CONSOL ENERGY INC.

Exhibit 10.21

CONSOL ENERGY INC.

SUPPLEMENTAL INDENTURE NO. 5

$250,000,000

7.875% Notes due 2012

THIS SUPPLEMENTAL INDENTURE NO. 5, dated as of October 21, 2005 (this “Supplemental Indenture No. 5”), by and among CONSOL ENERGY INC., a Delaware corporation (the “Company”), the Guarantors listed on Schedule I hereto and THE BANK OF NOVA SCOTIA TRUST COMPANY OF NEW YORK, a New York trust company, as trustee under the Indenture referred to below (the “Trustee”).

RECITALS OF THE COMPANY

WHEREAS, the Company has heretofore executed and delivered to the Trustee an Indenture dated as of March 7, 2002 (the “Indenture”), a Supplemental Indenture No. 1 dated as of March 7, 2002, a Supplemental Indenture No. 2 dated as of September 30, 2003, a Supplemental Indenture No. 3 dated as of April 15, 2005 and a Supplemental Indenture No. 4 dated as of August 8, 2005 (such Supplemental Indentures, collectively, the “Supplemental Indentures”) providing for the issuance of the 7.875% Notes due 2012 in the aggregate principal amount of $250,000,000;

WHEREAS, Article IX of the Indenture provides for various matters with respect to any series of Securities issued under the Indenture to be established in an indenture supplemental to the Indenture;

WHEREAS, certain subsidiaries of the Company, being CNX Gas Corporation and its subsidiaries CNX Gas Company LLC and Cardinal States Gathering Company, have recently entered into a Revolving Credit Facility, dated as of October 7, 2005, with a group of commercial lenders (the “Credit Facility”) under which CNX Gas Corporation may incur Indebtedness (as defined in the Indenture) by borrowing money, and its subsidiaries will guarantee such indebtedness;

WHEREAS, pursuant to Section 4.07 of the Indenture, upon a borrowing of money under the Credit Facility, CNX Gas Corporation and its subsidiaries would become Guarantor Subsidiaries within the meaning of that term in the Indenture and are required to deliver a Subsidiary Guarantee;

WHEREAS, Section 9.01(a)(11) of the Indenture provides that the Company, the Guarantor Subsidiaries and the Trustee may enter into an indenture supplemental to the Indenture to allow any Guarantor Subsidiary to execute a supplemental indenture in respect of a Subsidiary Guarantee;

WHEREAS, all the conditions and requirements necessary to make this Supplemental Indenture No. 5, when duly executed and delivered, a valid and binding agreement in accordance with its terms and for the purposes herein expressed, have been performed and fulfilled.

NOW THEREFORE, THIS SUPPLEMENTAL INDENTURE NO. 5 WITNESSETH:

For and in consideration of the premises, the Company, the Guarantor Subsidiaries and the Trustee mutually covenant and agree for the equal and proportionate benefit of the respective Holders of the Securities of such series as follows:

ARTICLE ONE

RELATION TO INDENTURE; DEFINITIONS; RULES OF CONSTRUCTION

SECTION 1.1 Relation to Indenture. This Supplemental Indenture No. 5 constitutes an integral part of the Indenture.

SECTION 1.2 Rules of Construction. For all purposes of this Supplemental Indenture No. 5:

(a) capitalized terms used herein without definition shall have the meanings specified in the Indenture;

(b) all references herein to Articles and Sections, unless otherwise specified, refer to the corresponding Articles and Sections of this Supplemental Indenture No. 5;

(c) the terms “herein,” “hereof,” “hereunder” and other words of similar import refer to this Supplemental Indenture No. 5; and

(d) in the event of a conflict with the definition of terms in the Indenture, the definitions in this Supplemental Indenture No. 5 shall control.

ARTICLE TWO

GUARANTOR SUBSIDIARIES

SECTION 2.1 Subsidiary Guarantees. Effective as of the date hereof, each of CNX Gas Corporation, CNX Gas Company LLC and Cardinal States Gathering Company hereby fully and unconditionally Guarantee the Company’s Obligations under the Indenture and under any Securities of any Series issued under the Indenture in accordance with Article XI of the Indenture.

SECTION 2.2 Guarantor Subsidiaries and Guarantors. Effective as of the date hereof, (i) the Guarantor Subsidiaries listed on Schedule I of the Indenture shall be as set forth on Schedule I attached hereto and (ii) the “Guarantors” as defined in the Supplemental Indentures shall mean those subsidiaries of the Company listed on Schedule I attached hereto.

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ARTICLE THREE

MISCELLANEOUS PROVISIONS

SECTION 3.1 Ratification. The Indenture, as supplemented and amended by the Supplemental Indentures and this Supplemental Indenture No. 5, is in all respects hereby adopted, ratified and confirmed.

SECTION 3.2 Trustee Not Liable for Recitals. The recitals contained herein are made by the Company and the Guarantors, and the Trustee assumes no liability for the correctness thereof. The Trustee makes no representations as to the validity or sufficiency of this Supplemental Indenture No. 5.

SECTION 3.3 Counterparts. This Supplemental Indenture No. 5 may be executed in any number of counterparts, each of which when so executed shall be deemed an original, and all such counterparts shall together constitute but one and the same instrument.

SECTION 3.4 Governing Law. THIS SUPPLEMENTAL INDENTURE NO. 5 SHALL BE GOVERNED BY THE INTERNAL LAWS OF THE STATE OF NEW YORK, AND FOR ALL PURPOSES SHALL BE CONSTRUED IN ACCORDANCE WITH THE LAWS OF SAID STATE.

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IN WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture No. 5 to be duly executed as of the day and year first above written.

SCHEDULE I

GUARANTOR SUBSIDIARIES AND GUARANTORS:

Central Ohio Coal Company

(Ohio corporation)

CNX Land Resources Inc.

(Delaware corporation)

CNX Marine Terminals Inc.

(Delaware corporation)

Conrhein Coal Company

(Pennsylvania general partnership)

Consol Docks Inc.

(Delaware corporation)

CONSOL Financial Inc.

(Delaware corporation)

CONSOL of Canada Inc.

(Delaware corporation)

CONSOL of Kentucky Inc.

(Delaware corporation)

CONSOL of WV LLC

(West Virginia limited liability company)

CONSOL Pennsylvania Coal Company

(Delaware corporation)

CONSOL Energy Sales Company

(Delaware corporation)

CONSOLIDATION COAL COMPANY

(Delaware corporation)

Eighty-Four Mining Company

(Pennsylvania corporation)

Helvetia Coal Company

(Pennsylvania corporation)

ISLAND CREEK COAL COMPANY

(Delaware corporation)

Keystone Coal Mining Corporation

(Pennsylvania corporation)

Laurel Run Mining Company

(Virginia corporation)

Leatherwood, Inc.

(Pennsylvania corporation)

McELROY COAL COMPANY

(Delaware corporation)

MTB Inc.

(Delaware corporation)

RESERVE COAL PROPERTIES COMPANY

(Delaware corporation)

Rochester & Pittsburgh Coal Company

(Pennsylvania corporation)

SOUTHERN OHIO COAL COMPANY

(West Virginia corporation)

Terra Firma Company

(West Virginia corporation)

TWIN RIVERS TOWING COMPANY

(Delaware corporation)

Windsor Coal Company

(West Virginia corporation)

WOLFPEN KNOB DEVELOPMENT COMPANY

(Virginia corporation)

CNX Gas Corporation

(Delaware corporation)

CNX Gas Company LLC

(Virginia limited liability company)

Cardinal States Gathering Company

(Virginia general partnership)