Here the term tzur, rock, is a casus pendens, a syntactical structure whereby the term the beginning of the sentence is syntactically independent of sentences that follow. Tzur stands alone at the beginning of this verse, its inherent qualities conveying the meaning of the term: strong, solid, unalterable, weighty. Tzur in the Song of Moses can only be God (though later, in verse 31, it is also used of other gods).
Tzur as a name for God is undoubtedly familiar to us from our liturgy. The phrase Tzur Yisrael is most recognisable from the paragraph immediately preceding the Amidah and Ma’oz Tzur, Rock of Ages, is the most famous of all the Chanucah hymns. The penultimate line of the Amidah, which is taken from Psalms 19:15, ends in Adonai, Tzuri V’goali, Eternal One, My Rock and My Redeemer. It is a name of God that passes our lips regularly, but how often do we stop to consider what it might mean?
The Rock is not an intimate name, like Beloved or Father. It is not a hierarchal name, like King or Lord of Hosts. It is neither fanciful nor imaginative. It is, like the rock itself, heavy, robust, substantial, even unyielding. The Rock is not emotive, quick to anger and jealous or even merciful and loving. The Rock, as the verse goes on to describe, is perfect, just, faithful, upright and true. This is God as immutable.
And curiously it is this image of God that found its way to the Israeli Declaration of Independence, in an historical fudge. Tzur Yisrael was deemed religious enough for the religious party, secular enough for the socialist Mapam and politically expedient enough for Ben Gurion to get the declaration signed before Shabbat. And most ambiguously of all, in English translation, it is rendered Strength of Israel.