Historical clue formulations

• Geological layers — 2026-05-03
• Levels — 2025-12-18
• Earth's crust and others — 2025-07-12
• Rock layers — 2025-01-30
• Geologic layers — 2024-07-30
• Sheets of rock — 2022-07-01
• Levels — 2021-01-27
• Layers of rock — 2020-03-30
• Echelons — 2018-11-28
• Rock bands? — 2017-10-03
• Geological layers — 2016-10-25
• Stacked beds — 2015-10-30
• Levels — 2013-07-04
• Levels — 2012-12-02
• Rock layers — 2012-05-16
• Geologic layers — 2011-12-21
• Layers — 2011-01-09
• Levels of society — 2009-10-20
• Rock layers — 2008-11-10
• Layers — 2008-03-24
• Echelons — 2008-01-19
• Layers — 2008-01-14
• Rock bands? — 2007-07-18
• Sheets — 2006-02-04
• Levels — 2005-04-03
• Layers — 2005-03-28
• Sheets — 2005-02-26
• Layers — 2004-01-19
• Levels — 2003-09-20
• Geological formations — 2003-06-07
• Levels — 2002-11-24
• Geologists' studies — 2001-08-26
• Layers — 2001-07-06
• Layers — 2001-01-30
• Sheets — 1999-03-05
• Geologic layers — 1997-11-04
• Layers — 1997-10-20
• Layers — 1996-02-23
• Class levels — 1995-01-12
• Rock layers — 1994-02-21