many many thanksA rockery is a type of feature found in a garden where plants are planted amongst rocks. long since = long ago.
A rockery had been started long ago (the rocks had been placed there) but had not been finished (the work of arranging the stones and planting had not been completed).
suresobhan, could you try to give full information about your source? Like this: The Girl on the Train. A novel by Paula Hawkins. Notice the use of capitals in the title and at the beginning of every sentence in my post. I also end my sentences with punctuation (. ! ?). Please do the same.
A rockery long since started - it has been a long time since somebody started to build the rockery. The work of making the rockery has not been completed.
(Cross-posted with Glasguensis, with whom I agree.)