???
I just did it last night in 0.951. used 'raise land' to raise a single tile 2 times, which turned the next tile by the shore into plains. So I did 2 times on that tile too, which turned the next one out into land...but good luck reaching that. Result was that 2 tiles became impassible mountain tiles. Not as tall as natural mountains, but it did the job.
and 50 mana is too cheap for such a powerful spell. Even with the 4 castings required, 200 mana is a small price to pay for security.