glaciers moving across a region underlain by incompetent bedrock or unconsolidated materials tend to churn and
pluck these materials, incorporating clay- to boulder-sized particles into the glacier itself. Excavation of the
substrate results in the formation of depressions that may later be filled with water, creating ponds and lakes.
(2) Depositional Features of Continentally Glaciated Regions. As the temperature rises, the ice
begins to melt and the material contained within a glacier is either deposited in place as an unstratified glacial till
or carried away by meltwaters and deposited in another location as a sorted, stratified glacial outwash. Figure 2-
19 depicts the major depositional features associated with continental glaciers.
Meltwaters flowing on the surface of a glacier during periods of increased temperature eventually percolate
downward through cracks in the ice to form streams. These streams, in turn, carve tunnels into the ice near the
base of the glacier. Water emerging from the tunnel at the terminal edge of a glacier is generally choked with
coarse-grained sediment; consequently, braided streams are common. In addition, during flood stages, fine-
grained overbank deposits may be laid down, forming a broad, flat outwash plain. If the flow of water from a
glacial tunnel is restricted by topography, a marginal lake may form. Often, deltas are deposited where glacial
streams enter a marginal lake. As melting continues, large chunks of ice may break away from the main glacier,
forming ice blocks when the detached ice occurs on land and icebergs when it occurs in marginal lakes. With
further glacial melting, the ice sheet begins to retreat and additional depositional features become visible. Ground
moraine is glacial till that has been spread evenly across the ground surface where the ice once flowed. Drumlins,
which periodically interrupt the ground moraine, are asymmetrical, streamlined hills of gravel till deposited at the
base of a glacier and oriented in a direction parallel to ice flow. Interlobate moraine is similar to the medial
moraine discussed in Lesson 2.B.3.a.(2), page 2-23. Likewise, terminal moraine deposited by continental
glaciation is similar to that deposited by alpine glaciation (see Lesson 2.B.3.a.(2), page 2-23). When there is a
pause in the rate of glacial retreat, the terminal edge of an ice mass remained stationary for a period of time,
allowing for the accumulation of recessional moraine. It is similar to terminal moraine except that it is fairly
discontinuous and usually contains less sediment. Eskers, as previously mentioned, are long sinuous ridges of
sand and gravel that mark the former pathways of ice tunnels. Once a glacier completely retreats from an area, a
water source is no longer available to supply a marginal lake; consequently, evaporation may expose a lake
bottom that is composed of fine-grained silts and clays. The delta that formed along the edge of the marginal lake
may also be left high and dry, resulting in the formation of an elevated region called a delta kame. Finally, ice
blocks that have been either partially or completely buried in the drift of the outwash plain may melt, forming
depressions known as kettles. Figure 2-20 shows several depositional features associated with continental
glaciation.
4. Gravity and Mass Wasting. Objects on the earth's surface tend to be attracted toward the center of the earth
by a natural force called gravity. Gravity is the primary force responsible for mass wasting, which consists of the
downslope movement and subsequent deposition of unstable material. Other factors that may contribute to the
mass movement of earth materials include steep slopes, the lack of vegetation, the addition of water, the increased
weight near the upper portion of a slope, the removal of material from the toe of a slope, and seismic activity. In
addition, the type of material composing a slope greatly influences that slope's tendency toward failure. For
example, cohesive sediments are generally stable and
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