The interval of geologic time between 541 million and 485 million years ago.
The interval of geologic time between 359 million and 299 million years ago.
A natural cavity or chamber beneath the surface of the earth that is large enough to permit entry to people.
One of the four large divisions of geologic time, it includes three geologic periods, from about 66 million years ago to the present.
A very fine grained material, smaller than silt (clay has a diameter of less than 1/256 mm). Clay is formed by the weathering and breaking down of rocks and minerals.
The breaking of a mineral along its crystallographic planes, mirroring crystal structure. Cleavage is one of the characteristics used to identify minerals.
Used to describe a smoothly curving fracture surface of certain rocks and minerals that is either concave or convex.
A section of rock a few inches in diameter and at least several meters long obtained by drilling and brought to the surface for examination and analysis.
The interval of geologic time between 145 million and 66 million years ago. It is named after the Latin word for chalk ("creta") because of the English chalk beds of this age. The well-known chalk of Kansas was deposited late in the Cretaceous, about 60 million years ago. The Dakota Formation is another famous Cretaceous deposit in Kansas, frequently called the Dakota sandstone because the most prominent beds—those that cap the hills and stand out as cliff formers—are sandstones.
A series of thin, inclined layers in a larger body of rock (usually sandstone) that form a distinct angle to the principal horizontal bedding plane. Formed by currents of water or wind, cross-bedding is found in dune, stream channel, or delta deposits.