WorldWide Drilling Resource®

44 OCTOBER 2020 WorldWide Drilling Resource ® Frequently-Used Geological Terms Part 90 Glossary Adapted from the Dictionary of Geological Terms S Sedimentary Trap - An area between a high- and low-energy environment where sedimentary materials accumulate. Sedimentation - The process of forming sediment in layers, including the separation of rock particles from its parent material, the transportation of the particles to the site of deposition, the actual deposition or settling, the diagenetic changes occurring in the sediment, and its ultimate consolidation into rock. Also used to describe the process of deposition of sediment. Sediment Concentration - The ratio of the dry weight of sediment in a water sediment mixture to the total weight of the mix- ture. It is usually expressed in percent for high concentration values, or in parts per million for low values. Sedimentology - The scientific study of sedimentary rocks and the processes by which they were formed. Also, the description, classification, origin, and interpretation of sediments. Seep - A spot where water or petroleum oozes from the earth, often forming the source of a small trickling stream. Also, to move slowly through small openings of a porous material. Seismic - Pertaining to an earthquake or earth vibration, including artificially induced. Seismic Activity - See Seismicity . Seismic Area - An earthquake zone or the area affected by a particular earthquake. Seismic Detector - An instrument, such as a seismometer or geophone, which receives seismic impulses and converts them into electrical voltage or otherwise makes them evi- dent. Seismic Exploration - The use of artificially generated seis- mic waves in the search for economic deposits such as salt or gas and oil, or in engineering studies. Also referred to as seismic prospecting. Seismicity - The likelihood of an area being subject to earth- quakes. The term is also used to describe the phenomenon of earth movements. Seismic Map - A contour map constructed from seismic data. Values may be in either time or depth. Data may be plotted with respect to the observing station or with respect to the subsurface reflecting or refracting locations. More terms next month! MIN Researcher in Iceland recording seismic information with a geophone. 60-Year-Old Core Sample Answers Ancient Question Compiled by Bonnie Love, Editor, WorldWide Drilling Resource ® Stonehenge, in the United Kingdom, has long been surrounded by a cloud of mystery. The monument of 52 massive, 25- ton stones, known as sarsens or druid stones, has long baffled scientists, archaeologists, geologists, and curious people for centuries. How old is Stonehenge? Where did the stones come from? Why are they arranged in such a unique pattern? Well, scientists have been able to lift the veil on at least one of those questions. This ancient ring of stones has been hailed as a masterpiece of engineering. Although the smaller bluestones in the middle of the monument have been studied a great deal, the sarsens haven’t received as much attention. The bluestones are believed to have traveled nearly 100 miles from various sites around Wales. So, where did the larger stones come from? The answer would take decades to reveal itself. Back in 1958, archaeologists were working to raise a fall- en trilithon (two upright sarsens with a third laying across the top) which had tumbled more than 150 years earlier. When workers lifted stone number 58, they noticed it was cracked, and decided to drill three cores from the stone and insert metal rods for reinforcement. One of the workers that day was Robert Phillips, who took one of the drilled cores as a souvenir. He proudly displayed the nearly three-foot core in his office in the U.K. for years until 1977, when he moved to the U.S., bringing the core with him. Robert eventually retired to Florida, and as he neared his 90th birthday, he wanted to return his little piece of history to English Heritage, a charitable organization in England, charged with managing over 400 his- toric buildings, monuments, and sites, including Stonehenge. After the core was returned to the U.K., it was analyzed by David Nash an archaeologist, geomorphologist, and professor of physical geography with the University of Brighton, and colleagues from leading universities. Although the stones are more than 99% silica, they also contain trace amounts of aluminum, carbon, iron, potassium, and magnesium. Using Phillips’ core, the team discovered 50 of the 52 sarsens had nearly identical chemistry, suggesting the stones all came from a single site. The next step was to match pieces of the core with samples taken from 20 sarsen boulder fields across southern England, each with is own geo- chemical signature. Armed with this knowledge, researchers were able to determine with nearly 100% confidence that the sarsens at Stonehenge came from a forest called West Woods about 15 miles away, in Marlborough Downs. Photo of Robert Phillips’ core from stone 58 courtesy of English Heritage © Photo by Juliet Brain. ENV