Unexpected result from contour plot - where is the color gradient?

When I plot a contour plot of my (in)famous code :P I get some unexpected results that I don't quite understand.

My code (the one that has NDSolve)that has been edited by several authors on mathematica.SE for ease of copying and running.

From the interpolating function polynomial that my NDSolve generates, I define a temperature T and density r and plot them as follows:

fac = 0.75;
T = 1 - (Bi*hSol[x, y, fac*TRup]/(Bi*hSol[x, y, fac*TRup] + 1));
a = 9.2*10^-4;
b = 4.5*10^-7;
r = 1.33/(1 + a *(T) +
b*(T)^2); (*Density as a function of temperature*)

Plot3D[
1 - (Bi*hSol[x, y, fac*TRup]/(Bi*hSol[x, y, fac*TRup] + 1)), {x,
0,
L}, {y, 0, L},

PlotRange -> {{0, L}, {0, L}, {0, 3.5}},
BaseStyle -> {FontWeight -> "Bold", FontSize -> 18}
]

ContourPlot[
Evaluate[\!$$\*SubscriptBox[\(\[PartialD]$$, $$x$$]r\) + \!$$\*SubscriptBox[\(\[PartialD]$$, $$\(y$$$$\$$\)]r\), {x, 0, L}, {y,
0, L}],

PlotRange -> {{0, L}, {0, L}, {0, 3.5}},
BaseStyle -> {FontWeight -> "Plain", FontSize -> 18},
ColorFunction -> Gray
]


The Plot3D plots the temperature and then a top view looks like this:

whilst the contour plot of density r, which is a function of T looks, bewilderingly, like this:

The little bumps as seen in the temperature plot aren't captured by my contour plot.

Now, I can't tell if I am missing any option in the contour plot. I assumed that I'd see a smooth gradient and not a binary black and white sudden transition in the contour plot of density.

Any suggestions? Thoughts? Recommendations?

-
Contour plots are not too good with depicting bumps; you might want to consider using a density plot instead of a contour plot... – J. M. Jun 29 '12 at 16:12
@J.M. Tried that. Density Plots give me the same kind of result. – drN Jun 29 '12 at 16:15
Did you try increasing the setting for PlotPoints? – J. M. Jun 29 '12 at 16:18
How high are the bumps? – Brett Champion Jun 29 '12 at 17:28
AFAK, Gray is just a fixed colour defined as GrayLevel[.5], not a function. You could try setting ColorFunction to an actual function such as for example Hue or GrayLevel. – Heike Jun 29 '12 at 20:32