A Decent Exposure 🌎 EarthCache

For purposes of logging this cache you must answer the following four questions:
1) Estimate how high is the line of angular unconformity located up the bank at WP1.
2) Using the interpretive sign at WP1, what are the names of the two plates that collided resulting in the formation of the Olympic Peninsula?
3) Also at WP1: Are the rocks pitted with holes caused by the Piddock clams more concentrated to the north or south of the end of the boardwalk?
4) After observing the chevron folds at WP2 would you say the anticlines or the synclines are more developed and prominent?






This earthcache is located within Olympic National Park and is placed with permission and a permit.

Note: An earthcache is the magical combination of geocaching and geological discovery. They vary greatly from traditional geocaches in that there is NO PHYSICAL CONTAINER! Once at the Earth Cache site, take in the scenery, the geological marvels or oddities, and read the cache description. You will likely be asked to answer some questions about the site and optionally take a picture. You can email the cache owner or use the Message Center to send answers to the required questions. Please don’t be intimidated about answering the questions. They are not graded and were meant to enrich your visit, open your eyes and provide you with new knowledge and insights about the location. Most importantly have fun!


Beach 4, north of Kalaloch on the wild coast of Olympic National Park, rewards the geologically-minded with some outstanding geology. With a name like beach 4 you might think it mundane. However, you will quickly change your mind after reaching the bottom of the stairs and witnessing an area rich in earth science. On the other hand there is an element of boring located there as well but we will get into that later on in the lesson!

Beach 4 provides textbook examples of folded rocks and overturned sedimentary rock layers. The well-exposed, tightly-folded rocks are very unusual for western Washington. Please note this geology lesson is tide dependent! You should check the tide tables before attempting this earthcache location. tide charts here:

Take the main trail downhill to the beach. WP1, or the posted coords is the outcrop just north of the foot of the Beach 4 trail and the interpretive sign at the bottom of the stairs. After going down to the beach look to where the vertical layers meet the horizontal ones. Whoa, what happened here?! Here is a fine example of an angular unconformity. Angular unconformity is an unconformity where horizontally parallel sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. . The steeply tilted sedimentary rock layers here rising out of the sandy beach were originally deposited as close-set horizontally stratified layers of sediment on the seafloor. The grains in the sediment were compressed and cemented together, by pressure within the crust and slowly became rock. Later, the layers were compressed, folded, and in many cases fractured to bits as they were subducted under North America. The rocks were then uplifted to sea level, where wave erosion could plane them off. The eroded rocks were later covered by flat-lying young sedimentary beds. Herein lies a lot of geology! Paleontologists have noticed that the few fossils that have been found in these rocks indicate that they were deposited in a deep ocean basin somewhere west of our present-day coastline.

Also of interest are the rocks nearby pitted with shallow holes. Notice how high they go. Do they appear to be above the high tide line? They are the remains of once-deeper burrows of the rock boring clam known as the Piddock clam. (I told you it might get boring later on!) When the clams are young and small, they burrow into softer rocks rather than the mud on the seafloor, as most clams do. The clam attaches its fleshy foot to the rock and pivots the hard edges of its shell back and forth to grind away the rock grain by grain. The resulting pit becomes deeper and deeper, as the clam grows, the cavity widens within the rock, eventually trapping the clam. These clams either live in the lower part of the intertidal zone or entirely subtidal. If you noticed some of the holes were above the high tide line you are right! These borings above sea level are further evidence for the uplift and erosion of these rocks.

Now proceed to WP2. The base of the bluff, where it meets the top of the beach, has a wonderful sequence of several chevron-shaped folds that look like zig zags. Formations like this are caused by the intense uplift and folding pressures exerted by one plate subducting beneath another. Anticlines and synclines are the most common up-and-down folds that result from compression. An anticline has a ∩-shape, with the oldest rocks in the center of the fold. A syncline is a U-shape, with the youngest rocks in the center of the fold.