Another NYC trivia fact is that the FDR drive around Bellevue and NYU (20th-34th st iirc) is built on the rubble of Bristol, England. There may be another section up around MSK/Cornell as well but I don't recall.
The ships returning from England would load up with rubble to use as ballast.
I'm not sure of a legitimate record, but searching for "Bristol Basin" suggests that it was supplies/weapons to assist with the German attacks on Bristol, England.
Edit to include text from the plaque commemorating the basin:
BENEATH THIS EAST RIVER
DRIVE OF THE CITY OF NEW YORK
LIE STONES, BRICKS AND RUBBLE
FROM THE BOMBED CITY OF BRISTOL
IN ENGLAND... BROUGHT HERE IN
BALLAST FROM OVERSEAS, THESE
FRAGMENTS THAT WERE ONCE HOMES
SHALL TESTIFY WHILE MEN LOVE
FREEDOM TO THE RESOLUTION AND
FORTITUDE OF THE PEOPLE OF BRITAIN.
THE SAW THEIR HOME STRUCK DOWN
WITHOUT WARNING.... IT WAS NOT
THEIR WALLS BUT THEIR VALOR
THAT KEPT THEM FREE ......
And broad-based under all
Is planted England's oaken-hearted mood,
As rich in fortitude
As e'er went worldward from the island-wall.
ERECTED BY THE
ENGLISH-SPEAKING UNION OF THE UNITED STATES
* 1942 *
Raw materials. Wool, tobacco, furs etc. The economy of the British Empire was based on extracting raw materials from the colonies and manufacturing finished goods in England.
The ships returning from England would load up with rubble to use as ballast.