多选题
1 The floor of a river valley develops in one of two ways: as a rock-floored valley bottom or as an accumulation valley floor. A rock-floored valley is formed by a stream that no longer incises by cutting downward but mther erodes latemlly in a course that winds from side to side across the valley floor. In a rock-floored valley, the valley slopes are undercut and steepened by the sideways erosion. The floor of the river channel lies in the bedrock, and on either side of the channel it is covered by only a thin layer of gmvel and sand. As the stream swings across the valley floor, it deposits material on the insides of the bends in the channel.
2 The second type of valley bottom, the accumulation floor, cannot easily be distinguished from a rock-floored valley on its surface. An accumulation valley floor is created by the continuous deposition of gmvel and sand in an existing incised valley where the accumulation of material has replaced the cutting action. Both the channel floor and the floodplain--the part of the valley floor flooded frequently at high water--are composed entirely of these gmvel and sand deposits. An accumulation floor is much less resistant to erosion than a rock floor since the gmvel and sand of its channel bed have already been transported and may easily be removed during the next flood.
2. Select the appropriate sentences from the answer choices and match them to the type of valley floor that they describe. TWO of the answer choices will NOT be used. This question is worth 3 points.
A. The river channel flows directly over the bedrock.
B. The top layer of rock is more resistant to erosion than the underlying rock.
C. Deposits of gravel and sand accumulate on the valley floor.
D. The river swings from side to side, leaving material on the insides of bends in the channel.
E. Sand and rock accumulate parallel to the coast but separated from it by a channel.
F. The sideways erosion of the river undercuts and steepens the valley slopes.
G. The channel floor and the floodplain are made entirely of gravel and sand deposits.
VALLEY FLOORS
1 The floor of a river valley develops in one of two ways: as a rock-floored valley bottom or as an accumulation valley floor. A rock-floored valley is formed by a stream that no longer incises by cutting downward but mther erodes latemlly in a course that winds from side to side across the valley floor. In a rock-floored valley, the valley slopes are undercut and steepened by the sideways erosion. The floor of the river channel lies in the bedrock, and on either side of the channel it is covered by only a thin layer of gmvel and sand. As the stream swings across the valley floor, it deposits material on the insides of the bends in the channel.
2 The second type of valley bottom, the accumulation floor, cannot easily be distinguished from a rock-floored valley on its surface. An accumulation valley floor is created by the continuous deposition of gmvel and sand in an existing incised valley where the accumulation of material has replaced the cutting action. Both the channel floor and the floodplain--the part of the valley floor flooded frequently at high water--are composed entirely of these gmvel and sand deposits. An accumulation floor is much less resistant to erosion than a rock floor since the gmvel and sand of its channel bed have already been transported and may easily be removed during the next flood.
2. Select the appropriate sentences from the answer choices and match them to the type of valley floor that they describe. TWO of the answer choices will NOT be used. This question is worth 3 points.
Answer Choices
A. The river channel flows directly over the bedrock.
B. The top layer of rock is more resistant to erosion than the underlying rock.
C. Deposits of gravel and sand accumulate on the valley floor.
D. The river swings from side to side, leaving material on the insides of bends in the channel.
E. Sand and rock accumulate parallel to the coast but separated from it by a channel.
F. The sideways erosion of the river undercuts and steepens the valley slopes.
G. The channel floor and the floodplain are made entirely of gravel and sand deposits.
【正确答案】
A、D、F
【答案解析】Rock floor: Thefloor of the river channel lies in the bedrock...; As the stream swings across the valley floor, it deposits material on the insides of the bends in the channel," In a rock-floored valley, the valley slopes are undercut and steepened by the sideways erosion.
C, G Accumulation floor: An accumulation valley floor is created by the continuous deposition of gravel and sand...; Both the channel floor and the floodplain.., are composed entirely of these gravel and sand deposits. Answers (B) and (E) are not mentioned.