Ledge Chert
“Ledge chert” is not a specific type of chert… it’s more a description of geological formation that can apply to many chert types. Most usually, the term is used to describe a relatively thick impermeable layer of chert that was originally underlying to, or sandwiched between, permeable layers of less resistant rock types such as chalk or limestone.
Subsequent erosion by oceans, rivers, ground-water or rainfall run-off degrades the softer permeable rocks, leaving the chert layer as a shelf or overhang protruding from its host rocks.
When the chert layer is the basal member of the formation, it is often associated with small springs arising from water percolating though or across the overlying permeable layers.
[amendment from “Ledger Chert” & addition by painshill]
These are pictures of a native rock quarry that is on a piece of land I used to own in Big Bend its about 300 ft up the south side of Black Mesa which has a large volcanic blowout on the north side. This area was once a shallow sea and this ledge of chert is held in place by limestone. The volcanic activity lifted the limestone bedrock up. There are large hammer stones near the outcropping and you can see where the natives were knocking out pieces, down below on level ground is an area that is covered with flakes, where the natives either flaked the material into preforms or points.
Ledge Chert (Big Bend, Texas):
Information and photos courtesy of Greywolf
“Ledge chert” is not a specific type of chert… it’s more a description of geological formation that can apply to many chert types. Most usually, the term is used to describe a relatively thick impermeable layer of chert that was originally underlying to, or sandwiched between, permeable layers of less resistant rock types such as chalk or limestone.
Subsequent erosion by oceans, rivers, ground-water or rainfall run-off degrades the softer permeable rocks, leaving the chert layer as a shelf or overhang protruding from its host rocks.
When the chert layer is the basal member of the formation, it is often associated with small springs arising from water percolating though or across the overlying permeable layers.
[amendment from “Ledger Chert” & addition by painshill]
These are pictures of a native rock quarry that is on a piece of land I used to own in Big Bend its about 300 ft up the south side of Black Mesa which has a large volcanic blowout on the north side. This area was once a shallow sea and this ledge of chert is held in place by limestone. The volcanic activity lifted the limestone bedrock up. There are large hammer stones near the outcropping and you can see where the natives were knocking out pieces, down below on level ground is an area that is covered with flakes, where the natives either flaked the material into preforms or points.
Ledge Chert (Big Bend, Texas):
Information and photos courtesy of Greywolf
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