We studied unusual growth macrolayers on the {100} faces of diamond crystals grown in Mg-C and Mg0.9Ge0.1C systems at 7.0 GPa and in the temperature range of 1800-1900 degrees C. The layers were shaped as rectangles highly elongated in one of the [110] directions. The layer elongation (length to width ratio) was different, but did not exceed 23. We found that the ends of the macrolayers in most cases were composed of {111} microfacets, and their growth occurred by layers along these microfacets. Using selective etching, we found that the macrolayers were not associated with outcrops of dislocations and any planar defects on the {100} faces. It is supposed that the formation of these highly elongated layers occurs by two-dimensional nucleation that becomes possible at high carbon supersaturation under conditions of kinetically limited diamond growth in Mg based systems.