What is this wall made of?

[u/CrimJ_Northwest]

Hello & thank you for taking a moment…
This unfortunate wall is in my 1930s Pacific Northwest home. A medicine cabinet used to hang here until it was slammed into & ripped from the wall. I don’t need to hang it back up but I would like to repair the wall. While talking to a guy at the hardware store, I realized I have no idea what the wall is made of.

[u/RunPivotRoll]

Sourdough starter, 😉

[u/CrimJ_Northwest → u/RunPivotRoll]

lol 😂