Subject: Help! Craters!!

I am doing an A-Level Earth Sciences project in England on the formation of
craters in sand by dropping glass marbles and measuring the diameter
of the craters formed.  I am using a rather simple method to get my
results. 

The marbles are being dropped (from heights of 2.5m to 0.5m) into 5cm
depths of sand.  In the sand with the smallest grains the craters tend
to stay the same size above about 1.2 meters.  Would this be because
the marble has reached it’s terminal velocity or because of the depth
of the sand or something else? 

Any other information which may be useful would also be GREATLY
appreciated!! 

Thanks!  

Re: Help! Craters!!