Glacial Erosion features - Glacial Valleys Objectives: To understand the formation of the following glacial erosion features: U – shaped valleys Hanging valleys To know the terms: truncated spurs, alluvial fan
Types of Glaciers – Valley Glacier Tongas National Forest, Alaska
<ul><li>Glaciers most commonly flow along pre-existing river valleys. </li></ul><ul><li>They carve relatively straight courses </li></ul><ul><li>Their strength enables them to form the following features: </li></ul><ul><ul><li>Glacial troughs </li></ul></ul><ul><ul><li>Truncated spurs </li></ul></ul><ul><ul><li>Hanging valleys </li></ul></ul><ul><ul><li>Ribbon lakes </li></ul></ul>
U shaped Valleys
U-Shaped Valley in Tracy Wilderness, Southeastern Alaska
U-shaped valley formation
U shaped Valley <ul><li>When a glacier erodes its valley , a classic U shape is formed. </li></ul><ul><li>U shaped valleys often start life as river valleys that existed before glaciation occurred. </li></ul><ul><li>The glaciers then followed the existing V shaped valleys, eroding the interlocking spurs, straightening the valley sides and deepening the valley floors as the ice moved. </li></ul><ul><li>U shaped valleys are also known as glacial troughs. </li></ul><ul><li>The flat floor is roughly shaped by the ice which tends to cut down more evenly than flowing water. A thick layer of glacial debris (ground moraine) is deposited as the ice retreats, creating a well drained and fertile soil </li></ul>
U-shaped valley (trough) The Nant Ffrancon Valley The once u-shaped valley has been distorted by moraine deposits and a collection of scree from weathering and mass movement.
U shaped Valley Glacial U shaped valley with steep sides, flat floor, and truncated spurs (middle left)
Langdale – a U-shaped valley Steep valley sides Flat valley floor
<ul><li>What is this feature? </li></ul><ul><li>Look at how this valley appears to be suspended above the valley below </li></ul><ul><li>It is a hanging valley </li></ul><ul><li>How is it created? </li></ul>
Hanging Valley A HANGING VALLEY is a glacial valley that runs above and perpendicular to another glacial valley
Receding tributary glacier Medial moraines Former height of glacier Main valley glacier Hanging valley Truncated spur U shaped valley ‘ Misfit’ stream Waterfall Corrie Pyramid peak Truncated spur
On an O.S. map
Can you identify a corrie, pyramid peak, hanging valley, terminal moraine, corrie lip, arete, glacial trough? In summary….
Glacial Landscapes <ul><li>1 is a Pyramidal Peak because it has steep, triangular faces divided by sharp ridges or arêtes. </li></ul><ul><li>2 is an Arête , because it is a sharp ridge between corries. </li></ul><ul><li>3 is a Corrie or Cirque , because it is an armchair shaped hollow with steep back and sides. </li></ul><ul><li>4 is a Corrie Lochan or Tarn , because water has gathered in the hollow in the floor of the corrie. </li></ul><ul><li>5 is an Alluvial Fan , because it is a fan shaped pile of rock remains (alluvium) washed down by the stream and piled up where the steep valley side meets the valley floor. </li></ul><ul><li>6 is a Ribbon Lake , because it is a long narrow lake in a part of the valley cut deeper by the glacier. </li></ul><ul><li>7 is a Truncated Spur , because the ridge has been cut off sharply by the ice that flowed down the main valley. </li></ul><ul><li>8 is a Misfit Stream , because it is far too small to have cut the valley. </li></ul><ul><li>9 is a Hanging Valley , because the valley floor is much higher than the floor of the main valley. </li></ul><ul><li>10 is a 'U' Shaped Valley , because it has steep sides and a nearly flat floor. (The other side of the valley is missing in this cut-away diagram. </li></ul>