Sand origin and its movement - continued
The sand, thrown onto the beach by the waves, dries and is driven by the wind to the coastal plain. The coastline by El-Arish is in an east-west direction (Fig. 4.1.12) and one can see that the dunes set along the coastline are perpendicular to the direction of the prevalent wind. They have a moderately slanted plain on their west side and a steep slope on their east side. The sand contains grains of varying dimensions, so in order to simplify the discussion we will use Fig. 4.1.13 to learn the names of the grain sections. Silt and clay particles are driven by the wind as "suspension" dust (Fig. 4.1.14). On days when the air is filled with these tiny particles the weather forecast warns us of hazy weather. The grains of fine sand move in saltation (Fig. 4.1.14). The wind velocity that causes this movement is sufficient to raise sand grains into the air layer close to the surface of the soil; when a grain falls after moving a few centimeters further, it moves a grain of a similar size and weight. Influenced by the movement energy transferred from one grain to the other, the affected grain rises up, in turn, and is moved a few centimeters by the wind.
Fig. 4.1.15 was photographed during wind blowing in the direction of the red arrow. The layer of fine sand moving in saltation looks like a light layer out of focus (Figs. 4.1.15, 4.1.16). The sand keeps moving, even beyond the slip-face of the dune, marked with a green arrow. Large particles that move a short distance on the soil surface as a result of being hit by fine grains, are regarded as "creeping" (Fig. 4.1.14). Sand that accumulates on the slip-face is held by the friction forces between the grains, whereas the gravitational force pulls the grains downwards. The result is the formation of an inclined plain influenced by the two forces. In many sandy areas the average angle between the inclined plain and the horizon is 33°. When the friction force cannot hold the accumulating sand, sliding starts from point 1 (in Fig. 4.1.16) towards point 2.
The entire dune proceeds to move in this manner one centimeter from left to right. The small dune in Fig. 4.1.17 was wet by rain; slipping started in the slip-face. The darker, wet layer tears away, exposing the dry, white sand beneath it. The newly arriving mobile sand is devoid of plants and often covers already vegetated landscape (Fig. 4.1.18). This feature of light sand "tongues' is discernable on a national scale when seen in a satellite image (Fig. 4.1.19).