Historical clue formulations

• Rock that's fracked to release oil — 2026-06-02
• Soft rock — 2026-05-02
• Cement ingredient — 2025-05-23
• Layered rock — 2025-02-17
• Burgess ___ (rock bed noted for its fossils) — 2024-05-11
• Layered rock — 2024-03-07
• Flaky rock — 2023-11-08
• Fracking target — 2023-07-14
• One source of oil — 2021-06-13
• Thin-layered sedimentary rock — 2021-03-31
• Rock in which fossils can be found — 2020-07-24
• Some mudrock — 2019-11-01
• Oil source — 2018-12-29
• Oil-producing rock — 2017-08-28
• Fracking target — 2017-06-23
• *Sedimentary rock — 2017-01-10
• Something fracked in fracking — 2016-12-24
• Fuel-yielding rock — 2016-11-02
• Fracking material — 2016-03-08
• Mineral layer involved in fracking — 2014-12-01
• Rock used for flagstones — 2013-10-06
• Oil-producing rock — 2012-10-15
• Oil source — 2009-06-27
• Fossil source — 2009-04-19
• Slate, originally — 2009-02-20
• Fossil-yielding rock — 2008-11-30
• Shelf material — 2006-03-03
• Fossil medium — 2004-11-12
• Oil source — 2003-10-01
• Brick ingredient — 2003-09-06
• Oil source — 2003-03-28
• Petroleum source — 2003-03-09
• Mudstone — 2002-08-02
• Oil source — 2001-12-05
• Rock that may hold fossils — 2000-08-10
• Synfuel source — 1999-10-01
• Oil source — 1998-10-14
• Kind of oil — 1997-10-25
• Laminated rock — 1996-02-19
• Laminated rock — 1995-04-10
• Oil source — 1994-01-16
• Easily split rock — 1993-11-25