Recent Ice Dropped Boulders

Icebergs which may have dropped debris, including boulders, sailed to our latitudes in the last little ice age, it wasn't just Frost Fairs on the Thames. 

Iceberg at the western harbour head of Delfshaven on January 2, 1565 during the cold wave in the Little Ice Age - Cornelis Jacobsz van Culemborch 1565 - (The grounded iceberg was measured at more than 6 meters high and almost 70 meters long). https://museumrotterdam.nl/collectie/item/11113
 

So maybe some of those ice dropped boulders on our coasts haven't been there that long.  

As Bing imagines it.