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Bowlder clay, the unstratified clay deposit of the Glacial or Drift epoch, often containing large numbers of bowlders.

These deposits may have been made on the rock which underlies the drift, or they may have been made on stratified or unstratified drift already deposited.

From this arrangement they have been termed “stratified,” in contrast to the unstratified or igneous series.

The unstratified class are all colored red, to indicate their igneous origin.

Further observation showed that stratified tumuli occupy a certain fixed position with regard to other works, which the unstratified tumuli do not.