set up raster stack
```{r}
library(raster)
r <- raster(ncols=3000, nrows=3000)
r1 <- setValues(r, runif(ncell(r)))
r2 <- setValues(r, runif(ncell(r)))
r3 <- setValues(r, runif(ncell(r)))
r4 <- setValues(r, runif(ncell(r)))
r5 <- setValues(r, runif(ncell(r)))
r6 <- setValues(r, runif(ncell(r)))
stk <- stack(r1,r2,r3,r4,r5,r6)
```