sand dunes are explained to be underwater sand dunes.
I've challenged many creationists on this board to do this very simple experiment, and to report the results back to the EvC community:
Get a glass pie plate, a protractor, and some sand. Make a pile of sand in the plate, and measure the maximum angle of the cone that dry sand can make.
Repeat the experiment, but making a cone os sand under water, as it would necessarily be in a Global Flood.
Compare angles in in-the-air and in-the-water cases. Look at buried dunes in geology, say, in the Coconino Sandstone in the Grand Canyon. Report those angles back to us, too.
The reality is, had you not have any preconceived notions on how chalk forms, this particular explanation certainly wouldn't jump at you when you would look at chalk for the first time.
T H Huxley put the lie to that before my grandfather was born. Read "On a Piece of Chalk" here:
http://aleph0.clarku.edu/huxley/CE8/Chalk.html