to 25.6 milligrams per liter and the iron concentration ranges from 18.9 to 60.7 milligrams per liter. The boron concentration ranges from an estimated 12 to 16 milligrams per liter. The concentration of fluoride in Darby Creek is less than 0.01 milligrams per liter and the chloride concentration ranges from 26.2 to 33.6 milligrams per liter. The sulfate concentration ranges between 18.1 and 19.9 milligrams per liter. The silica concentration was twice measured to be 15.7 milligrams per liter. Geography, geology, and climate The elevation near the mouth of Darby Creek is above sea level. The elevation of the creek's source is between above sea level. The gradient of the creek for its first is per . For the next , the gradient of the creek is per . For the last , the gradient is only a few feet per mile. The valley of Darby Creek is considerably narrower in its upper reaches, but widens out as it approaches the Delaware River. Numerous reaches of the creek have been armored with boulders or riprap. Upstream of the tributary Cobbs Creek, the stream's channel is sinuous and the creek flows through a narrow valley bordered by low, steep hills. From Cobbs Creek downstream to its mouth, the creek is in a tidal flat. The northern section of Darby Creek is within the Piedmont Uplands physiographic province, while the southern section is part of the Atlantic Coastal Plain province. The Piedmont Uplands section has generally old, hard upland rocks that eroded from the Appalachian Mountains. The rocks from the northern portion of the watershed date to the Precambrian Era and Lower Paleozoic Era. The rocks from the southern portion of the watershed are newer, dating from the Tertiary and Quaternary Periods. Atlantic Coastal Plain rock is generally softer than Piedmont Uplands rock and was deposited in the area about 1.6 million years ago through glacial erosion. Several geologic formations can be found within watershed of Darby Creek. The Wissahickon Formation is the most prevalent formation in the watershed. It is derived from derived from sandstones and mudstones and is composed of mica schist. Metamorphic felsic gneiss and mafic gneiss formations are common in the northern parts of the watershed. The Bryn Mawr Formation and the Bridgeton Formation are also present and are unconsolidated deposits of rock that rest on top of the dense crystalline bedrock. Mica slate is present in Marple