the present Atlantic Ocean margin of the United States lay to the east. An algal mat-covered carbonate bank was deposited, forming the Noonday Dolomite. Subsidence of the region occurred as the continental crust thinned and the newly formed Pacific widened, forming the Ibex Formation. An angular unconformity (an uneven gap in the geologic record) followed. A true ocean basin developed to the west, breaking all the earlier formations along a steep front. A wedge of clastic sediment then began to accumulate at the base of the two underwater precipices, starting the formation of opposing continental shelves. Three formations developed from sediment that accumulated on the wedge. The region's first known fossils of complex life are found in the resulting formations. Notable among these are the Ediacara fauna and trilobites, the evolution of the latter being part of the Cambrian Explosion of life. The sandy mudflats gave way about 550 million years ago to a carbonate platform (similar to the one around the present-day Bahamas), which lasted for the next 300 million years of Paleozoic time (refer to the middle of the timescale image). Death Valley's position was then within ten or twenty degrees of the Paleozoic equator. Thick beds of carbonate-rich sediments were periodically interrupted by periods of emergence. Although details of geography varied during this immense interval of time, a north-northeastern coastline trend generally ran from Arizona up through Utah. The resulting eight formations and one group are thick and underlay much of the Cottonwood, Funeral, Grapevine, and Panamint ranges. Compression and uplift In the early-to-mid- Mesozoic the western edge of the North American continent was pushed against the oceanic plate under the Pacific Ocean, creating a subduction zone. A subduction zone is a type of contact between different crustal plates where heavier crust slides below lighter crust. Erupting volcanoes and uplifting mountains were created as a result, and the coastline was pushed to the west. The Sierran Arc started to form to the northwest from heat and pressure generated from subduction, and compressive forces caused thrust faults to develop. A long period of uplift and erosion was concurrent with and followed the above events, creating a major unconformity, which is a large gap in the geologic record. Sediments worn off the Death Valley region were carried both east and west by wind and water. No Jurassic- to Eocene-aged sedimentary formations exist in the area, except for some possibly