This Gibraltar Rock is Really Lying Out There
In Wisconsin, United States
Size:  (not chosen)
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This is not your typical geocache; it is an EarthCache. Instead of looking for a "cache container" you will be brought to a geological feature.
ABOUT THIS LOCATION This EarthCache is in southcentral Wisconsin in southwestern Columbia County. The entrance to Gibraltar Rock State Natural Area is on Highway V about one mile west of Highway 113. Walk up Gibraltar Rock using the old roadway to the small monument for Richmond Memorial Park. You need go no further toward the cliff for this EarthCache (the drop off is considerable). However, you may just find some St. Peter sandstone to admire: its texture, the layering, all the colors. At this time there is no sign explaining the geology of Gibraltar Rock. The following text provides brief information about its long, intriguing past.
THE route up this erosional feature passes through five hundred million years of geologic time. The lower part of Gibraltar Rock is composed of late Cambrian sandstone deposited when the late Cambrian Sea covered much of this continent, and what would become Wisconsin was just south of the Equator. There was plenty of sand for this from the eroding Precambrian shield.
THE two layers above it are of Ordovician age. The first is Prairie du Chien dolomite that formed in a shallow sea beginning about 495 million years ago in the early Ordovician Period. Above that is the whitish St. Peter sandstone deposited by wind and rivers during the Middle Ordovician Period when the sea was absent in this area. Although St. Peter sandstone is often a soft rock, here it is the cap rock and forms this cliff. (Take Hwy. V west and south on J for a view of this bluff.)
THESE rock layers were like the mattresses for the princess, however, that pea began to grow since the Cambrian Period into a dome and arch through Wisconsin. This slowly raised those layers, causing them to tilt and erode and form escarpments and cuestas such as at the leading edge of harder layers, as the Prairie du Chien cuesta (formerly Lower Magnesium cuesta) and St. Peter cuesta. Gibralter Rock was not a separate, indentifiable entity until erosion removed the layers around it and left it lying out away from its respective receding cuestas. It is justifiably and geologically called an outlier.
OTHER layers were deposited here during th Silurian Period, but eroded away. That, however, is not the end of the geologic story here. The continental Laurentide Ice Sheet began its advance out of Canada about three million years ago. Of its numerous advances and retreats during the Pleistocene Epoch, the western edge of the Green Bay Lobe during the late Wisconsin Stage left evidence of its appearance here. It rounded off part of Gibraltar Rock, left gravel on top of it, and would have carved grooves/striations in the direction of ice movement. Then the receding glacier and its meltwater left many feet of deposits in this area. Now, there's all this geology for the EarthCaching hiker!
EARTHCACHE INFORMATION Bring your camera and compass. To receive credit for this EarthCache please do the following requirements (enclosed text will help you):
1. Take elevation: trailhead _______, memorial _______, so Gibraltar Rock is ______ high. The Precambrian shield is 300 feet above sea level here (use your trailhead elevation) leaving _______ feet for Cambrian sandstone and glacial deposits.
2. The three rock layers here with respective periods are from the bottom: _______, _______, and _______.
3. Take a photo of your GPS (and you/group if possible) showing part of the memorial, rock, and valley (only if you can from the memorial). This photo is taken facing the _____ direction. Send photo with log-in. As of 1/1/11 photos are optional but appreciated.
Send answers to me with EarthCache name.
You do NOT need to wait for confirmation from me before logging your find. Please do not make any reference to these answers in your log.
(No hints available.)
Last Updated: on 03/12/2016 13:18:08 (UTC-08:00) Pacific Time (US & Canada) (21:18 GMT)
Coordinates are in the WGS84 datum