An isopleth map shows the distribution of a variable by lines of equal magnitude called isopleths. Till-pebble isopleth maps show the areal distribution of pebbles of different rock types in till.

Four till-pebble isopleth maps were prepared during a reconnaissance study of the Pleistocene geology of northeastern Montana and northwestern North Dakota. Three of the maps are concerned with erratic pebbles; the fourth, with locally derived pebbles.

The isopleth maps of the erratic pebbles reveal an area in northwestern North Dakota where the erratic content of the till differs significantly from that of the till elsewhere in the region. Lithologic relations are described which suggest that the drift of this small area may differ in age from that to the north and south.

A till-pebble isopleth map of distinctive, locally derived fluvial pebbles reveals three large areas of till in which these pebbles are abundant. Two of the areas, southwest and west of extensive exposures of the source gravels, suggest that the ice moved generally southwestward across the area. The third area is unrelated to known gravel exposures and may indicate a body of gravel below the till.

The study indicates that till-pebble isopleth maps may sometimes prove useful in suggesting the possibility of multiple drifts and in helping to delimit the drifts once substantiated. They may also provide information on direction of ice movement and on the identity of concealed bedrock.

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