In this paper we propose an algorithm to detect ridges in the finite time Lyapunov exponent field obtained form a continuous dynamical system or flow. These ridges represent time-dependent separatrices of the flow and are also called Lagrangian coherent structures (LCS). LCS have been demonstrated to be an effective way to analyze realistic time-chaotic flows, although they can be quite complex. Therefore, in order to exploit the information that LCS can provide it is important to locate and characterize these structures in a systematic way. This can be accomplished by interpreting the FTLE as a height field and detecting the LCS as ridges of this graph incorporating methods from image processing. Here we focus our attention on 2-D fluid flows. We review the definition of a ridge, propose an algorithm for detecting ridges and provide a measure of their strength. Finally we show results on a simple analytical model case as well as a noisy LCS from realistic geophysical fluid data.