1. AstmCommittee-D 18 (1964 ). Procedures for Testing Soils , 4th edn , pp.95 -106 . American Society for Testing and Materials, Philadelphia.
2. B. B. Berglund(1979 ). International Geological Correlation Programme, Subproject 158B, Lake and Mire Environments, Project Guide vol. 1 Lund, 123 p.