From AMS Glossary
"Freshwater ice consisting of numerous long crystals and hollow tubes having variable form, layered arrangement, and a content of air bubbles. This ice often forms at the bottom of an ice layer near its contact with water."[from Glossary of Arctic and Subarctic Terms (1955)].
Arctic, Desert, Tropic Information Center (ADTIC) Research Studies Institute 1955. Glossary of Arctic and Subarctic Terms. ADTIC Pub. A-105, Maxwell AFB, AL, . 90 pp.