"Between her and the port lay the great flat reef"
by cm
Whitby Harbour
Creative Commons AttributionWhitby Harbour - Credit: Chris Robertshaw


The correspondent is probably referring to the rocks due east of the Whitby piers.  According to visitmyharbour.com:

Most directions of approach are clear of danger and straightforward... the exception to this is from the East. When approaching from that direction a good offing is necessary to clear Whitby Rock and a shoal patch that extends outwards from the Abbey Cliffs just to the east of the harbour entrance.


Google Map