Variations in the pebble composition of surface till in southern Alberta indicate the presence of three contemporaneous lobes in the last continental glacier. The ice lobe domains have similar pebble associations and differ only in the frequency of occurrence of rock constituents. A large part of the percentage variability within each domain is caused by irregular entrainment of locally derived stones, which dilutes pebble contents and results in a lithology substantially different from the provenance composition. The study has shown that the regional trends are preserved and can be recognized in the locally modified lithologies, but the latter have to be isolated and evaluated differently from the regular provenance lithologies. The common diluting agent in the area is fluvial gravel that is widespread on erosional bedrock surfaces and in preglacial valleys. Because the gravel is generally coarse, pebble fractions of larger size are more affected by local dilution than are successively finer fractions. Quantitative comparison of rock contents in two size ranges helps to identify the regional and local types of pebble composition and to segregate petrological data into two corresponding sets. Source indices are calculated then as a function of three lithologic variables, for each of the sets separately. The indices have similar values in nondiluted and diluted samples of a given area and distinguish well between tills deposited by glaciers with different directions of regional flow.