What the heck is in the Kirkland Signature Organic Milk?

I noticed that all three cartons had dark specks in the bottom of the carton. On the last one, I rinsed out the empty carton with a little water and poured it into the clean glass and looked at the bottom of the glass and saw this. Has anyone else noticed this stuff? What is it?

I noticed that all three cartons had dark specks in the bottom of the carton. On the last one, I rinsed out the empty carton with a little water and poured it into the clean glass and looked at the bottom of the glass and saw this. Has anyone else noticed this stuff? What is it?