Matted Needles are natural blocks that the ground of some areas in Taigas and Tundras sometimes is made of, often covered with a thin layer of Snow. These blocks can be harvested/mined without requiring any Power Cells.
You can often collect Rimecones from these blocks, since rather large versions of Elderwood trees (with 4 blocks thick trunks) "grow" Matted Needles. However you cannot plant Elderwood Saplings on these blocks.
No Creatures are known to spawn on these blocks, neither during the day nor during the night (tested this a lot). They cannot be plowed as of yet (May 2017) and such also cannot be turned into tilled land for Farming.
Below Matted Needles, there is often a thick layer of Dirt (with Brown Mushrooms mixed in regularly) above the typical Fossil layer blocks.