Rare Birthmark

I apparently have a rare birthmark that I havent paid much attention to until recently.  I've tried searching it on the internet and medical books but have yet to identify it. Here is a breif description:

It is located on my middle-to-lower back, just left of my spine.  It is a cluster of small indentations, or holes that have been described to looks like "fork stabs." there is about 6 larger ones, and 10 very small holes. My mother has it also, same location and has always referred to it as a "subdermal birthmark"

Any ideas?!