I saw Mary in bath tubs last Sept when I went through Kentucky Indiana and Iowa. The bath tubs were standing up and sunk into the ground about halfway to form a little grotto like the clam shell you picture. I had never seen those before.
In NC we mostly see very large gaudily painted statues (Mary or Jesus) in the front yards of Mexicans. There is one I pass by on the way to the grocery store that is Jesus. He is painted only in gold and white.