Life In The Jovian Atmosphere III

Which book am I describing? -

an imaginary world;
diverse kinds of intelligent beings;
a major conflict;
fantastic events;
a long, perilous journey.

Three Worlds To Conquer by Poul Anderson.

Theor's progress along the top of a very high cliff is impeded by a wall - natural or artificial? I think that it must be the former. Blown against the wall is a strange organism:

a tough, ragged, square sheet, twenty feet on a side;
thick, fuzzy strands joining the corners of the sheet to something resembling a large, heavy, wart-covered log.

Theor deduces that:

usually, the organism drifts;
the top absorbs energy;
the cilia absorb ammonia and minerals;
the sheet and the log balance each other;
the organism is a druga species, currently encysted (in a cyst) during metamorphosis.

Theor tries unsuccessfully to open the shell - presumably the "log" or cyst - for meat. Then he tries to parachute down the cliff with the sheet but instead is pushed upward by a thermal updraft. Updrafts, weak and infrequent near the surface, are powerful when pressure and density are lower and the temperature gradient is greater.