Ice covers a significant part of the Earth’s surface and is one key component of the global climate system. A thorough understanding of ice flow is crucial for modeling the response of ice sheets to past and upcoming climate changes. Glen’s law, a experimentally derived exponential relationship between stress and strain rate, is usually applied. However, it does not adequately capture microstructural changes observed for high total shear strains and long time scales which cannot be reproduced by laboratory experiments. Deep ice cores provide insights into the natural evolution of microstructure (grain boundary networks). Large sets of sublimation groove images, mapping grain boundaries in high resolution, are available along the EDML (East Antarctica) and NEEM (Greenland) ice cores. A digital image processing approach has been developed to derive grain size, grain shape, shape of grain boundaries, and density of sub-grain boundaries in a consistent way. An automatic assignment to c-axes orientation measurements allows estimating the error of calculated parameters. Depth profiles along both ice cores are presented and interpreted based on variations in impurity content, temperature differences, and involved time scales. Furthermore, the presence of effective negative pressures caused by air bubbles and clathrate hydrates is taken into account.