![]() The finer the grains, the sharper these edges can be- with the most extreme being obsidian (volcanic glass), which can form from either basaltic or rhyolitic lavas.Īphanitic ( a = not, phaner = visible) rocks in contrast to phaneritic rocks, typically form from lava which crystallizes rapidly on or near the Earth's surface. When basalt is broken, it leaves very sharp edges. Basalt is from a melt that cooled very rapidly- in other words, on the surface of the earth or in the ocean. Diabase and gabbro have the same general mineral composition as basalt, but they have a larger grain size.īasalt is identical to diabase and gabbro, but is very fine-grained. The grain size of diabase is on the smaller side. Erosion is still occurring (modified from ).Ī “flume” is a geological feature formed when a plug of basalt is worn away from between two walls of harder granite-the result is a torrent of water in a steep gorge.ĭiabase is actually a variety of gabbro which consists mainly of labradorite feldspar, augite, magnetite, and olivine. As you walk through the Flume, look at the floor of the Gorge and you many notice remnants of the main basalt dike, and on the walls of the gorge, small trees are growing. The highly fractured granite and basalt have been eroded by frost action as well as by the brook’s water. After the Ice Age, Flume Brook began to flow through the valley again. It partially filled the valley with glacial debris and removed soil and weathered rock from the vicinity. The gorge was covered by glaciers during the Ice Age, but the ice sheet did not greatly change the surface. The basalt dikes eroded faster than the surrounding Conway granite, creating a deepening valley where the gorge is now. As the overlying rock was worn away, pressure was relieved and horizontal cracks developed, allowing water to get into the rock layers. Had this material ever reached the surface, it would have become lava flows.Įrosion gradually exposed the dikes. Because of this quick cooling, the basalt is a fine- grained rock. The basalt crystallized quickly against the relatively cold granite. The basalt came from deep within the earth as a fluid material, and because of pressure, was able to force the Conway granite aside. Sometime after the fractures were formed, small dikes of basalt were forced up along the fractures. ![]() As it cooled, the granite was broken by closely spaced vertical fractures which lay nearly parallel in a northeasterly direction. Nearly 200 million years ago in Jurassic times, the Conway granite that forms the walls of the Flume was deeply buried molten rock. The walls of Conway granite rise to a height of 70 to 90 feet and are 12 to 20 feet apart. Like settling, corrections for submergence have been developed.įinally, if there is a flow that is used with the flume, the location and calibration of that meter should be checked.The Flume is a natural gorge extending 800 feet at the base of Mount Liberty in Franconia State Park, on I-93 in New Hampshire. Submergence will result in the flume overreading the actual flow rate. If upstream and downstream flow lines are close in elevation, the flume has been experiencing submergence for an extended period of time. Check the flow line of the flume - the high water mark along the wall.If the flume has a staff gauge, take a look to make sure that gauge is readable - cleaning or replacing as necessary.Verify that the channel hasn’t experienced overtopping since the last inspection.Maintenance may have to be performed to remove sediment and debris. That the channel upstream and downstream of the Parshall Flume hasn’t changed and that vegetative growth isn’t impacting flow - particularly out.Verify the sidewalls are vertical - that frost heave, settlement, or soil conditions haven't bowed the walls.The Parshall Flume is still in dimension - within 2% of the nominal dimensions. ![]() That the Parshall Flume’s flow surfaces are free of algae and other biological growth.If the flume has shifted - but otherwise in dimenision - corrections for settelment have been developed to still allow you to use the flume.The Parshall Flume hasn’t shifted out of position due to frost heave, animal traffic, or subsidence.Generally, before the flow season begins, it’s recommended that you do a site inspection. Now that doesn’t meant that you don’t need to periodically check to make sure the Parshall Flume isn’t clean and level, or that the upstream / downstream conditions haven’t changed - because they do! One of the advantages of using a device like a Parshall Flume is that, properly installed and applied, there is NO NEED TO CALIBRATE THE FLUME! ![]()
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