What is the function of the description of the "Great Irish Elk" skeleton and the butter?

The Great Irish Elk and butter description serves to illustrate the archaeological value of the bog in terms of Irish identity, culture, and history.
In the poem, the reference to the Great Irish Elk also highlights Heaney's own childhood memory of an elk fossil found in a bog near his hometown. To Heaney, the archaeological find is evidence that the bog is a repository of memories gleaned from centuries of Irish culture and history. The Great Irish Elk is at present time extinct; however, skeletons and fossils of these elks have been found in numerous locations around the world. The elk skeleton serves to underline the vastness and depth of Irish history; centuries upon centuries of Irish history lie in the bogs of Heaney's poem.
Additionally, the preserved butter in the poem ("sunk under/ More than a hundred years/...recovered salty and white") references the social practice of preserving or storing food in peat bogs for extended periods of time. It is remarkable that the hundred-year old butter is so well preserved that it emerges "white" from the bog.
So, Heaney uses the two items to describe how bogs are able to preserve generations of rich Irish history within the depths of their "kind, black butter."