For photo comparison, here's the link to Lepidium virginicum:
Virginia Peppergrass (Lepidium virginicum) which doesn't quite look like a match to me but my eyes are sooo Not good at comparing photo's.
Leftwood's suggestion of Thlaspi arvense ("Field Pennycress") sure looks more like a match to my eyes but then again, my eyes are getting old.
http://www.google.com/search?q...
If it turns out to be Thlaspi arvense the ATP database sure could use photo's!:
Field Pennycress (Thlaspi arvense)