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A boulder train consists of rock fragments of boulder size (greater than 10 inches or 256 mm) arranged in the alignment of ice-flow, usually in a row or fan. Most material in a till (see definition of ice-laid material) is derived from the local bedrock; however, the till will contain rock fragments and boulders that differ from the underlying bedrock. Many times the origin of these rocks is not known, but in areas where the source can be identified, the fragments or erratics (rock fragments of unknown origin) will be called indicators. These indicators plotted on a map will show the direction and distance of transport as well as any scatter or fanning of the material from the source. Such indicator fans are called boulder trains, although boulders are only one of several particles used to trace the origin of the erratics.
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