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Mapping Geomorphological Environments Phần 5

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Các bản đồ địa chất thường được vẽ chồng lên trên bản đồ địa hình (và các bản đồ nền khác) cùng với việc tô màu và các kí hiệu khác để thể hiện các kiểu đơn vị địa chất khác nhau. | Mapping Geomorphological Environments between 5-12 while the lee side has a higher inclination which can reach up to 20 to 30 . Regarding their shape it also has high variety from very simple to composite forms. Thus there are crescent longitudinal matterhorn dunes and also dunes which are created by very complicated combinations. Dunes can develop in every subaerial environment where loose material of sand grain size are exposed to the wind s activity and can easily migrate and accumulate in large masses. Every obstacle on the ground such as protrusions ditches the presence of vegetation etc contribute to this accumulation. Furthermore the presence of humidity may stabilize the sand accumulation beginning the creation of a dune. It is a fact that the coastal dunes cover a very small area compared to the vast areas of dunes which are generated in the deserts. Coastal dunes are generated where a broad sandy coast exists this is usually characterised by a great tidal width and rarely by rocky and steep coasts. Many sand dunes originate from older geological eras when sea level was much lower than it is today. The sand is transported towards the land in a bouncing way during which the movement of grains is accelerated after they are raised by the wind. The wind speed is slower close to the ground and faster as we rise since the ground s friction is reduced. When a cloud of sand passes over an uneven surface its speed is reduced due to the loss of energy. This results to the deposition of transported material and leads in the creation of a new dune. The development and stabilisation of a dune depend on vegetation which entraps the sand and reduces the activity of aeolian energy. After the development and stabilisation of the dune its migration towards the land s interior may possibly follow as its seaward side might be eroded depositing its material along the shoreline. When the Aeolian energy exceeds the cohesion of the sand layer or the resistance of vegetation large