation, mostly sandstone, but with certain localities having finer grained sediments. There are leaf print fossils and coal in the Swauk. This formation is made-up of the detritus of the weathering of the Mt Stuart Batholith which was deposited in a basin which once stretched from Table Mountain west about 60 miles. Geologists date the formation from 54-48 million years ago. Two significant events occured at the end of this time. The second was the rupture of the Straight Creek Fault (not visible at this location) which transported the western half of the Swauk about 50 miles to the north. There it is called the Chukanaut Formation because early geologists did not recognize that the two formations were two parts of the same deposit. This fault is most visible from I-90 traveling east as cliffs of Teanaway Basalt rising above Lake Kachess.
The first event contributed to the roadcut before you. The darker rock is basalt. Specifically: Teanaway Basalt. This is the third largest flood basalt province in Washington, (Largest is Siletzia, or Cresent Basalts, second largest is the Columbia River Basalt Group). Rather than flowing out of a single vent the basalt oozed out through many many fissures visible throughout the Teanaway region. As magma nears the surface it becomes known as "lava" Usually we think of "lava" in the form of a flow, but it can also solidify underground. Lava filling a fissure is referred to as a dike and lava which intrudes between layers of rock is referred to as a "sill" (see my drawing).
It is important to know that the cut exposes a two dimensional image of a 3 dimensional reality.
Congratulations to Kiekefretter for the first to find!
To log this Earthcache please message me the answers to the following questions.
At WP 1
1. What is the length and width of the large light colored rectangular patch in the cut?
2. What is the approximate angle of the long axis from horizontal and which direction does it dip, (north or south)?
3. What is the approximate angle and dip of the contact between the basalt and sandstone?
4. In your opinion is the rectangular block best imagined as attached to the Swauk Formation, or is it completely surrounded by basalt?
At WP 2
5. What is the width of the basalt as indicated in the illustration?
At WP 3
6. Is the basalt seen here part of a dike or a sill, what evidence would you use to support your conclusion?
Where to measure at WP 2 
Further Information:
About the Straight Creek Fault: https://en.wikipedia.org/wiki/Straight_Creek_Fault
Abstract from a paper on the Teanaway Basalt: https://ui.adsabs.harvard.edu/abs/2013AGUFM.V33E2832I/abstract
A Masters Thesis describing the Swauk Formation and the Teanaway Basalt":
https://scholarworks.sjsu.edu/cgi/viewcontent.cgi?article=4709&context=etd_theses
Nick Zentner's video about Lake Kachess and the Straight Creek Fault:
https://www.youtube.com/watch?v=23QDHUfbK74
Nick Zentner's video lecture which connects The Teanaway Basalts and the Challis Magmas:
https://www.youtube.com/watch?v=BQlHamTVAgg