I recently posted Seamus Heaney's "The Peninsula," which ends with the following stanza:
And drive back home, still with nothing to say
Except that now you will uncode all landscapes
By this: things founded clean on their own shapes,
Water and ground in their extremity.
Seamus Heaney, Door into the Dark (1969).
The idea that certain landscapes seen on certain days end up staying with us, and thereafter serve as a sort of reference point throughout our lives, is one that Derek Mahon has considered as well. The following poem was first published in 1968. Thus, it is not unlikely that Mahon and Heaney were separately writing along similar lines within a year or so of each other.
Thinking of Inis Oirr in Cambridge, Mass.
A dream of limestone in sea-light
Where gulls have placed their perfect prints.
Reflection in that final sky
Shames vision into simple sight;
Into pure sense, experience.
Atlantic leagues away tonight,
Conceived beyond such innocence,
I clutch the memory still, and I
Have measured everything with it since.
Derek Mahon, Collected Poems (1999). The poem first appeared in Mahon's Night-Crossing (1968) under the title "Recalling Aran." Inis Oirr (anglicized as "Inisheer") is one of the Aran Islands.