Union Brick…any ideas?

As I waited for a wedding rental to get set up this morning, I walked around the Patapsco Female Jnstitute grounds to a place a have never been up close. There is a large pile of bricks off to the side in an overgrown area, I am guessing left over from the stabilization project. They are not modern day bricks, but I saw something I have never come across before: a brownish white brick with the word UNION imprinted on its face. A quick web search yielded nothing so I am hoping when I have time to delve a little deeper I will find out more !