The coordinates of this Earthcache lead you to an overlook on the southern end of the island. You will follow a short path to the overlook, please be careful as you approach the lookout spot.
According to URI Geologist Jon Boothroyd, Block Island was formed by the deposits from the lower edges of two glaciers, the first one more than 120,000 years ago, the second one about 22,000 years ago. “The first glacier brought rocks and sediment from Narragansett Bay to the north; the second came across Connecticut and southwest Rhode Island from the northwest”, writes Dr. Boothroyd. “bringing with it a finer, grainier sediment”. Some of the debris, according to geologist Alonzo Quinn, traveled the ice-covered glacial highway from as far away as Iceland and Great Britain.
A glacial erratic is a piece of rock that differs in size and type native to an area that were carried by the glacier over distances of many hundreds of miles and dropped in place as the glacier retreated. Erratics can range in size from pebbles to large boulders.
Once you arrive at the posted coordinates, you will notice a number of large boulders at the bottom of the bluff. These boulders are examples of glacial ‘erratics’.
Please also visit “Glaciers on the Beach?” ( GC2BE6W), by TerraViators.
Resources:
1) Bedrock Geology of Rhode Island GEOLOGICAL SURVEY BULLETIN 1295 Prepared in cooperation with the State of Rhode Island Development Council
2) Rocks, stones and erratics: the 'bedrock' of life. By Kim Gaffett, Mon, 04/11/2011
3) Rhode Island Geology for the Non Geologist Paperback – 1973 by Alonzo W. Quinn
Please answer the following. Do not include the answers in your log; send them separately to me. Correct answers earn a smiley. After sending your answers to me, log your find. I will only contact you if I have questions about the responses. Rock on!
1. In the field of geology, what does the term ‘erratic’ commonly refer to? (Please see content above, and for additional information, read “Bedrock Geology of Rhode Island”.
2. At the listed coordinates you will notice a large boulder about a third of the way down the bluff:
a. Please describe its size, color and shape.
b. Based on what you’ve learned about erratics and what you’ve read in Bedrock Geology of Rhode Island and/or Rhode Island Geology for the Non Geologist, would you consider this rock to be an erratic and why/why not?
3. What evidence is there as you look down the bluff toward the shoreline to support the belief that there were two glacial periods arriving from two different directions and each depositing debris?