checkered maps appeared

hi guys. please help me. i have this scene with ice material on it. there seems to be a
checkered part at the upper and bottom part of it. check this out.



anybody
knows what causes it?and also..any advice on how to render this much
faster?this is a 640x480 image and it took 22 mins. im using i5 760, 4gb
ddr3.

IRR -4,-4
LC 300

Thanks in advance!

thank you very much sir bokkins!always helpful!