I believe that the accepted crest for the Caldwell Coat of Arms is "A Fountain Throwing up Water" which is relatively unusual. It is obviously a reference to water and the heraldic device for a well is known as a fountain. I also read that a fountain on a crest is supposed to signify that the bearer had achieved their desired position in life.

An English Caldwell Coat of Arms has a cross patee fitche which is a conventional maltese cross with the lower limb sharpened like a stake. It was apparently known as the Cross de Maltho Caldwell. It was the original device of the Knights of St John (1092).
"Azure, a cross patee fitche argent between an orle of eight estoiles or. Crest: a cockerel's head with wings erect or, holding in its beak a cross of the same"
(Not much sign of water or wells there