If roots won't penetrate it then neither will water so the soil is likely to stay very wet, not a good growing medium for any plants! Is this the remains of an old tarmac road and are you able to chop it out?

Hi, welcome to GoY, if you were to enlarge the area you've dug out, you may find that you have a large stone, I would use a mattock to dig around the sides of it, or maybe a sledge hammer to break it up, as Bulbaholic says, it's not good for plants, Derek.