Looking at the Grimsby Sandstone, compare the texture with the Whirlpool Sandstone. Does it feel similar? Are the height of the rocks similar or is one larger than the other? Describe any other obvious differences you note between the three rock types
Silurian Period in geologic time, was the third period of the Paleozoic Era. It began 443.4 million years ago and ended 419.2 million years ago, extending from the close of the Ordovician Period to the beginning of the Devonian Period.
During the Silurian period, continental elevations were generally much lower than in the present day, and global sea level was much higher. Sea level rose dramatically as the extensive glaciers from the Late Ordovician ice age melted. This rising prompted changes in climatic conditions that allowed many faunal groups to recover from the extinctions of Late Ordovician times. Large expanses of several continents became flooded with shallow seas, and mound-type coral reefs were very common.
As occurs with present day water bodies such as Hudson Bay or the Gulf of Mexico, rivers flowing into this ancient sea carried sand, silt and clay to be deposited as thick layers of sediment. At the same time lime-rich organic material from the abundant sea life was also accumulating. Over millions of years these materials became compressed into massive layers of sedimentary rocks and ancient reef structures now visible along the Escarpment. Some rock layers now consist of soft shales and sandstones while others are made up of dolostone (a rock similar to limestone which contains magnesium and is more durable).
Lockport Dolomite
Lockport Dolomite overlays weaker shale layers and forms the top layer (Caprock) or crest of the falls. Formed during the Silurian Period over 400 million years ago, this dolostone is a magnesium rich sedimentary rock resembling limestone. While the composition is similar to limestone, it is very hard and more resistant than limestone. Lockport Dolomite has been quarried in the area for many years and is used for crushed fill, drainage, cement aggregate, macadam, riprap, fertilizer, ceramic material and in particular is used as reinforcing walls along lake Erie property due to being highly resistant to erosion.
Clinton Group
The Clinton Group is made up of the a combination of dolomite, shales and dolomite limestone. The top layer of this group is DeCew Dolomite, the uppermost formation of the Clinton Group, which is dark gray to olive gray in colour, is argillaceous to sandy, fine grained dolomite that locally contains shaly partings and interbeds. A middle bed of up to 3 ft thick is present in the southern Niagara River Gorge. This layer overlies a layer of Rochester Shale.
Rochester shale is a sedimentary rock made up of shale and mudstone, though it encompasses a number of sedimentary rock types including conglomerate, sandstone, limestone, dolostone and ironstone. Shale is relatively soft, and readily weathers. Given that it is overlaid by the harder and more resilient Lockport Dolostone, this strata layer tends to erode quicker where it is exposed and is a major contributor to the formation of the Niagara Gorge.
Also included in the Clinton Group is a layer of Irondequoit limestone, Reynales dolomite, Neagha shale and Thorold sandstone.
Medina Group
Grimstby Sandstone is primarily a hematitic quartz standstone with subordinate greywacke, siltstone and shale. Sand grains are well rounded and are typically cemented with silica and sporadically with calcite. Red color dominates with some green-gray mottles and layers.
Power Glen shale is comprised of green-gray siltstones with alternating shale bands
Whirlpool Sandstone is composed of two main units: a fluvial unit and an estuarine to transitional marine unit. Whirlpool sandstone is light-gray to white, with medium to coarse grained, trough cross-bedded, quartzose sandstone with thin, dark-gray to greenish-gray shale clasts and interbeds
It is a major source of building stone. Two distinct mapping units are recognizable within the sandstone. Only the upper unit is marine in origin, deposited in a nearshore environment. The lower unit is probably of braided fluvial origin. Both are very fine to fine-grained subarkose to quartz arenite, with no notable compositional differences between the upper and lower units. Salt and pepper appearance is due to the presence of phosphatized fossil fragments. The Whirlpool sandstone is thin and rarely exceeds 9 m in thickness. It extends a total of 275 km in outcrop along the Niagara Escarpment from Medina, NY, northward to Mitchell's Mill, near Duncan, Ontario.
References
http://ngmdb.usgs.gov/Geolex/UnitRefs/WhirlpoolRefs_4375.html
http://www.escarpment.org/Geology/about_geology.htm
http://www.buffaloah.com/a/DCTNRY/mat/lock/lock.html
http://www.uwgb.edu/dutchs/GeologyWisconsin/niagesc.htm
