Bark of rivers and roof of the wave

Is
Is byþ oferceald, ungemetum slidor,
glisnaþ glæshluttur gimmum gelicust,
flor forste geworuht, fæger ansyne.

These lines come from one of the old rune poems, in which descriptive verses were associated with letters of the runic alphabet.  They refer to the rune Isa, which means 'ice', and can be translated from the Anglo-Saxon as follows

Ice is very cold and immeasurably slippery;
it glistens as clear as glass and most like to gems;
it is a floor wrought by the frost, fair to look upon.

That second line is nice but it does sound better in alliterative Old English: "glisnaþ glæshluttur gimmum gelicust..."  Two other rune poems have survived, in Norwegian and Icelandic, and here are their descriptions of this same ice rune:

Isa
Ice we call the broad bridge;
the blind man must be led.

Ice
Bark of rivers
and roof of the wave
and destruction of the doomed.

These imagistic definitions of natural phenomena can be seen as an early form of nature poetry.  Here are three more examples from the Anglo-Saxon rune poem, which dates from the 8th/9th century and is preserved in a copy made in 1705 (the original manuscript perished in a fire in 1731).  The first, for the rune called Berkana, describes a poplar, although it is given as a birch in the other two rune poems.  The second, for Eihwaz, praises the yew tree, 'greenest of trees in winter' according to the Norwegian version, and in the Icelandic, source of the bow, speeder of arrows.  In his guide to the runes, Bernard King writes that 'the hunting God Ull built his hall in Ydalir, Yewdale, and the bow was regarded as his sacred weapon'.   The third, for Algiz, describes 'eolh-sedge', a kind of sedge grass which would cut you if you brushed against it.  According to King, this rune 'implies defence and protection, possibly even in the form of an amulet or temple sanctuary, and related words are the Gothic alhs, temple, and the Old English ealgian, to protect. There may also be a relationship here with the mysterious runeword alu. The meaning has also been equated with the elk, mentioned by Caesar as sleeping upright leaning against a tree to elude the hunter more easily, and thus in some measure a symbol of preservation in the face of adversity.'

Beorc
The poplar bears no fruit; yet without seed it brings forth suckers,
for it is generated from its leaves.
Splendid are its branches and gloriously adorned
its lofty crown which reaches to the skies. 

Eoh
The yew is a tree with rough bark,
hard and fast in the earth, supported by its roots,
a guardian of flame and a joy upon an estate.

Eolh
The Eolh-sedge is mostly to be found in a marsh;
it grows in the water and makes a ghastly wound,
covering with blood every warrior who touches it.