problem with table structure

Hi,

I am having some uncertainty while designing the following structure:

I have two sets of data:

* arqueology sites (can be natural):

id
name
description
id_category
id_period
x
y

* natural sites (can be arqueological also - bear with me -, so there will
be duplicate records in the above table and this):

id
name
description
altitude
x
y

and i would like to put these two "sites" in the same data set and maybe add
a new table called "site types" to categorize each record (maybe a relation
table to allow many to many): how can i go about doing it? is this solution
decent enough:

* sites (generic):

id_site
name
description
x
y

* site_natural
id
id_site
altitude

* site_arqueology
id
id_site
id_category
id_period

But i seem to be missing something. How can i have this in a way that its
easy to list only "arqueology sites" for example. I feel the solution is
simple enough, even for me, but its eluding me. Any help in the right
direction would be very appreciated.

Thanks

Pag