The interior wall, high and smooth, a quite effective barrier, served well to keep their slaves within.

In the longer term no effective barriers can be devised.

The former, surrounding it from three sides, provides an effective barrier.

Whether this will form an effective barrier, no one knows.

Most legislatures that did add a test of injury severity couldn't bring themselves to make it an effective barrier.

The only option seemingly available to them will be to install some form of suitable and effective barrier.

But it remained an effective barrier to further exploration just the same.

The passes are almost two kilometers up or higher, and it's an effective barrier.

The first two are particularly effective barriers to livestock.

However, they have no effective barriers to keep them out.