Here's an extract on the natural environment of Ubin.
"Pulau Ubin is the second largest of Singapore's offshore islands. It lies in the Straits of Johor between Singapore and the Malaysia mainland. It has a linear east-west alignment and is about 7km long and a maximum of 2km wide, giving a total area of about 10 square kilometres. ...
The topography of Pulau Ubin is undulating and comprises a series of low hills and small river valleys. The hills have slopes of less than 10 degrees generally, and rise to a high point of 74m above sea level at Bukit Puaka in the centre of the island. Other high points include Bt Tinggi (55m) and Tanjong Tajam (55m) in the west, and Pulau Ubin (hill) 11 in the east. In between the small hills are valleys which have been penetrated by small Sungei (rivers) which meander a kilometre or so inland. (see figure 1 - click to enlarge)
Figure 1 - Topography of Pulau Ubin (Source: Waller, 2001)
A study of the geological map of the island (figure 2) reveals that the underlying reason for the varied relief is that these low hills are made up of hard granite and associated intrusive rocks of post-Jurassic age [see Geologic Time Table]. The granite hills would have once stood out as small islands within the sea. Now littoral, transitional and alluvial sediments have filled the spaces in between.
Figure 2 - Geology of Pulau Ubin (Source: Waller, 2001)