92 FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT.
Matson has described* what he believes to be a Pleistocene terrace bordering the St. Marys River in Nassau County. A similar abrupt rise in passing onto the upland may be observed in many places bordering the coast and the valley of the St. Johns River. It may be observed that a subsidence of 23 feet would submerge the entire St. Johns valley and would allow the sand dunes once more to face the ocean.
TOPOGRAPHY AND DRAINAGE.
The section of the State to which this report relates borders the Atlantic Ocean. From sea level the rise in elevation is as a rule gradual and the country in general level or rolling. It is probable that with the exception of sand dunes all of St. Lucie, Brevard and St. Johns Counties, as well as the eastern one-half or more of Nassau, Duval, Clay, Putnam, Volusia and Orange Counties and the entire St. Johns River valley lie below the 50-foot contour line. Elevations exceeding 50 feet occur in the western part of Nassau, Duval, Clay, Putnam and Orange Counties and as a ridge extending from northwest to southeast through Volusia County. The maximum elevation for eastern Florida is found in the northwestern part of Clay County approaching "Trail Ridge." On this ridge are found, according to levels made by the Seaboard Ait Line Railway, elevations exceeding 200 feet.
RIVERS.
The St. Johns River rises from the lakes of southern Brevard County within a few miles of the Atlantic coast. From this point it flows north or slightly west of north about 200 miles, entering the Atlantic Ocean within 25 miles of the north line of the State. The elevations along this river at no point exceed 25 feet above sea, the entire valley lying within -the artesian flow area of the State. The principal tributaries of the St. Johns are Black Creek and Ocklawaha River. The former heads in the uplands of Clay County, while the latter is fed from numerous lakes of,.Lake County and receives tributaries from Silver Springs in Marion County and from the lakes of southeastern Alachua County.
The St. Marys River, forming a part of the northern boundary of the Statg, rises in or near Okefinokee Swamp in Georgia. From its origin it flows south until on a parallel with the mouth of the St. Johns River. From this point it bends abruptly and flows north for
*Florida Geol. Survey, Second Annual Report, p. 39, 1Q09.