The answer to 18 is simply the wind. The wind will blow a light material such as the sand quite easily, so the dunes size and shape will constantly be different.
19 Is due to the lake and tectonic plates respectively. Underground rocks are usually specific and variant because the magma and crushing of tectonic plates that form them can be very random at times. The lakes and overall waterways will cause many pebbles and rocks to collide (assuming their is running water, not a still water), cause the rocks to scrape each other many times until the surfaces are fine and rounded.
#19 due to weathering and erosion the rock from the stream was smoothened and without as much the rock from deep underground was/is very rugged and natural.
#18 Due to wind patterns the sand dunes are blew around and the sands shift and fly around with nothing to keep the sand in place the sand can go long distances or come long distances.
