This came out better than expected, photographically. It was an accident of course.
This is garlic mustard or Jack-by-the-hedge, Alliaria petiolata. A common plant, especially on chalk. It smells garlicky when bruised and was widely used in cooking, either as an addition to salads or boiled to make a sauce.
It was introduced to the US in the nineteenth century for this purpose and has become an invasive weed, producing chemicals that suppress the mycorrhizal fungi there (but not here in its native range).
It is also the food plant of orange tip butterflies, the year's first of which I saw a week ago.