R按块拆分列并重新组合

Question

我有一个data.frame

> blah
  Col1 Val1 Col2 Val2
1    A    1    B   10
2    A    2    B   20
3    A    3    B   30

structure(list(Col1 = structure(c(1L, 1L, 1L), .Label = "A", class = "factor"), 
    Val1 = structure(1:3, .Label = c("1", "2", "3"), class = "factor"), 
    Col2 = structure(c(1L, 1L, 1L), .Label = "B", class = "factor"), 
    Val2 = structure(1:3, .Label = c("10", "20", "30"), class = "factor")), .Names = c("COl1", 
"Val1", "Col2", "Val2"), row.names = c(NA, -3L), class = "data.frame")

其中col1和col2是重复的id列。

我希望按列名拆分数据框并重新组合它们，以便最终结果如下所示。

> blah
   Col  Val
1    A    1
2    A    2
3    A    3
4    B   10
5    B   20
6    B   30

我一直在尝试使用融化和重塑，但没有太多运气。