The Golan Heights is composed of two geologically distinct areas divided by the Sa'ar River: the Golan Heights proper (approximately 1,070 sq. km.) and the slopes of the Mount Hermon range (approximately 100 sq. km.).
While the Mt. Hermon range is mostly limestone, the Golan Heights proper is mostly basalt and other types of volcanic rock, forming a plateau that drops off to the west, to the Jordan River and Lake Kinneret (in the Syrian-African Rift Valley), and to the south, to the Yarmouk River. The plateau is crossed by a number of seasonal streams which run through valleys, sometimes very deep, and flow west into the Jordan or the Lake. The Golan proper may be divided into three regions: northern (between Nahals Sa'ar and Gilabon), central (between Nahals Gilabon and Dilayot), and southern (between Nahal Dilayot and the Yarmouk Valley).
The northern Golan has double the average rainfall of the southern Golan, and often receives snow in the winter, as does the Mt. Hermon area. Hydrologically, nearly the entire Golan lies within the Lake Kinneret catchment basin, which supplies 30% of Israel's water requirements. Two of the Jordan River's main sources, the Dan and the Banias Rivers, rise on the slopes of Mt. Hermon -- in addition to many seasonal streams that rise on the Heights and flow into the Lake, either directly or via the Jordan. In 1964, Syria sought to divert the sources of the Jordan and prevent their waters from reaching Israel, provoking a series of border incidents; the Syrian plan was ultimately thwarted by IDF operations in the spring of 1965.
In ancient and classical times, the Golan was heavily forested (see Ezekiel 27:5-6). Today, small remnants of these forests survive near Odem and Mt. Avital in the north, and near Yehudiya in the central Golan. Half of Israel's mammal and reptile species, and all of its amphibians, can be found on the Heights.
Sources: Israeli Government Press Office