  We report on ALMA ACA observations of a high-density region of the Corona
Australis cloud forming a young star cluster, and the results of resolving
internal structures. In addition to embedded Class 0/I protostars in continuum,
a number of complex dense filamentary structures are detected in the C18O and
SO lines by the 7m array. These are sub-structures of the molecular clump that
are detected by the TP array as the extended emission. We identify 101 and 37
filamentary structures with a few thousand AU widths in C18O and SO,
respectively, called as feathers. The typical column density of the feathers in
C18O is about 10^{22} cm^{-2}, and the volume density and line mass are ~ 10^5
cm^{-3}, and a few times M_{sun} pc^{-1}, respectively. This line mass is
significantly smaller than the critical line mass expected for cold and dense
gas. These structures have complex velocity fields, indicating a turbulent
internal property. The number of feathers associated with Class 0/I protostars
is only ~ 10, indicating that most of them do not form stars but rather being
transient structures. The formation of feathers can be interpreted as a result
of colliding gas flow as the morphology well reproduced by MHD simulations,
supported by the the presence of HI shells in the vicinity. The colliding gas
flows may accumulate gas and form filaments and feathers, and trigger the
active star formation of the R CrA cluster.
