English: Wallbridge Unconformity in Kentucky, USA.

The rocks in the upper part of the picture are cherty limestones of the Middle Devonian Boyle Formation. The gray rocks in the lower part of the picture are the Lower Silurian Estill Shale (also known as the Crab Orchard Shale). The entire Upper Silurian and Lower Devonian are missing at the contact, representing about 40 to 50 million years.

The Boyle-Estill contact is a megasequence boundary called the Wallbridge Unconformity - it's the Tippecanoe-Kaskaskia Megasequence boundary. Regionally recognizable packages of sedimentary rocks bounded by major unconformities are known as megasquences, or Sloss sequences (named after geologist L.L. Sloss, 1913-1996). The bounding unconformities were caused by erosion during regressions. Continental margins have more complete sedimentary successions, whereas craton interiors have more incomplete successions. The names for North America's megasequences are (from oldest to youngest): Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuni, and Tejas.

Stratigraphy: Casey Member of the Boyle Formation (Middle Devonian) over the Estill Shale (Lower Silurian)

Locality: Drowning Creek West outcrop - roadcut along the northern side of Route 52, ~1.2 miles west of Drowning Creek, far-eastern Madison County, east-central Kentucky, USA