El Capitan Moraine EarthCache

Welcome to Yosemite National Park

A long, low ridge crosses the floor of Yosemite Valley at this location, quietly recording the retreat of the last valley glacier. Known as the El Capitan moraine, this glacial landform marks a former position of ice and reveals how sediment, meltwater, and changing climate shaped the valley after the Ice Age. This EarthCache explores the processes responsible for the moraine’s formation and explains what this landform reveals about Yosemite Valley’s glacial history.

This EarthCache was created in correspondence with and with permission from the Yosemite National Park Service. Respect and follow all park regulations during your visit. Please help protect Yosemite by picking up your trash, staying on designated trails, and never feeding wildlife.

The Story of the El Capitan Moraine

Yosemite Valley sits atop a deep bedrock basin sculpted primarily by ancient glaciers, and in many locations as much as 2,000 feet of sediment now rests above the underlying rock, filling this depression. Most of the deep excavation occurred during early glaciations nearly one million years ago, when ice was thicker, more extensive, and far more erosive than during later advances. Following this major carving phase, rivers, streams, rockfalls, and smaller glaciers gradually deposited sediment across the valley, creating a broad and gently sloping floor prior to the last major glaciation. This process left a complex mixture of materials of varying sizes and compositions, including sand, gravel, and cobbles, which together record the long history of glacial and fluvial activity in the valley. By the time the Tioga-age glacier arrived, much of the valley was already partially filled with sediment, leaving only a shallow depression in the east where ice could reshape the landscape.

During the Tioga glaciation, ice again flowed through Yosemite Valley, but it was far smaller than earlier glaciers and did not cut down to bedrock in most areas. As the glacier slowed and began to retreat, debris carried within the ice was deposited across the valley floor, forming the El Capitan moraine. This ridge marked a temporary stand of the glacier margin, extending across much of the valley and creating a low but continuous barrier. Its long, narrow form, smoothly convex surface, and cross-valley orientation distinguish it from river deposits, since rivers rarely build ridges perpendicular to their flow. The moraine contains a poorly sorted mixture of sediments, ranging from fine material to gravel, cobbles, and large boulders, and includes rocks transported from far up the valley, providing clear evidence that ice, rather than flowing water, was responsible for their deposition.

As the glacier continued to retreat, meltwater pooled behind the moraine, forming a shallow lake referred to as Lake Yosemite. Rather than a single deep body of water, the lake likely consisted of interconnected ponds, wetlands, and braided channels, similar to landscapes observed in recently deglaciated regions today. The moraine acted as a natural dam, slowing water flow and trapping sediment delivered by the Merced River and its tributaries. Over time, sediment accumulated behind the ridge until it reached the level of the outlet, eventually forming the relatively flat valley floor visible today. Observing the moraine from this location, one can see both the convex shape of the ridge and the wide variety of rock types and sizes it contains, which together reveal the processes of ice transport, deposition, and the shaping of the valley after the Tioga glacier’s retreat.

Segment of the El Capitan moraine.

Map of Lake Yosemite showing the El Capitan moraine as a natural dam.

Types of Glacial Moraines

Glacial moraines form when debris carried by ice is deposited as a glacier advances, pauses, or retreats. Different types of moraines reflect where sediment accumulated relative to the ice.

• Lateral moraine — forms along the sides of glaciers where rockfall and debris collect against valley walls.

• Recessional moraine — develops when a retreating glacier pauses, leaving ridges that record stepwise ice retreat.

• Terminal moraine — marks the farthest point a glacier reaches during its maximum advance.

• Medial moraine — forms when two glaciers merge and their lateral moraines combine along the center of the ice.

• Ground moraine — consists of widespread, gently rolling deposits left behind as ice melts and releases sediment across the landscape.

The El Capitan moraine is best interpreted as a recessional moraine, recording a temporary stillstand during the retreat of the Tioga-age glacier.

Types of glacial moraines.

Tasks for This EarthCache

To log this EarthCache, visit the site and answer the following questions. Send your answers via Geocaching or email. You are NOT answering questions about or taking your photo with El Capitan. All questions refer to the low ridge at the posted coordinates.

1. Include “El Capitan Moraine - GCBJXVJ” on the first line of your message.

2. Look at the ridge in front of you and describe its (a) height, (b) length, (c) shape, and (d) slope. Which features indicate it was formed by glacial ice rather than by a river?

3. Examine the sediment on and around the moraine. What range of particle sizes and rock types do you see? How does this mixture support a glacial origin?

4. Observe the moraine’s alignment relative to the Merced River to the north and the valley as a whole. Based on what you’ve learned and what you see, what type of moraine do you think this is?

5. In your log, attach a photo of yourself or a personal belonging at the posted coordinates with the El Capitan moraine in the background. (Note: photos predating the publication of this EarthCache are not accepted.)

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Works Cited

https://www.yosemite.ca.us/library/matthes/lake_yosemite.html

https://www.internetgeography.net/geotopics/which-landforms-result-from-moving-or-melting-ice/

www.geocaching.com