Such symbolism does not mean that the actual event was merely allegorical and not also literal.
"The term 'firmament' does not compel us to imagine a stationary heaven: we may understand this name as given to indicate not that it is motionless but that it is solid, and that it constitutes an impassable boundary between the water above and the waters below..."
-St. Augustine, 'on the Literal Interpretation of Genesis'
"But as far as concerns the separation of the waters I am obliged to contest the opinion of certain writers in the Church who, under the shadow of high and sublime conceptions, have launched out into metaphor, and have only seen in the waters a figure to denote spiritual and incorporeal powers. In the higher regions, above the firmament, dwell the better; in the lower regions, earth and matter are the dwelling place of the malignant. So, say they, God is praised by the waters that are above the heaven, that is to say, by the good powers, the purity of whose soul makes them worthy to sing the praises of God. And the waters which are under the heaven represent the wicked spirits, who from their natural height have fallen into the abyss of evil. Turbulent, seditious, agitated by the tumultuous waves of passion, they have received the name of sea, because of the instability and the inconstancy of their movements."
"And God said, let there be a firmament in the midst of the waters, and let it divide the waters from the waters. And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament." Before laying hold of the meaning of Scripture let us try to meet objections from other quarters. We are asked how, if the firmament is a spherical body, as it appears to the eye, its convex circumference can contain the water which flows and circulates in higher regions? What shall we answer? One thing only: because the interior of a body presents a perfect concavity it does not necessarily follow that its exterior surface is spherical and smoothly rounded. Look at the stone vaults of baths, and the structure of buildings of cave form; the dome, which forms the interior, does not prevent the roof from having ordinarily a flat surface. Let these unfortunate men cease, then, from tormenting us and themselves about the impossibility of our retaining water in the higher regions."
"Therefore we read: "Let there be a firmament in the midst of the waters, and let it divide the waters from the waters." I have said what the word firmament in Scripture means. It is not in reality a firm and solid substance which has weight and resistance; this name would otherwise have better suited the earth. But, as the substance of superincumbent bodies is light, without consistency, and cannot be grasped by any one of our senses, it is in comparison with these pure and imperceptible substances that the firmament has received its name. Imagine a place fit to divide the moisture, sending it, if pure and filtered, into higher regions, and making it fall, if it is dense and earthy; to the end that by the gradual withdrawal of the moist particles the same temperature may be preserved from the beginning to the end."
-St. Basil the Great, On the Firmament
"But, boasting as they do that they know these things, let them first tell us about the things of inferior nature; what they think of the body of the heavens, of the machinery which conveys the stars in their eternal courses, or of the sphere in which they move; for, however far speculation may proceed, when it comes to the uncertain and incomprehensible it must stop. For though any one say that another body, like in fashion (to that body of the heavens), fitting to its circular shape, checks its velocity, so that, ever turning in its course, it revolves conformably to that other upon itself, being retained by the force that embraces it from flying off at a tangent, yet how can he assert that these bodies will remain unspent by their constant friction with each other? And how, again, is motion produced in the case of two coeval bodies mutually conformed, when the one remains motionless (for the inner body, one would have thought, being held as in a vice by the motionlessness of that which embraces it, will be quite unable to act); and what is it that maintains the embracing body in its fixedness, so that it remains unshaken and unaffected by the motion of that which fits into it?"
-St. Gregory of Nyssa, Answer to Eunomius's Second Book
“...the vault of heaven prolongs itself so uninterruptedly that it encircles all things with itself, and that the earth and its surroundings are poised in the middle, and that the motion of all the revolving bodies is round this fixed and solid center...
“...on whatever side the sun's rays may fall on some particular point of the globe, if we follow a straight diameter, we shall find shadow upon the opposite point, and so, continuously, at the opposite end of the direct line of the rays shadow moves round that globe, keeping pace with the sun, so that equally in their turn both the upper half and the under half of the earth are in light and darkness...”
-St. Gregory of Nyssa, On the Soul and Resurrection
"Resuming then, let us first lay bare, in speaking of those things according to our power, the imposture of those who boast as though they alone had comprehended from what forms the heaven is arranged, in accordance with the hypothesis of the Chaldeans and Egyptians. For they say that the circumference of the world is likened to the turnings of a well-rounded globe, the earth having a central point. For its outline being spherical, it is necessary, they say, since there are the same distances of the parts, that the earth should be the center of the universe, around which, as being older, the heaven is whirling. For if a circumference is described from the central point, which seems to be a circle, for it is impossible for a circle to be described without a point, and it is impossible for a circle to be without a point,--surely the earth consisted before all, they say, in a state of chaos and disorganization."
-St. Methodius, Banquet of the Ten Virgins