Glacier National Parks sedimentary rocks are known for their distinguishing markings, which also bear an historic record of ancient times and the conditions under which they were formed.
Salt Casts bear square indentations from large salt crystals that have long since disappeared. Sometimes, these casts are filled in by other minerals.
Rain Casts are rarer, but we did find one large specimen that still bore the splash marks of an ancient rain storm.
Ripple Marks are very common in Glacier Park. You can see modern examples at the shores of any moving bodies of water. The ripple marks found in the rocks of northwest Montana indicate that this was once the shoreline of a vast inland sea.
Mudcracks form when sediments dry out. Fossilized mudcracks were preserved when other sediments filled the cracks. If the sediments were darker or lighter, or of a different consistency, they clearly delineate the mudcracks. You can also see specimens where the filling sediments bump out of the rock, because the filler is harder and more durable.
Worm Burrows are fairly common in Glacier Park. They are all that is left of long vanished species from the Cretaceous. Because all the known Cretaceous animal species were soft-bodied, fossils of them are rare. They have been found in Australia, but the Cretaceous sediments of the United States haven't yielded any. Like mudcracks, worm burrows were filled in with other sediments. We've found them in mudstone, claystone and limestones. They make interesting patterns in polished specimens.
Strata - Also called layers or beds. Rock-forming sediments are often deposited in layers, which can be distinguished by differences in texture, hardness, cementation, color, internal structure, or composition in the successive layers. The layering is called stratification.
Cross-bedding - Arrangement of minor beds or laminae, within a stratified rock, more or less inclined to the depositional surface, with straight sloping or concave surfaces at various angles; cross-bedding may result from the deposition of sediments by flood waters, buildup movement of sand dunes by wind, or accumulation of sediments in a delta.
Concretions - Roughly spherical or rounded, more or less symmetrical structures in clastic sedimentary rocks; concretions are formed by the accumulations and concentration of mineral matter (usually present in the rock as cement) around a nucleus (often a fossil) in the approximate center of the structure.
Oolites - Small, round, concretionary bodies in a sedimentary rock, resembling fish roe, usually between 0.5 and 1mm in diameter; oolites are composed of calcite, dolomite, quartz, hematite, limonite, pyrite, or other minerals arranged in concentric layers, commonly about a nucleus minerals arranged in concentric layers, commonly about a nucleus (a shell fragment, sand grain, etc); some oolites have internal radial structures, and others are virtually structureless.
Pisolites - essentially the same as oolites, but have larger diameters (1-10mm; pin size) and are somewhat more irregular in shape; the interiors are more commonly structureless or filled with radial mineral fibers.
Fossils - any remains, trace, or imprint of a plant or animal preserved by natural processes in the earth’s crust (particularly in sedimentary rocks), fossiliferous rocks that give evidence of life during past geologic periods. structures, and others are virtually structureless.
Pisolites - essentiallly the same as oolites, but have larger diameters (1-10mm; pin size) and are somewhat more irregular in shape; the interiors are more commonly structureless or filled with radial mineral fibers.
Fossils - any remains, trace, or imprint of a plant or animal preserved by natural processes in the earth’s crust (particularly in sedimentary rocks), fossiliferous rocks that give evidence of life during past geologic periods.
I. Pelagic SedimentsCLASSIFICATION OF DEEP-SEA SEDIMENTS
A. Brown clay (chiefly composed of inorganic particles of very small size. Some biogenous material may be present).
B. Authigenic deposits (minerals formed by crystallization in seawater).
C. Biogenous sediments (chief constituent is material derived from
marine organisms, either calcareous or siliceous in composition. Non biogenous material may also be present.l. Pelagic oozes (chiefly microscopic organisms).
a. Calcareous ooze (contains abundant remains of calcareous skeletons of one-celled animals, called Foraminifera).
b. Siliceous ooze (contains abundant remains of the siliceous one-celled animals called Radiolaria, or one-celled plants
called Diatoms, or a mixture of both).
II. Terragenous SedimentsA. Terragenous Muds (contain abundant silt and clay derived chiefly from land areas).
B. Turbidites (produced by turbidity currents) - These are composed of silt, sands, and coarser grains that are delivered to the deep sea floor, expecially the abyssal plains, by turbidity currents. In some places thay are overlain by a thin veneer of Terragenous mud or pelagic ooze. They have characteristics of a shallow-water origin, but are found in the deep ocean.
C. Glacial marine sediments (contain clastics of various size ranges derived from glaciers or icebergs.