Flat Sand Dunes are at the northern end of the valley floor and are nearly surrounded by mountains on all sides. Due to their easy access from the road and the overall proximity of Death Valley to Hollywood, these dunes have been used to film sand dune scenes for several movies including films in the Star Wars series. The largest dune is called Star Dune and is relatively stable and stationary because it is at a point where the various winds that shape the dunes converge. The depth of the sand at its crest is but this is small compared to other dunes in the area that have sand depths of up to deep. The primary source of the dune sands is probably the Cottonwood Mountains which lie to the north and northwest. The tiny grains of quartz and feldspar that form the sinuous sculptures that make up this dune field began as much larger pieces of solid rock. In between many of the dunes are stands of creosote bush and some mesquite on the sand and on dried mud, which used to cover this part of the valley before the dunes intruded (mesquite was the dominant plant here before the sand dunes but creosote does much better in the sand dune conditions). Mesquite Spring Mesquite Spring is located in the northernmost part of Death Valley. This part of the valley has numerous cotton top cactus, blister beetles and cholla cactus. On the alluvial fan above the springs there are 2-3 thousand year old petroglyphs from the extinct Mesquite Spring culture. The petroglyphs here are made possible because many of the rocks in these arid conditions have desert varnish on them. This particular form of desert varnish takes 10,000 years to make 1/100th of an inch of varnish and is deposited by a certain type of bacteria that collects the iron, manganese and clay needed to make the varnish. Also, since varnish is created at a predictable rate, it is possible to date petroglyphs based on the amount of re-varnishing that has taken place since the marks were made. Varnish does not normally form on carbonate rocks because their surfaces weather too easily. In a wash near some of the petroglyphs there is a fault scarp that exposes some fanglomerate which is a type of sedimentary rock which looks like concrete with large rocks intermixed. In fact it is lithified