Welcome to Clear Creek
At this point along Clear Creek, you’ll find a landscape carved by time, water, and fire. Flowing from the western slopes of the Oregon Cascades, Clear Creek has cut through ancient volcanic bedrock and transported sediment downstream for thousands of years, shaping the valley you see today. This EarthCache invites you to explore how volcanic rock, erosion, and sediment transport have worked together to form the Clear Creek Watershed, a geologically rich part of the Clackamas River Basin.
Clear Creek Watershed
The Clear Creek Watershed is part of the larger Clackamas River Basin in Oregon. It starts high in the mountains at over 4,200 feet elevation and flows down to meet the Clackamas River at 79 feet. Most of the watershed lies below 1,500 feet. As the creek moves from higher to lower ground, the landscape and soil change—from volcanic and sedimentary rock in the upper part to softer, alluvial soils downstream. These changes affect how the water flows and shapes the land. Clear Creek’s watershed includes forests, valleys, and floodplains, all connected by the water that drains through this area.
Clear Creek flows through a landscape shaped by ancient volcanic activity linked to Olallie Butte, a shield volcano located to the southeast in the Oregon Cascades. Olallie Butte last erupted over 25,000 years ago and is primarily composed of basaltic andesite, a type of volcanic rock that formed from thick, slow-moving lava. These lava flows spread across the landscape and hardened into dense rock, becoming part of the bedrock that underlies much of the Clear Creek watershed today.
As thousands of years passed, tectonic uplift and erosion gradually exposed and broke down this volcanic bedrock. Gravity, rainfall, and flowing water all worked together to wear away the rock, and Clear Creek began carving through the hardened lava, forming valleys and transporting fragments downstream. The rounded volcanic cobbles you see in the creekbed today are pieces of that once-solid bedrock, shaped and smoothed over time by the creek’s movement.
Volcanic Rock Types
Volcanic rocks form when molten rock, called magma or lava, cools and solidifies at or near the Earth’s surface. The speed of cooling, gas content, and chemical composition all influence how these rocks look and behave. Below are some of the most common volcanic rock types you may encounter:
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Basalt — Dark gray to black, fine-grained volcanic rock formed from low-silica, iron-rich lava that cools quickly at the surface. It often looks dense and uniform, sometimes with small gas bubble holes called vesicles.
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Andesite — Medium gray volcanic rock with intermediate silica content. It commonly shows a fine-grained texture with small visible crystals, giving it a speckled or porphyritic appearance.
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Rhyolite — Light-colored volcanic rock, usually white, pink, or light gray, formed from silica-rich lava. It often appears fine-grained or slightly glassy and may contain small gas cavities.
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Scoria — Dark red, brown, or black volcanic rock filled with large vesicles from escaping gas. It is rough, lightweight, and commonly associated with cinder cones and explosive eruptions.
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Pumice — Very light-colored, extremely porous volcanic rock formed from gas-rich frothy lava. It is so full of tiny holes that it can sometimes float on water and has a rough, chalky texture.
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Obsidian — Natural volcanic glass formed when silica-rich lava cools very rapidly. Usually black with a smooth, glassy surface and sharp edges, it often breaks with curved conchoidal fractures.
Geological Significance
Clear Creek offers an ideal setting to study:
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Volcanic Origins – The type and structure of the bedrock.
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Sediment Movement – How rocks are broken down and transported over time.
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Watershed Dynamics – How water flow shapes the landscape and moves sediment.
Tasks for This EarthCache
To log this EarthCache, visit the site and complete the following tasks. Send me your answers via Geocaching or email.
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Include “Clear Creek Cobbles - Viola - GCB7AEY” on the first line of your message.
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Look down into the creekbed. What sizes of rocks do you notice? Are the rocks mostly angular or rounded? What does this indicate about the distance they have traveled?
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Describe the (a) colors and (b) textures of the rocks. Based on these features, what types of rocks might they be? Where do you think they originally came from?
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Based on your observations, do you think this site is located in the upper, middle, or lower part of the watershed? Explain your reasoning.
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In your log, attach a photo of yourself or a personal item with the Your Clackamas River Watershed: Clear Creek sign in the background. (Note: photos predating the publication of this EarthCache are not accepted.)
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Also Nearby: Original Stash Tribute Plaque
