From my memory of Luton that hill of chalk was formed when Vauxhall extended their works, late 1950s?. The top plateau is chalk. The middle plateau has an impermeable membrane and crushed concrete (recycled material) over it.
Going back to the 1968 time when the apron next to Autair was built a lagoon on the chalk hill was used to store nasty liquids in. The lagoon burst and the 'liquid' flowed down the hill into Spitalsea Lane and down onto Kimpton Road /Vauxhall Way junction. When the thousands of gallons of nasty sludge had been sucked up the road had disappeared. The junction was closed for several days.