Proceedings of the Particle Physics and Early Universe Conference (PPEUC). For this paper: postscript PPEUC CMB index 1 Introduction .. ... A new statistic is discussed, designed to detect isolated coherent step-like discontinuities produced by cosmic strings present at late times. ... The statistical variable considered is the Sample Mean Difference (SMD) between large neighbouring sectors of CMB maps, separated by straight lines in two dimensional maps and points in one dimensional maps. ...