Mendenhall Glacier is a stunning natural wonder located in the Tongass National Forest near Juneau, Alaska. Stretching approximately 12 miles from its source in the Juneau Icefield, this magnificent glacier showcases the raw power and beauty of nature. Its ice face, characterized by shades of blue and deep crevasses, attracts visitors from around the world who come to witness the breathtaking sight. Over the years, Mendenhall Glacier has been retreating due to climate change, serving as a stark reminder of the environmental challenges facing our planet. This iconic glacier not only provides valuable insights into Earth's geological history but also serves as an important symbol of the urgent need for conservation efforts.
Glacial landforms provide a fascinating insight into the powerful forces at play during the movement of glaciers. Among these features, glacial striations are prominent and easily recognizable. These are the result of rocks and sediments embedded in the glacier's ice grinding against the bedrock, leaving grooves and scratches in their wake. Glacial striations offer crucial information about the direction and strength of ancient glaciers, aiding researchers in reconstructing past glacial movements. While glacial striations are often linear and uniform, chatter marks provide a more intricate story. These smaller, irregular depressions and gouges on bedrock surfaces beneath glaciers emerge from the repeated bouncing and impacts of rocks trapped in the glacier's ice. Chatter marks vary in shape and orientation, reflecting the chaotic nature of glacier dynamics. They help us understand the complexity of glacier movement, including its intermittent surges and variations in velocity. Moving on, lunate fractures are distinctive features characterized by crescent-shaped fractures in bedrock. These fractures radiate outward from a central point and are formed under the immense pressure exerted by advancing glaciers. Lunate fractures are significant because they reveal the direction of glacier movement and the forces that glaciers exert on the landscape. In regions with substantial lateral glacier motion, such as valleys, we often encounter crescentic gouges. These polished, curved grooves result from the rotational movement of glacier ice. They can be classified based on their size, depth, and orientation, providing insights into the glacier's past behavior. Crescentic gouges tell a tale of glaciers carving their way through valleys and shaping the terrain over time.
Finally, crescentic cracks are another outcome of glacier-induced stress on bedrock. These curved fractures mirror the bending and flexing of rock layers due to glacier movement. Like crescentic gouges, they can offer clues about the glacier's activity and its impact on the surrounding geology. By studying crescentic cracks, geologists gain a deeper understanding of the mechanical interactions between glaciers and bedrock. In summary, these glacial features are essential in deciphering the history and dynamics of glaciers. They provide a valuable record of past ice movements, helping scientists reconstruct ancient landscapes and track changes in glacial behavior over time.
Logging Requirements:
- Describe the glacial striations visible in front of you. Do the striations run parallel to each other, or do some of them intersect? What compass direction are they oriented in?
- Using the image and description on the cache page as a guide, are you able to identify any other geologic features on the exposed bedrock in front of you?
- Proceed about 20 feet down the trail where you will find a sign titled, 'A Waterfall that Moved'. How is the shape of the hollows in the rock described, and what was it eroded by? Looking at the spot where the waterfall used to be, how does its orientation compare to that of the striations?
- Upload a photo taken with either the glacial striations or the Mendenhall Glacier in the background. You don't have to be in the photo though it is strongly encouraged.