from igneous intrusions. Other times, heat from magma migrating close to the surface would superheat overlaying groundwater until it exploded, not unlike an exploding pressure-cooker, creating blowout craters and tuff rings. One example of such a feature is the roughly 2000-year-old and deep Ubehebe Crater (photo) in the northern part of the park; nearby smaller craters may be less than 200 to 300 years old. Sediment filled the subsiding Furnace Creek Basin as the area was pulled apart by Basin and Range extension. The resulting -thick Furnace Creek Formation is made of lakebed sediments that consist of saline muds, gravels from nearby mountains and ash from the then-active Black Mountain volcanic field. Boron, which is abundant in this formation, is dissolved by ground water and flows out onto the northern end of the Death Valley playa. Today this formation is most-prominently exposed in the badlands at Zabriskie Point. Additional subsidence of the Furnace Creek Basin was filled by the four-million-year-old Funeral Formation, which consists of of conglomerates, sand, mud and volcanic material. Another smaller basin to the south was filled by the Copper Canyon Formation around the same time. Footprints and fossils of camels, horses, and mammoths are in all three of these Pliocene formations. About 2–3 Ma, in the Pleistocene, continental ice sheets expanded from the polar regions of the globe to cover lower latitudes far north of the region, starting a series of cold glacial periods that were interrupted by warmer interglacial periods. Snowmelt from alpine glaciers on the nearby Sierra Nevada during glacial periods fed rivers that flowed into the valleys of the region year round. Since the topography of the Basin and Range region was largely formed by faulting, not by river erosion, many of the basins have no outlets, meaning they will fill up with water like a bathtub until they overflow into the next basin. So during the cooler and wetter pluvial climates of the glacial periods, much of eastern California, all of Nevada, and western Utah were covered by large lakes separated by linear islands (the present day ranges). Lake Manly was the lake that filled Death Valley during each glacial period from at least 240,000 years ago to as late as 10,500 years ago; the lake typically dried up during each interglacial period, such as the current one. Lake Manly was the last in a chain of lakes that were fed by