stupa rituals

[Gary M Tartakov]

At Sanchi this winter I noticed some pilgrims, probably Tibetans, doing
things, as they processed around the stupas, that I do not understand.
They went in groups and as they proceeded around some individuals seemed
to be picking out or leaving bits of sand, or I don't know what, from
between stones at iregular intervals.  

I am interested in the fact of going in groups as well as the touching /
picking activity.  I am also interested in the doing of repititive
prostrations that I witnessed here as well as in Bodh Gaya.

Can anyone tell me anything about this or lead me to literature on
rituals associated with stupa pradakshina?  

Gary Tartakov