We've discovered a similar situation at daughter's new, to her, garden.
Some bright spark has dumped a layer of tarmac, luckily thin & breaking up, underneath what had previously been a bed in front of where we know a greenhouse had stood.
So the job for this season/yr is to try & remove as much as possible & eventually will need a small skip- joy- as lots of other rubble/rubbish is coming to light as we explore.
Oh well, planters this yr for her veg/salad crops & hopefully by the Autumn we can have achieved some clearance. With my dodgy knee, back & arm & daughter's back this could take a long time.....
Good luck. J.